Apparatus for generating an image from a digital video signal

Abstract

An apparatus for generating a high quality image from a digital video signal includes a system for gamma correcting the digital video signal with a digital look up table and for converting the resultant digital signal to an analog video signal. Another circuit generates a triangular wave reference pattern signal and a comparator compares the analog video signal with the triangular wave reference pattern signal to form a pulse-width-modulated signal. A raster scanning print engine producing, for example, a laser beam, scans over a recording medium in accordance with the pulse-width-modulated signal, thereby forming an image of high quality on the recording medium of a print engine. This apparatus can also be used with an analog video signal by first converting the analog video signal to a digital video signal with an analog to digital converter.

1. An image processing apparatus responsive to a digital input signal, said apparatus comprising:
 a  raster scanning print engineimage forming means for generating a series of successive scan lines;  
 means for generating an analog pattern signal;  
 means for generating a pulse-width-modulated signal from a digital input signal input to said apparatus and from said analog pattern signal; and  
 means for applying said pulse-width-modulated signal to said print engine  image forming means to cause said print engine  image forming means to generate each said scan line as a succession of line-segments, the lengths of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, said line screen comprising a plurality of columns of said line segments.  
 
     
     
       2. An image processing apparatus responsive to a digital input signal, said apparatus comprising:
 a raster scanning print engine for generating a series of successive scan lines;  
 means for generating a pulse-width-modulated signal from a digital input signal input to said apparatus; and  
 means for applying said pulse-width-modulated signal to said print engine to cause said print engine to generate each said scan line as a succession of line-segments, the lengths of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, said line screen comprising a plurality of columns of said line segments, wherein said digital input signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital input signal has the minimum value.  
 
     
     
       3. An image processing apparatus according to  claim 2 , wherein the axes of said columns comprising said line screen are substantially perpendicular to said scan lines. 
     
     
       4. An image processing apparatus according to  claim 2 , wherein the axes of said columns comprising said line screen extend at an oblique angle to said scan lines. 
     
     
       5. Image processing apparatus according to  claim 2 , wherein said reference pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal. 
     
     
       6. An image processing apparatus according to  claim 2 , wherein said generating means includes converting means for converting said digital input signal to an analog video signal, reference pattern signal generating means for generating an analog reference pattern signal of predetermined period, and comparing means for comparing said converted analog video signal with said analog reference pattern signal and for generating said pulse-width-modulated signal on the basis of said comparison. 
     
     
       7. Image processing apparatus according to  claim 6 , wherein said reference pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       8. An image processing apparatus responsive to a digital input signal, said apparatus comprising:
 a raster scanning print engine for generating a series of successive scan lines;  
 means for generating a pulse-width modulated signal from a digital input signal input to said apparatus; and  
 means for applying said pulse-width-modulated signal to said print engine to cause said print engine to generate each said scan line as a succession of line-segments, the lengths of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, said line screen comprising a plurality of columns of said line segments wherein said digital input signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital input signal has the maximum value.  
 
     
     
       9. Image processing apparatus according to  claim 8 , wherein the axes of said columns comprising said line screen are substantially perpendicular to said scan lines. 
     
     
       10. Image processing apparatus according to  claim 8 , wherein the axes of said columns comprising said line screen extend at an oblique angle to said scan lines. 
     
     
       11. Image processing apparatus according to  claim 8 , wherein said generating means includes converting means for converting said digital input signal to an analog video signal, reference pattern signal generating means for generating an analog reference pattern signal of predetermined period, and comparing means for comparing said converted analog video signal with said analog reference pattern signal and for generating said pulse-width-modulated signal on the basis of said comparison. 
     
     
       12. Image processing apparatus according to  claim 11 , wherein said reference pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal. 
     
     
       13. Image processing apparatus according to  claim 11 , wherein said reference pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       14. An image processing apparatus for forming an image on a recording medium, said apparatus comprising:
 video signal output means for generating an analog video signal;  
 pattern signal generating means for producing a pattern signal of predetermined period;  
 a pulse-width-modulated signal generating means for generating a pulse-width modulated signal in accordance with the analog video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and  
 image forming means for scanning lines on a recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width modulated signal generating means thereby forming an image on said recording medium; said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal.  
 
     
     
       15. An image processing apparatus according to  claim 14 , wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       16. An image processing apparatus according to  claim 14 , further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal. 
     
     
       17. Image processing apparatus according to  claim 14 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       18. Image processing apparatus according to  claim 14 , wherein said pattern signal generating means includes means for freely varying a period of the pattern signal generated. 
     
     
       19. Image processing means according to  claim 14 , wherein said pulse-width-modulated signal generating means includes means for comparing said analog video signal with said pattern signal and for generating said pulse-width modulated signal on the basis of the comparison result. 
     
     
       20. Image processing apparatus according to  claim 14 , wherein said pattern signal generating means includes timing changing means for changing a timing for generation of the pattern signal in association with the scanning line. 
     
     
       21. Image processing apparatus according to  claim 14 , wherein one period of said pattern signal corresponds to a plurality of pixels of the analog video signal. 
     
     
       22. An image processing apparatus according to  claim 14 , wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal. 
     
     
       23. An image processing apparatus according to  claim 14 , wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       24. Image processing apparatus according to  claim 14 , further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into the analog video signal. 
     
     
       25. Image processing apparatus according to  claim 24 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value. 
     
     
       26. Image processing apparatus according to  claim 24 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value. 
     
     
       27. Image processing apparatus according to  claim 24 , wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a firSt clock for generating said pattern signal by dividing the frequency of said reference clock, and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock. 
     
     
       28. Image processing apparatus according to  claim 27 , wherein each of said first and second clocks is generated in accordance with the synchronizing signal. 
     
     
       29. Image processing apparatus according to  claim 24 , wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into the analog video signal. 
     
     
       30. Image processing apparatus according to  claim 29 , wherein said characteristic converting means includes means for changing a factor for converting the characteristic of said digital video signal for each line scanned by said image forming means. 
     
     
       31. Image processing apparatus according to  claim 30 , wherein said factor changing means changes the factor in accordance with the synchronizing signal. 
     
     
       32. An image processing apparatus comprising:
 digital video signal generating means for generating a digital video signal having a characteristic;  
 characteristic converting means for converting the characteristic of said digital video signal generated by said digital video signal generating means and for producing a converted digital video signal therefrom;  
 digital-to-analog converting means for converting the converted digital video signal generated by said characteristic converting means into an analog video signal;  
 pattern signal generating means for generating a pattern signal of predetermined period; and  
 pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with said analog video signal and said pattern signal.  
 
     
     
       33. An image processing apparatus according to  claim 32 , wherein said digital video signal ranges between maximum and minimum values, and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the minimum value. 
     
     
       34. An image processing apparatus according to  claim 32 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the maximum value. 
     
     
       35. An image processing apparatus according to  claim 32 , wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal. 
     
     
       36. Image processing apparatus according to  claim 32 , wherein said pattern signal generating means includes means for freely varying a period of the pattern signal generated. 
     
     
       37. Image processing apparatus according to  claim 32 , wherein said pulse-width-modulated signal generating means includes means for comparing said analog video signal with said pattern signal and for generating said pulse-width modulated signal on the basis of the comparison result. 
     
     
       38. Image processing apparatus according to  claim 32 , wherein one period of said pattern signal corresponds to a plurality of pixels of the digital video signal. 
     
     
       39. An image processing apparatus according to  claim 32 , wherein said characteristic converting means comprises storage means containing digital information for providing at least one non-linear transformation of said digital video signal. 
     
     
       40. An image processing apparatus according to  claim 39 , wherein said storage means comprises a read only memory for storing a digital look-up table for gamma correction. 
     
     
       41. An image processing apparatus according to  claim 32 , further comprising image forming means for scanning successive lines on a recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, and wherein said characteristic converting means includes means for changing the factor converting the characteristic of said digital video signal for each of the successive lines scanned by said image forming means. 
     
     
       42. Image processing apparatus according to  claim 41 , wherein said pattern signal generating means includes timing changing means for changing a timing for generation of the pattern signal in association with the scanning line. 
     
     
       43. Image processing apparatus according to  claim 41 , wherein said image forming means includes means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal. 
     
     
       44. Image processing apparatus according to  claim 37 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       45. Image processing apparatus according to  claim 43 , wherein said factor changing means changes the factor in accordance with the synchronizing signal. 
     
     
       46. Image processing apparatus according to  claim 43 , further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal. 
     
     
       47. An apparatus for generating an image from a digital video signal comprising:
 A. a raster scanning print engine for generating a series of successive scanning lines;  
 B. a digital look up table addressable by said digital video signal;  
 C. means for generating from said digital video signal a pulse-width-modulated signal, said pulse-width-modulated signal generating means comprising: 
 (1) means for applying said digital video signal as an address to said digital look up table for producing in accordance with said digital look up table a resultant gamma corrected digital signal;  
 (2) means for converting said resultant gamma corrected digital signal to an analog video signal;  
 (3) means for generating a periodic analog reference signal of predetermined period; and  
 (4) means for comparing said analog video signal to said periodic analog reference signal thereby to produce said pulse-width-modulated signal; and  
 
 D. means for applying said pulse-width modulated signal to said print engine for generating each said scanning line as a succession of line-segments, the length of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, an axis of symmetry of the line screen being substantially normal to the scanning lines.  
 
     
     
       48. An apparatus for generating an image from a digital video signal according to  claim 47 , wherein said periodic analog reference signal generating means generates as said analog reference signal a triangular wave signal of a predetermined period. 
     
     
       49. An apparatus for generating an image according to  claim 47 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value. 
     
     
       50. An apparatus for generating an image according to  claim 47 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the maximum value. 
     
     
       51. An apparatus for generating an image according to  claim 47 , wherein periodic analog reference signal generating means includes means for adjusting at least one of the amplitude and offset of said analog reference signal. 
     
     
       52. Apparatus according to  claim 47 , further comprising table changing means for changing the digital look up table utilized in association with the scanning line generated by said raster scanning print engine. 
     
     
       53. Apparatus according to  claim 47 , wherein one period of the analog reference signal corresponds to a plurality of pixels of the digital video signal. 
     
     
       54. Apparatus according to  claim 19 , wherein said raster scanning print engine includes image forming means for scanning lines on a recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a.synchronizing signal for each line scanned on the recording medium, said periodic analog reference signal generating means generating the periodic analog reference signal of predetermined period in accordance with said synchronizing signal. 
     
     
       55. Image processing apparatus according to  claim 54 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       56. An image processing apparatus responsive to a digital video signal input thereto, said apparatus comprising:
 a digital-to-analog converting means for converting the digital video signal input to said apparatus into an analog video signal;  
 a pattern signal generating means for generating a pattern signal of a predetermined period; and  
 a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with said converted analog video signal and said pattern signal.  
 
     
     
       57. An image processing apparatus according to  claim 56 , wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       58. Image processing apparatus according to  claim 56 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the minimum value. 
     
     
       59. Image processing apparatus according to  claim 56 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value. 
     
     
       60. Image processing apparatus according to  claim 56 , wherein said pulse-width-modulated signal generating means includes means for comparing said analog video signal with said pattern signal and for generating said pulse-width modulated signal on the basis of the comparison result. 
     
     
       61. Image processing apparatus according to  claim 56 , wherein one period of said pattern corresponds to a plurality of pixels of the digital video signal. 
     
     
       62. Image processing apparatus according to  claim 56 , further comprising image forming means for forming an image by lines on a recording medium in accordance with said pulse-width-modulated signal generated by said pulse-width modulated signal generating means, said image forming means including means for generating a synchronizing signal for each line on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal. 
     
     
       63. Image processing apparatus according to  claim 62 , wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal, thereby forming the image on the recording medium, and wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam, and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       64. Image processing apparatus according to  claim 62 , further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal. 
     
     
       65. Image processing apparatus according to  claim 62 , wherein said pattern signal generating means includes timing changing means for changing a timing for generation of the pattern signal in association with each line. 
     
     
       66. Image processing apparatus according to  claim 56 , further comprising digital video signal input means for inputting the digital video signal having a characteristic and characteristic converting means for converting the characteristic of said digital video signal input by said digital video signal input means and for producing a converted digital video signal therefrom, wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into the analog video signal. 
     
     
       67. Image processing apparatus according to  claim 66 , wherein said characteristic converting means includes a table for entering as an address the digital video signal input from said digital video signal input means, the converted digital video signal being produced from said table. 
     
     
       68. Image processing apparatus according to  claim 67 , wherein a plurality of tables for entering as an address the digital video signal input from said digital video signal input means are provided, and further comprising image forming means for scanning lines on a recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, and table changing means for changing the table utilized in association with the scanning line. 
     
     
       69. Image processing apparatus according to  claim 68 , wherein said image forming means includes means for generating a synchronizing signal for each line scanned on the recording medium, and said table changing means changes the table in accordance with the synchronizing signal. 
     
     
       70. Image processing apparatus according to  claim 69 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       71. Image processing apparatus comprising:
 digital video signal generating means for generating a digital video signal having a characteristic;  
 characteristic converting means for converting the characteristic of said digital video signal generated by said digital video signal generating means and for producing a converted digital video signal therefrom, said characteristic converting means including changing means for periodically changing a plurality of factors for converting the characteristic of said digital video signal at a predetermined interval thereby to convert the characteristic of said digital video signal generated by said digital video signal generating means and to produce said converted digital video signal by using said plurality of factors; and  
 image forming means for forming an image in accordance with said converted digital video signal, said image forming means forming said image by scanning lines on a recording medium in accordance with said converted digital video signal;  
 wherein said characteristic converting means uses the same ones of said plurality of factors for at least one scanning line scanned by said image forming means, and said changing means changes said plurality of factors after scanning of said at least one scanning line scanned by said scanning means.  
 
     
     
       72. Image processing apparatus according to  claim 71 , wherein said image forming means includes pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the converted video signal. 
     
     
       73. Image processing apparatus according to  claim 71 , wherein said characteristic converting means comprises storage means containing digital information for providing at least one non-linear transformation of said digital video signal. 
     
     
       74. Image processing apparatus according to  claim 73 , wherein said storage means comprises a read only memory for storing a digital look-up table for gamma correction. 
     
     
       75. Image processing apparatus according to  claim 71 , wherein said characteristic converting means includes a table for entering as an address the digital video signal generated from said video signal generating means, the converted digital video signal being produced from the table. 
     
     
       76. Image processing apparatus according to  claim 75 , wherein a plurality of tables are provided for entering as an address the digital video signal generated from the video signal generating means, and wherein said changing means changes the table for each line scanned by said image forming means. 
     
     
       77. Image processing apparatus according to  claim 76 , wherein said image forming means includes digital-to-analog converting means for converting the converted digital video signal generated by said characteristic converting means into an analog video signal, pattern signal generating means for generating a pattern signal of predetermined period, pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with said analog video signal and said pattern signal, and a raster scanning print engine for generating a series of successive scan lines with a beam in accordance with said pulse-width-modulated signal. 
     
     
       78. Image processing apparatus according to  claim 77 , wherein said digital video signal ranges between maximum and minimum values, and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the minimum value. 
     
     
       79. Image processing apparatus according to  claim 77 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the maximum value. 
     
     
       80. Image processing apparatus according to  claim 77 , wherein said pattern signal generating means includes an integrator for entering a clock having a predetermined period and for generating a triangular wave signal as the pattern signal on the basis of the clock having a predetermined period. 
     
     
       81. Image processing apparatus according to  claim 77 , wherein said image forming means includes means for generating a synchronizing signal for each line scanned on the recording medium, and said changing means changes the table in accordance with the synchronizing signal. 
     
     
       82. Image processing apparatus according to  claim 81 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means. 
     
     
       83. Image processing apparatus according to  claim 81 , wherein said pattern signal generating means generates the pattern signal of predetermined period in accordance with said synchronizing signal. 
     
     
       84. Image processing apparatus according to  claim 81 , wherein one period of said pattern corresponds to a plurality of pixels of the digital video signal. 
     
     
       85. Image processing apparatus for forming an image, said apparatus comprising:
 video signal output means for generating an analog video signal;  
 pattern signal generating means for producing a pattern signal of predetermined period, one period of said pattern signal corresponding to a plurality of pixels of the analog video signal;  
 a pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the analog video signal generated by said video signal output means and said pattern signal generated by said pattern signal generating means; and  
 image forming means for forming an image in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means.  
 
     
     
       86. Image processing apparatus according to  claim 85 , wherein said pattern signal generating means generates as said pattern signal a triangular wave signal of predetermined period. 
     
     
       87. Image processing apparatus according to  claim 85 , wherein said pattern signal generating means includes means for adjusting at least one of the amplitude and offset of said pattern signal. 
     
     
       88. Image processing apparatus according to  claim 85 , wherein said image forming means scans lines on the recording medium with a beam in accordance with said pulse-width-modulated signal generated by said pulse-width-modulated signal generating means thereby forming an image on said recording medium, said image forming means including means for generating a synchronizing signal for each line scanned on the recording medium, said pattern signal generating means generating the pattern signal of predetermined period in accordance with said synchronizing signal. 
     
     
       89. Image processing apparatus according to  claim 88 , wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the bases of a detection output from said detecting means. 
     
     
       90. Image processing apparatus according to  claim 88 , further comprising reference clock generating means for generating a reference clock, said pattern signal generating means producing a clock for generating said pattern signal by dividing the frequency of said reference clock in accordance with said synchronizing signal. 
     
     
       91. Image processing apparatus according to  claim 85 , further comprising digital video signal generating means for generating a digital video signal, wherein said video signal output means includes digital-to-analog converting means for converting the digital video signal generated by said digital video signal generating means into the analog video signal. 
     
     
       92. Image processing apparatus according to  claim 91 , wherein said digital video signal generating means includes digital video signal output means for outputting a digital video signal having a characteristic, and characteristic converting means for converting the characteristic of said digital video signal output from said digital video signal output means and for producing a converted digital video signal therefrom and wherein said digital-to-analog converting means converts the converted digital video signal generated by said characteristic converting means into the analog video signal. 
     
     
       93. Image processing apparatus according to  claim 91 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width modulated signal having a predetermined pulse width when said digital video signal has the minimum value. 
     
     
       94. Image processing apparatus according to  claim 91 , wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having a predetermined pulse width when said digital video signal has the maximum value. 
     
     
       95. Image processing apparatus according to  claim 91 , wherein one period of the pattern signal corresponds to a plurality of pixels of the digital video signal. 
     
     
       96. Image processing apparatus according to  claim 91 , wherein said apparatus further comprises reference clock generating means for generating a reference clock, said pattern signal generating means producing a fIrst clock for generating said pattern signal by dividing the frequency of said reference clock and wherein said apparatus further comprises means for generating a second clock by dividing the frequency of the reference clock, said digital video signal generating means generating the digital video signal in synchronism with the second clock. 
     
     
       97. Image processing apparatus according to  claim 85 , wherein one period of said pattern signal corresponds to two pixels of the analog video signal.  
     
     
       98. Image processing apparatus according to  claim 85 , wherein one period of said pattern signal corresponds to three pixels of the analog video signal.  
     
     
       99. An image processing apparatus responsive to a digital input signal, said apparatus comprising:
 raster scanning image forming means for generating a series of successive scan lines;    means for digitally converting a characteristic of a digital input signal input to said apparatus to produce a converted digital signal;    means for generating a pulse-width-modulated signal from said converted digital signal; and    means for applying said pulse-width-modulated signal to said image forming means to cause said image forming means to generate each said scan line as a succession of line-segments, the lengths of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, said line screen comprising a plurality of columns of said line segments.   
     
     
       100. An image processing apparatus according to  claim 99 , wherein said characteristic converting means includes means for gamma-correcting said digital input signal.  
     
     
       101. An image processing apparatus responsive to a digital input signal, said apparatus comprising:
 raster scanning image forming means for generating a series of successive scan lines;    means for digitally nonlinearly generating a generated digital signal from a digital input signal input to said apparatus;    means for generating a pulse-width-modulated signal from said generated digital signal; and    means for applying said pulse-width-modulated signal to said image forming means to cause said image forming means to generate each said scan line as a succession of line-segments, the lengths of which are controlled in accordance with said pulse-width-modulated signal to produce a variable density line screen from said line segments, said line screen comprising a plurality of columns of said line segments.    
     
     
       102. An image processing apparatus according to  claim 101 , wherein said nonlinear generating means includes means for gamma-correcting said input signal.  
     
     
       103. In a pixel recording pulse signal generation method in which tone data signals representing respective pixels of an image are transformed into respective pixel recording pulse signals each having a time width proportional to the value of the respective tone data signal, and in which said pixel recording pulse signals are used to print respective dots in respective pixels along scanning lines to reproduce said image, wherein only one dot is printed in each pixel of the reproduced image, the improvement wherein the pixels along each scanning line of the reproduced image are divided into pairs of adjacent pixels, each pair consisting of a first pixel and a second pixel which succeeds the first pixel, and wherein the end of the pixel recording pulse corresponding to the first pixel is made to coincide with the beginning of the pixel recording pulse signal corresponding to the second pixel, thus causing the dots printed in the first and second pixels to be contiguous over the boundary in the first and second pixels.  
     
     
       104. A scanning recording type printing device comprising:
 memory means for storing tone data signals for a scanning line;    means for generating a triangular comparison data signal for each of a plurality of pairs of pixels of an image to be recorded;    means for comparing said stored tone data signals with said comparison data signal and for generating a pixel recording pulse based on the comparison;    a semiconductor laser circuit for producing laser light based on said pixel recording pulse signal;    means for recording an image corresponding to said pixel recording pulse signal on a recording medium by sweeping said laser light across said recording medium in a scanning direction to record a dot in each of said plurality of pixels; and    timing treatment means for controlling said memory means, said comparison data signal production means, said pixel recording pulse signal production means and said image recording means such that no interruptions in the scanning direction exist between dots in first ones of said plurality of pixels and dots in respective succeeding ones of said plurality of pixels thereby producing dots which are continuous in the scanning direction across the boundaries of the said first and succeeding pixels, wherein each of said plurality of dots has a width in a direction perpendicular to the scanning direction that is constant and a length in the scanning direction that is proportional to a respective one of said tone data signals.    
     
     
       105. A scanning recording type printing device comprising:
 memory means for storing tone data signals for a scanning line;    means for generating a triangular comparison data signal for each of a plurality of pairs of pixels of an image to be recorded;    means for comparing said stored tone data signals with said comparison data signal and for generating a pixel recording pulse based on the comparison;    a semiconductor laser circuit for producing laser light based on said pixel recording pulse signal;    means for recording an image corresponding to said pixel recording pulse signal on a recording medium by sweeping said laser light across said recording medium in a scanning direction to record a dot in each of said plurality of pixels; and    timing treatment means for controlling said comparison data signal production means, said pixel recording pulse signal production means and said image recording means such that no interruptions in the scanning direction exist between dots in first ones of said plurality of pixels and dots in respective succeeding ones of said plurality of pixels thereby producing dots which are continuous in the scanning direction across the boundaries of the said first and succeeding pixels, wherein each of said plurality of dots has a width in a direction perpendicular to the scanning direction that is constant and a length in the scanning direction that is proportional to a respective one of said tone data signals.    
     
     
       106. A scanning recording type printing method in which an image is printed by printing a dot in each of a plurality of pixels of the printed image, wherein the length of each dot in a scanning direction is proportional to respective tone data sampled from an original image and the widths of each dot in a direction perpendicular to the scanning direction is constant, said method comprising the step of:
 locating dots within selected pixels in a scanning line of the printed image such that the dots in the selected pixels are printed at second ends of the selected pixels, said second ends of said selected pixels being adjacent to first ends of respective succeeding pixels, wherein no interruptions in the scanning direction exist between the dots in the selected pixels and dots in the respective succeeding pixels.    
     
     
       107. A scanning type printing method according to  claim 106 , wherein said selected pixels in said scanning line comprise odd-numbered pixels in said scanning line and the succeeding pixel comprises even-numbered pixels in said scanning line.  
     
     
       108. A scanning type printing method according to  claim 106 , wherein said selected pixels in said scanning line comprise even-numbered pixels in said scanning line and the succeeding pixels comprise odd-numbered pixels in said scanning line.  
     
     
       109. A scanning recording type printing device comprising:
 memory means for storing tone data signals for a scanning line;    means for generating a triangular comparison data signal for each of a plurality of pairs of pixels of an image to be recorded;    means for comparing said stored tone data signals with said comparison data signal and for generating a pixel recording pulse based on the comparison;    a semiconductor laser circuit for producing laser light based on said pixel recording pulse signal;    means for recording an image corresponding to said pixel recording pulse signal on a recording medium by sweeping said laser light across said recording medium in a scanning direction to record a tone dot in each of said plurality of pixels; and    timing treatment means for controlling said memory means, said comparison data signal production means, said pixel recording pulse signal production means and said image recording means such that tone dots within selected pixels in a scanning line of the recorded image are recorded at second ends of said selected pixels, said second ends of said selected pixels being adjacent to first ends of respective succeeding pixels, wherein said tone dots have a constant width in a direction perpendicular to the scanning direction that is constant and each tone dot has a length in the scanning direction that is proportional to a respective one of said tone data signals.    
     
     
       110. A scanning recording type printing device according to  claim 109 , wherein said selected pixels in said scanning line comprise odd-numbered pixels in said scanning line and the succeeding pixel comprise even- numbered pixels in said scanning line.    
     
     
       111. A scanning recording type printing device according to  claim 109 , wherein said selected pixels in said scanning line comprises even-numbered pixels in said scanning line and the succeeding pixels comprise odd-numbered pixels in said scanning line.  
     
     
       112. A scanning recording type printing device according to  claim 109 , further comprising means for controlling a screen angle at which the tone dots are recorded by controlling said comparison data signal production means.  
     
     
       113. A scanning recording type printing device comprising:
 memory means for storing tone data signals for a scanning line;    means for generating a triangular comparison data signal for each of a plurality of pairs of pixels of an image to be recorded;    means for comparing said stored tone data signals with said comparison data signal and for generating a pixel recording pulse based on the comparison;    a semiconductor laser circuit for producing laser light based on said pixel recording pulse signal;    means for recording an image corresponding to said pixel recording pulse signal on a recording medium by sweeping said laser light across said recording medium in a scanning direction to record a tone dot in each of said plurality of pixels; and    timing treatment means for controlling said comparison data signal production means, said pixel recording pulse signal production means and said image recording means such that tone dots within selected pixels in a scanning line of the recorded image are recorded at second ends of said selected pixels, said second ends of said selected pixels being adjacent to first ends of respective succeeding pixels, wherein said tone dots have a constant width in a direction perpendicular to the scanning direction that is constant and each tone dot has a length in the scanning direction that is proportional to a respective one of said tone data signals.    
     
     
       114. A scanning recording type printing device according to  claim 113 , wherein said selected pixels in said scanning line comprise odd-numbered pixels in said scanning line and the succeeding pixel comprise even-numbered pixels in said scanning line.  
     
     
       115. A scanning recording type printing device according to  claim 113 , wherein said selected pixels in said scanning line comprise even-numbered pixels in said scanning line and the succeeding pixels comprise odd-numbered pixels in said scanning line.  
     
     
       116. A scanning recording type printing device according to  claim 113 , further comprising means for controlling a screen angle at which the tone dots are recorded by controlling said comparison data signal production means.  
     
     
       117. A scanning recording type printing method in which an image is printed by printing a dot in each of a plurality of pixels of the printed image, wherein the length of each dot in a scanning direction is proportional to respective tone data sampled from an original image and the width of each dot in a direction perpendicular to the scanning direction is constant, said method comprising the steps of:
 locating second ends of first pixels and second ends of dots in said first pixels in coincidence with each other, and locating first ends of respective second pixels succeeding said first pixels in a scanning line and first ends of dots in said second pixels in coincidence with each other such that no interruptions in the scanning direction exist between said dots in said first pixels and said dots in said second pixels, thereby causing the printed image to have one continuous dot per pair of pixels; or    locating first ends of first pixels and first ends of dots in said first pixels in coincidence with each other and locating second ends of respective second pixels succeeding said first pixels in a scanning line and second ends of dots in said second pixels in coincidence with each other such that no interruptions in the scanning direction exist between white areas without said dots recorded within said first pixels and white areas without said dots recorded within said second pixels, thereby causing the printed image to have one continuous white area per pair of pixels.    
     
     
       118. A method according to  claim 103 , wherein said pixel recording pulse signals are formed based on the tone data signals and a comparison data signal.  
     
     
       119. A method according to  claim 118 , wherein said comparison data signal is a triangular comparison signal for each of a plurality of pairs of pixels of an image to be recorded.  
     
     
       120. A method according to  claim 103 , wherein the tone data signals are obtained by characteristic-conversion of an input digital video signal.  
     
     
       121. A method according to  claim 118 , wherein said pixel recording signals are obtained using a signal obtained by analog-conversion of the tone data signals.  
     
     
       122. An image processing apparatus operable in conjunction with a photosensitive member, said apparatus comprising:
   digital video signal generating means for generating a digital video signal having a characteristic;      characteristic conversion means for converting the characteristic of the digital video signal generated by said digital video signal generating means to generate a characteristic-converted digital video signal; and    electro-photographic system image forming means for forming an image by line-scanning said photosensitive member with an optical beam in accordance with the characteristic-converted digital video signal,    said characteristic conversion means comprising changing means for periodically changing a plurality of gamma-conversion characteristics for converting the digital video signal, during formation of an image, and said plurality of characteristics are set so that a change in characteristics among the plurality of gamma-conversion characteristics in a light area of an image may be greater than that in a dark area of an image.    
     
     
       123. An apparatus according to  claim 122 , wherein said changing means is arranged so as to change said gamma-conversion characteristics according to line-scanning by said image forming means.  
     
     
       124. An image processing apparatus, comprising:
 video signal input means for inputting a digital video signal capable of representing a gradation level with a plurality of bits for each pixel of an image;    pattern signal generating means for generating a plurality of different pattern signals, each of which may have one of a plurality of different periods;    selection means for selecting one of the plurality of periods of the pattern signal generated by the pattern signal generating means;    pulse-width modulated signal generating means for generating a pulse-width modulated signal having a pulse width corresponding to a gradation level of the digital video signal input by said digital video signal input means based on the digital video signal and a pattern signal generated by said pattern signal generating means; and    image forming means for forming an image in accordance with the pulse-width modulated signal generated by said pulse-width modulated signal generating means.    
     
     
       125. An apparatus according to  claim 124 , wherein at least one of the periods of said pattern signal generated by said pattern signal generating means corresponds to a plurality of pixels of said digital video signal.