Method of Controlling an Image Capturing System, Image Capturing System and Digital Camera

Abstract

A method of controlling an image capturing system comprising an interface for receiving an external trigger to capture an image, and an image capturing device provided with a photosensitive area and an array of pixel cells, each pixel cell including a device for generating a signal indicative of the intensity of light falling on an associated part of the photosensitive area, which image capturing device is further provided with readout circuitry for generating an array of pixel values to capture an image frame at a set spatial resolution, such that each pixel value represents an integral of the signal or signals generated in at least one of the pixel cells in an associated one of a number of areas over an exposure time interval, the number of areas being determined by the set spatial resolution, the areas together covering a region of the photosensitive area corresponding to a region in the image, comprises receiving an external trigger to capture an image, and, in response to the external trigger, directing the image capturing device to capture at least two image frames by generating respective arrays of pixel values representing integrals over respective consecutive exposure time intervals. The spatial resolutions of at least two of the captured image frames are set to different values.

1 . Method of controlling an image capturing system comprising an interface for receiving an external trigger to capture an image, and an image capturing device provided with a photosensitive area and an array of pixel cells, each pixel cell including a device for generating a signal indicative of the intensity of light falling on an associated part of the photosensitive area, which image capturing device is further provided with readout circuitry for generating an array of pixel values to capture an image frame at a set spatial resolution, such that each pixel value represents an integral of the signal or signals generated in at least one of the pixel cells in an associated one of a number of areas over an exposure time interval, the number of areas being determined by the set spatial resolution, the areas together covering a region of the photosensitive area corresponding to a region in the image, which method comprises receiving an external trigger to capture an image, and, in response to the external trigger, directing the image capturing device to capture at least two image frames by generating respective arrays of pixel values representing integrals over respective consecutive exposure time intervals, wherein the spatial resolutions of at least two of the captured image frames are set to different values. 
   
   
       2 . Method according to  claim 1 , wherein at least the lower of the spatial resolution values is set by directing the image capturing device to generate an array of pixel values in such a manner that each pixel value is representative of the integral of the sum of the signals generated by at least two devices in pixel cells. 
   
   
       3 . Method according to  claim 1 , including retrieving a desired exposure time for a combined final image, determining the number of image frames to be captured, for each image frame, calculating settings determining an exposure level applicable to the image frame, the settings including the length of the exposure time interval, wherein the settings are calculated so that the sum of the lengths of the exposure time intervals over the number of image frames is equal to or less than the desired exposure time. 
   
   
       4 . Method according to  claim 2 , wherein at least the length of the exposure time interval applicable to the image frame at the lower of the spatial resolution values is calculated in dependence of the spatial resolution value. 
   
   
       5 . Method according to  claim 1 , including the step of generating a set of arrays of pixel values, each based on one of the captured image frames, in such a manner that each encodes at least a region of an adjusted frame at the same spatial resolution. 
   
   
       6 . Image capturing system comprising an interface for receiving an external trigger to capture an image, an image capturing device provided with a photosensitive area and an array of pixel cells, each pixel cell including a device for generating a signal indicative of the intensity of light falling on an associated part of the photosensitive area, which image capturing device is further provided with readout circuitry for generating an array of pixel values to capture an image frame at a set spatial resolution, such that each pixel value represents an integral of the signal or signals generated in at least one of the pixel cells in an associated one of a number of areas over an exposure time interval, the number of areas being determined by the set spatial resolution, the areas together covering a region of the photosensitive area corresponding to a region in the image, which image capturing system comprises a control system for controlling the operation of the image capturing device and for processing commands received through the interface, wherein the control system is configured to, in response to the external trigger, direct the image capturing device to capture at least two image frames by generating respective arrays of pixel values representing integrals over respective consecutive exposure time intervals, wherein the control system is further configured to set the spatial resolutions of at least two of the captured image frames to different values. 
   
   
       7 . Image capturing system according to  claim 6 , wherein the control system is configured to execute a method according to  claim 1 . 
   
   
       8 . Method of forming a combined final image from a plurality of image frames, including the steps of: obtaining a first and at least one further array of intensity values, each array of intensity values encoding light intensity levels at each of a respective number of pixel positions in the respective image frame, the number determining the spatial resolution of the image frame concerned, generating a set of derived arrays of intensity values, each derived array being based on a respective one of the obtained arrays of intensity levels and encoding light intensity levels at each of a common number of pixel positions in at least a region of overlap of the respective image frames, generating an array of combined intensity values, each element in the array based on a sum of intensity values represented by the corresponding element in each of the respective derived arrays of intensity values, and providing an array of intensity values encoding the combined final image, the array being based on the array of combined intensity values, wherein a first array of intensity values encoding at least the region of overlap at a higher resolution than the further arrays of intensity values is obtained, an array of intensity values encoding at least the region of overlap in the combined final image at a higher spatial resolution than the further arrays of intensity values is provided, and the array of intensity values encoding the combined final image is based on a sufficient number of intensity values in the first array of intensity values to encode the region of overlap at a higher resolution than the further arrays of intensity values. 
   
   
       9 . Method according to  claim 8 , including obtaining first and further arrays of intensity values in which each intensity value represents a light level in an area surrounding a pixel position, wherein at least one derived array of intensity values is obtained by adjusting the number of intensity values in an array by a multiplication factor, such that each derived array encodes at least the region of overlap at the same spatial resolution. 
   
   
       10 . Method according to  claim 9 , wherein the number of intensity values in at least one array based on an obtained further array of intensity values is adjusted by a multiplication factor larger than one. 
   
   
       10 . Method according to  claim 8 , wherein each derived array of intensity values is generated by transforming an image frame encoded by an array of intensity values based on one of the obtained arrays of intensity values and in which each intensity value represents a light level in an area surrounding a pixel position in an image frame, into the spatial frequency domain, such that each intensity value in a derived array of intensity values represents an intensity of a spatial frequency component of the image frame. 
   
   
       12 . Method according to claim  11 , wherein the step of providing the array of intensity values encoding the combined final image includes replacing at least one intensity value representing a low spatial frequency component in the derived array of intensity values based on the first obtained array of intensity values by an intensity value based at least partly on the intensity value representing the corresponding spatial frequency component in the array of combined intensity values. 
   
   
       13 . Method according to claim  11 , wherein the transformation is carried out by a co-processor comprising at least a partial implementation in hardware of an image compression algorithm, or by a digital signal processor programmed to implement an image compression algorithm. 
   
   
       14 . Method according to  claim 8 , wherein the step of generating an array of combined intensity values is preceded by a step of aligning the image frames, such that each derived array encodes light intensity levels at each of substantially corresponding pixel position in at least the region of overlap. 
   
   
       15 . Method according to  claim 8 , wherein at least one array of intensity values, based on an obtained array of intensity values encoding at least the region of overlap in the respective image frame at a higher spatial resolution than at least one further array of intensity values, is subjected to a digital filter operation having a characteristic of passing high spatial frequency components of the image encoded by the array. 
   
   
       16 . Image processing system for forming a combined final image from a plurality of image frames, which image processing system includes an arrangement for loading a first and at least one further array of intensity values,
 each array of intensity values encoding light intensity levels at each of a respective number of pixel positions in the respective image frame, the number determining the spatial resolution of the image frame concerned, and a data processing arrangement for processing the intensity values, wherein the system is configured to direct the data processing arrangement to perform the steps of generating a set of derived arrays of intensity values, each derived array being based on a respective one of the obtained arrays of intensity levels and encoding light intensity levels at each of a common number of pixel positions in at least a region of overlap of the respective image frames, generating an array of combined intensity values, each element in the array based on a sum of intensity values represented by the corresponding element in each of the respective derived arrays of intensity values, and providing an array of intensity values encoding the combined final image, the array being based on the array of combined intensity values, wherein the system is configured to load a first array of intensity values encoding at least the region of overlap at a higher resolution than the further arrays of intensity values, to provide an array of intensity values encoding at least the region of overlap in the combined final image at a higher spatial resolution than the further arrays of intensity values, and to base the array of intensity values encoding the combined final image on a sufficient number of intensity values in the first array of intensity values to encode the region of overlap at a higher resolution than the further arrays of intensity values.   
   
   
       17 . Image processing system according to  claim 16 , configured to direct the processor to execute a method according to  claim 8 . 
   
   
       18 . Computer program configured, when loaded into a programmable processing device to enable the programmable processing device to carry out a method according to  claim 1 . 
   
   
       19 . Digital camera comprising an image capturing system and/or an image processing system according to  claim 6 .