Display of images

1. A method for transforming an initial image into a target image for display by a display device, said method comprising:

a first calculation for calculating a tone curve which maps luminance levels of an original image to luminance levels of the transformed image;

a first transformation for transforming luminance levels of the original image according to the tone curve thereby to provide a first transformed image for display by said display device according to a peak luminance for display; wherein the calculating includes determining the tone curve that involves an optimization process which optimises a match between a visual contrast of the original image and a visual contrast of the first transformed image in which the peak luminance of the tone curve is less or equal to the peak luminance of the first transformed image for display by the display device, and wherein the optimising the match includes minimising a difference between the visual contrast of the original image and the visual contrast of the first transformed image; and

a third transformation for transforming said first transformed image having a first luminance to adjust a colour saturation thereof for display by said display device having a second luminance, said third transformation comprising: calculating a colour saturation adjustment transform for adjusting colour values of said first transformed image; and, adjusting the colour values ({tilde over (X)}) of said first transformed image according to the colour saturation transform thereby to provide a third transformed image for display by the display device at the second luminance; wherein the adjusted colour value is defined according to the value of the first luminance (Y) and the value of the second luminance ({tilde over (Y)}) and a saturation correction factor (s( . . . )) in accordance with the following transform:

X

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=

Y

˜

×

(

X

˜

Y

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)

s

⁡

(

Y

)

/

s

⁡

(

Y

~

)

in which the saturation correction factor is a function of luminance and approaches a value of zero as the value of luminance approaches zero and monotonically approaches a value of one (1.0) asymptotically as luminance increases.

2. The method of claim 1 , wherein said first calculation is performed on a piecewise basis in which the tone curve is represented as a piecewise linear function with a plurality of linear sections each representing a specific range of tones and in which the determination of that linear section is performed according to said optimization process.

3. The method of claim 1 , wherein said method comprises:

a second transformation for transforming said first transformed image locally within a sub-area of said first transformed image to adjust image contrast for display by said display device, said second transformation comprising:

a second calculation for calculating a contrast adjustment factor for adjusting a contrast within a sub-area of said first transformed image; and

transforming said contrast within said sub-area of the first transformed image according to the contrast adjustment factor thereby to provide a second transformed image for display by said display device;

wherein said second calculation includes determining a measure of local contrast within said sub-area and therewith determining a contrast adjustment factor that optimises a match between said contrast of said first transformed image and said contrast of said second transformed image within said sub-area wherein the luminance in said sub-area of the first transformed image dismatches the luminance in said sub-area of the second transformed image for display by the display device.

4. The method of claim 3 , wherein said measure of local contrast is in terms of a measure of local variation of pixel luminance levels within a defined local area.

5. The method of claim 1 , wherein said first calculation includes decomposing the original image into an image pyramid which comprises a plurality of different component images which each represent the original image via spatial frequencies within a respective one of a plurality of different spatial frequency bands.

6. The method of claim 1 , wherein said method comprises:

a fourth transformation for transforming an image of an initial luminance to adjust a perceived colour hue thereof for display by said display device according to a target luminance, said fourth transformation comprising:

calculating a colour adjustment factor for adjusting colour values of said image; and

adjusting said colour values of said image according to the colour adjustment factor thereby to provide a transformed image for display by the display device at the second luminance, said calculating including representing numerically a cone photoreceptor response to the colour values in terms of a corresponding contributory rod photoreceptor response to luminance, and said cone photoreceptor response per unit luminance at the second luminance being constrained to substantially match the cone photoreceptor response per unit luminance at the initial luminance.

7. The method of claim 1 , wherein the tone curve is determined using a model for matching contrast.

8. The method of claim 7 , wherein the model for matching contrast is Kulikowski's model of matching contrast.

9. A method for transforming an initial image into a target image for display by a display device, said method comprising:

a first calculation for calculating a tone curve which maps luminance levels of an original image to luminance levels of the transformed image; and

a first transformation for transforming luminance levels of the original image according to the tone curve thereby to provide a first transformed image for display by said display device according to a peak luminance for display; wherein the calculating includes determining the tone curve that involves an optimization process which optimises a match between a contrast of the original image and a contrast of the first transformed image in which the peak luminance of the tone curve is less or equal to the peak luminance of the first transformed image for display by the display device;

wherein said method comprises:

a second transformation for transforming said first transformed image locally within a sub-area of said first transformed image to adjust image contrast for display by said display device, said second transformation comprising:

a second calculation for calculating a contrast adjustment factor for adjusting a contrast within a sub-area of said first transformed image; and

transforming said contrast within said sub-area of the first transformed image according to the contrast adjustment factor thereby to provide a second transformed image for display by said display device;

wherein said second calculation includes determining a measure of local contrast within said sub-area and therewith determining a contrast adjustment factor that optimises a match between said contrast of said first transformed image and said contrast of said second transformed image within said sub-area wherein the luminance in said sub-area of the first transformed image dismatches the luminance in said sub-area of the second transformed image for display by the display device; and

wherein said sub-area is defined by a spatial window function centred thereupon in which the width of the window function is proportional to the inverse of a spatial frequency of the original image such that the width of the window function is smaller for higher spatial frequencies.

10. A system for transforming an initial image into a target image for display by a display device, the system comprising at least one computer having computer-executable instructions that, when executed by the at least one computer, cause the system to:

calculate a tone curve which maps luminance levels of an original image to luminance levels of the transformed image; and

transform luminance levels of the original image according to the tone curve thereby to provide a first transformed image for display by said display device according to a peak luminance for display; wherein the calculating includes determining the tone curve that involves an optimization process which optimises a match between a visual contrast of the original image and a visual contrast of the first transformed image in which the peak luminance of the tone curve is less or equal to the peak luminance of the first transformed image for display by the display device, and wherein the optimising the match includes minimising a difference between the visual contrast of the original image and the visual contrast of the first transformed image;

wherein said system is configured so that, when the computer-executable instructions are executed by the at least one computer, the system:

transforms said first transformed image locally within a sub-area of said first transformed image to adjust image contrast for display by said display device, said second transformation includes:

calculating a contrast adjustment factor for adjusting a contrast within a sub-area of said first transformed image; and

transforming said contrast within said sub-area of the first transformed image according to the contrast adjustment factor thereby to provide a second transformed image for display by said display device;

wherein said calculating the contrast adjustment factor includes determining a measure of local contrast within said sub-area and therewith determining a contrast adjustment factor that optimises a match between said contrast of said first transformed image and said contrast of said second transformed image within said sub-area wherein the luminance in said sub-area of the first transformed image dismatches the luminance in said sub-area of the second transformed image for display by the display device;

wherein said sub-area is defined by a spatial window function centred thereupon in which the width of the window function is proportional to the inverse of a spatial frequency of the original image such that the width of the window function is smaller for higher spatial frequencies.

11. The system of claim 10 , wherein said calculating the tone curve includes decomposing the original image into an image pyramid which comprises a plurality of different component images which each represent the original image via spatial frequencies within a respective one of a plurality of different spatial frequency bands.

12. The system of claim 10 , wherein the tone curve is determined using a model for matching contrast.

13. The system of claim 12 , wherein the model for matching contrast is Kulikowski's model of matching contrast.