Image formats and related methods and apparatuses
Image data may be color graded, distributed and viewed on target displays. Mappings that preserve mid-range points and mid-range contrast may be applied to view the image data for color grading and to prepare the image data for display on a target display. The image data may be expanded to exploit the dynamic range of the target display without affecting mid-tone values.
1. A method for displaying an image on a target display, the method comprising: processing image data that has been reviewed on a reference display and encoded such that a predetermined image value of the image data corresponds to a predetermined mid-tone value displayed on the reference display, the processing comprising applying a mapping to the image data that maps the predetermined image value to the predetermined mid-tone value when viewed on the target display,
wherein a dynamic range of the target display differs from a dynamic range of the reference display;
wherein the mapping sets the slope of the mid-tone region of a response function when viewed on the target display to match a corresponding mid tone region of a response function of the reference display; and
wherein the mapping comprises a mapping function given by:
L
OUT
=
c
1
+
c
2
L
IN
n
1
+
c
3
L
IN
n
where L OUT is an output of the mapping L IN is an input of the mapping and c 1 , c 2 , c 3 and n are numeric parameters.
2. A method according to claim 1 comprising adjusting the mapping in response to ambient lighting.
3. A method according to claim 2 wherein adjusting the mapping comprises increasing the mid-tone value to which the predetermined image value is mapped in response to an increase in the ambient lighting.
4. A method according to claim 3 wherein increasing the mid-tone value comprises adjusting the value of c 2 in the mapping function.
5. A method according to claim 2 wherein adjusting the mapping comprises decreasing mid-tone contrast of the mapping function.
6. A method according to claim 5 wherein decreasing the mid-tone contrast comprises adjusting the value of n.
7. A method according to claim 2 comprising modeling the effect of the ambient lighting on light adaptation of the human visual system to obtain an estimated adaptation and adjusting the mapping based on the estimated adaptation.
8. A method according to claim 7 wherein modeling the effect of the ambient lighting on the adaptation of the human visual system comprises modeling a plurality of adaptation aspects to obtain a corresponding plurality of estimated adaptations and the method comprises adjusting different characteristics of the mappings based on different ones of the estimated adaptations.
9. A method according to claim 8 wherein modeling the effect of the ambient lighting on light adaptation of the human visual system is based at least in part on inputs from one or more ambient light sensors and on image content displayed on the target display.
10. A method according to claim 8 comprising controlling a luminance to which the predetermined image value is mapped based at least in part on an estimate of a general brightness adaptation.
11. A method according to claim 8 comprising controlling mid-range contrast based at least in part on an estimate of lateral brightness adaptation.
12. A method according to claim 8 comprising controlling contrast in highlight and shadow regions based at least in part on an estimate of local brightness adaptation.
13. A method according to claim 1 wherein the target display has a greater peak luminance than the reference display and the mapping is configured to expand the luminance corresponding to image values corresponding to a highlight region above the mid-tone region.
14. A method according to claim 1 wherein the target display has a lower black level than the reference display and the mapping is configured to expand the luminance corresponding to image values corresponding to a shadow region below the mid-tone region.