Method for tone adapting an image to a target peak luminance L
An image processing method is described. The method includes obtaining low dynamic range expansion exponents E LDR (p), obtaining target expansion exponents E T (p) as a weighted sum of the high dynamic range expansion exponents E HDR (p) and of the low dynamic range expansion exponents E LDR (p), applying obtained target expansion exponent E T (p) to low dynamic range luminance values Y LDR of a low dynamic range version of the image, resulting in target luminance values Y T , and building a tone-adapted version of said image based on said target luminance values Y T .
1. An image processing method for tone adapting an image to a target peak luminance L T of a target display device from a low dynamic range version of this image adapted to be displayed on a display device having a low peak luminance L LDR , and from high dynamic range expansion exponents E HDR (p) to be applied to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image to get expanded luminance values Y HDR of a high dynamic range version of said image adapted to be displayed on a display device having a high peak luminance L HDR , said method comprising:
obtaining low dynamic range expansion exponents E LDR (p) such that, when applied to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image, low dynamic range luminance values Y LDR are linearly scaled within a high dynamic range [0, L HDR ],
obtaining target expansion exponents E T (p) as a weighted sum of the high dynamic range expansion exponents E HDR (p) and of the obtained low dynamic range expansion exponents E LDR (p), with a weight 0≤α in ≤1 of the high dynamic range expansion exponent E HDR (p) and with a weight of the low dynamic range expansion exponent E LDR (p) equal to (1−α in ), wherein the weight α in of the high dynamic range expansion exponent E HDR (p) is proportional to a difference between the target peak luminance L T of the target display device and the low peak luminance L LDR , this difference being scaled by another difference between the high peak luminance L HDR and the low peak luminance L LDR ,
applying obtained target expansion exponent E T (p) to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image, resulting in target luminance values Y T ,
building a tone-adapted version of said image based on said target luminance values Y T .
2. An image processing device for tone adapting an image to a target peak luminance L T of a target display device from a low dynamic range version of this image adapted to be displayed on a display device having a low peak luminance L LDR , and from high dynamic range expansion exponents E HDR (p) to be apply to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image to get expanded luminance values Y HDR of a high dynamic range version of said image adapted to be displayed on a display device having a high peak luminance L HDR , said device comprising at least one processor configured for:
obtaining low dynamic range expansion exponents E LDR (p) such that, when applied to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image, low dynamic range luminance values Y LDR are linearly scaled within a high dynamic range [0, L HDR ],
obtaining target expansion exponents E T (p) as a weighted sum of the high dynamic range expansion exponents E HDR (p) and of the obtained low dynamic range expansion exponents E LDR (p), with a weight 0≤α in ≤1 of the high dynamic range expansion exponent E HDR (p) and with a weight of the low dynamic range expansion exponent E LDR (p) equal to (1−α in ), wherein the weight α in of the high dynamic range expansion exponent E HDR (p) is proportional to a difference between the target peak luminance L T of the target display device and the low peak luminance L LDR , this difference being scaled by another difference between the high peak luminance L HDR and the low peak luminance L LDR ,
applying obtained target expansion exponent E T (p) to low dynamic range luminance values Y LDR or to low pass filtered luminance values of said low dynamic range version of said image, resulting in target luminance values Y T ,
building a tone-adapted version of said image based on said target luminance values Y T .
3. An electronic device incorporating the image processing device according to claim 2 .
4. The electronic device according to claim 3 selected from the group consisting of a camera, a TV set, a monitor, a head mounted display, a set top box, a gateway, a smartphone and a tablet.
5. A non-transitory storage medium carrying instructions of program code for executing steps of the method according to claim 1 , when said program is executed on a computing device.