Method and device for encoding a high-dynamic range image and/or decoding a bitstream

1. Method for encoding an image, performed in an encoder, characterized in that it comprises:

encoding a backlight image determined from the image;

obtaining a residual image by dividing the image by a decoded version of the backlight image;

obtaining a tone-mapped residual image by tone-mapping the residual image; and

encoding the tone-mapped residual image.

2. The method according to claim 1 , wherein tone-mapping the residual image comprises either a gamma correction or a SLog correction according to the pixel values of the residual image.

3. The method according to claim 1 , wherein obtaining the backlight image from the image comprises:

obtaining a backlight image from a luminance component of the image; and

modulating the backlight image with a mean luminance value of the image.

4. The method according to claim 3 , wherein obtaining the backlight image from the image further comprises:

normalizing the backlight image by its mean value before modulating the backlight image.

5. The method according to claim 1 , wherein the method further comprises, before encoding, scaling the residual image by multiplying each component of the residual image by a scaling factor.

6. The method according to claim 1 , wherein the method further comprises, before encoding, clipping the residual image to limit its dynamic range to a targeted dynamic range.

7. The method according to claim 1 , wherein said image is obtained from a source belonging to a set comprising:

a local memory;

a storage interface,

a broadcast interface;

a communication interface; and

an image capturing circuit.

8. The method according to claim 1 , wherein the image is encoded into an encoded image, the encoded image being sent to a destination belonging to a set comprising: a local memory; a storage interface, and a communication interface.

9. Method for decoding a bitstream representing a residual image calculated by dividing an image by a backlight image, the method, performed in a decoder, comprising:

obtaining a backlight image and a decoded residual image by a at least partially decoding of the bitstream, and

obtaining a decoded image by multiplying the decoded residual image by the backlight image;

characterized in that the decoded residual image is inverse-tone-mapped before multiplying the decoded residual image by the backlight image.

10. Device for encoding an image, characterized in that it comprises a processor configured to:

encode a backlight image determined from the image;

obtain a residual image by dividing the image by a decoded version of the backlight image,

obtain a tone-mapped residual image by tone-mapping the residual image; and

encode the tone-mapped residual image.

11. The device according to claim 10 , wherein said device belongs to a set comprising:

a mobile device;

a communication device;

a game device;

a tablet;

a laptop;

a still image camera;

a video camera;

an encoding chip;

a still image server; and

a video server.

12. The device according to claim 10 , wherein said device comprise means to obtain said image, belonging to a set comprising:

a local memory;

a storage interface,

a broadcast interface,

a communication interface; and

an image capturing circuit.

13. The device according to claim 10 , wherein said device comprises means to send an encoded image to a destination, which belongs to a set comprising:

a local memory;

a storage interface,

a broadcast interface,

a display and

a communication interface.

14. Device for decoding a bitstream representing a residual image calculated by dividing an image by a backlight image, wherein it comprises a processor configured to:

obtain a backlight image and a decoded residual image by a at least partially decoding of the bitstream, and

obtain a decoded image by multiplying the decoded residual image by the backlight image;

characterized in that the processor is further configured to inverse tone-map a decoded residual image before multiplying the decoded residual image by a backlight image.

15. The decoding device according to claim 14 , wherein said device belongs to a set comprising:

a mobile device;

a communication device;

a game device;

a set top box;

a TV set;

a tablet;

a laptop; and

a decoding chip.