ADAPTIVE RATIO IMAGES IN HDR IMAGE REPRESENTATION

• US 20160205405A1
• Filed: 03/23/2016
• Published: 07/14/2016
• Est. Priority Date: 08/08/2012
• Status: Active Grant

• Current Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Original Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Inventors: TEN, Arkady
• Granted Patent: US 9,467,704 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06T 2207/20008 Globally adaptive
• CPC Class Codes: G06T 2207/20024 Filtering details
• CPC Class Codes: G06T 2207/20208 High dynamic range [HDR] image processing
• CPC Class Codes: G06T 5/50 using two or more images, e.g. averaging or subtraction
• CPC Class Codes: G06T 5/92 based on global image properties
• CPC Class Codes: G06T 7/90 Determination of colour characteristics
• CPC Class Codes: H04N 19/117 Filters, e.g. for pre-processing or post-processing sub-band filter banks H04N19/635
• CPC Class Codes: H04N 19/119 Adaptive subdivision aspects, e.g. subdivision of a picture into rectangular or non-rectangular coding blocks
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/136 Incoming video signal characteristics or properties
• CPC Class Codes: H04N 19/182 the unit being a pixel
• CPC Class Codes: H04N 19/184 the unit being bits, e.g. of the compressed video stream
• CPC Class Codes: H04N 19/186 the unit being a colour or a chrominance component
• CPC Class Codes: H04N 19/33 in the spatial domain
• CPC Class Codes: H04N 19/44 Decoders specially adapted therefor, e.g. video decoders which are asymmetric with respect to the encoder
• CPC Class Codes: H04N 19/46 Embedding additional information in the video signal during the compression process H04N19/517, H04N19/68, H04N19/70 take precedence

Abstract

Image encoding and decoding are described, An input HDR image is coded using a coded tone-mapped image, a coded invertible function of a ratio image, and a look-up table representing the inverse of the invertible function, wherein the ratio image represents the pixel by pixel ratio of the luminance pixel values of the input image and the corresponding luminance values in the tone-mapped image. A method to generate the invertible function based on a cumulative histogram of the ratio image is presented.

Claims

1. A method to code a high dynamic range (HDR) image with a processor, the method comprising:

receiving an input image with a first dynamic range;

generating a tone-mapped image based on the input image, wherein the tone-mapped image has a dynamic range lower than the first dynamic range;

generating a ratio image (YR), the ratio image comprising luminance pixel values of the input image divided pixel by pixel by corresponding luminance pixel values in the tone-mapped image;

applying an invertible function (F) to the ratio image to generate a modified ratio image (F(Y_R));

generating a look-up table to represent the inverse of the invertible function (F<sup>−

1</sup>), wherein applying the look-up table (F<sup>−

1</sup>) to the modified ratio image generates an approximation of the ratio image; and

generating a coded HDR image based on the tone-mapped image and the modified ratio image.

2. The method of claim 1, wherein the invertible function is selected so that when applying its inverse to the modified ratio image, the distance between the ratio image and the approximation of the ratio image is minimized according to a predetermined measure.

3. The method of claim 2, wherein the distance between the ratio image and the approximation of the ratio image is minimized according to a mean square error criterion.

4. The method of claim 2, wherein the predetermined measure comprises the signal to noise ratio between the ratio image and the approximation of the ratio image.

5. The method of claim 2, wherein the predetermined measure comprises the peak signal to noise ratio of the approximation of the ratio image.

6. The method of claim 1, wherein a file format of the coded HDR image comprises a coded tone-mapped image, a coded version of the modified ratio image, and the look-up table representing the inverse of the invertible function.

7. The method of claim 6, wherein, in a decoder, decoding the coded HDR image comprises:

receiving the coded HDR image;

extracting the tone-mapped image, the modified ratio image, and the look-up table from the coded HDR. image;

applying the look-up table to the modified ratio image to generate the approximation of the ratio image; and

generating an output HDR image based on the tone-mapped image and the approximation of the ratio image.

8. The method of claim 1, further comprising quantizing the modified ratio image to generate a quantized ratio image.

9. The method of claim 8, wherein quantizing the modified ratio image comprises computing

10. The method of claim 9, wherein scale=2^B−

1 where B denotes the number of bits for representing a pixel of the quantized ratio image.

11. The method of claim 1, further comprising:

computing a histogram of pixel values of the ratio image;

computing a cumulative histogram based on the histogram; and

generating the invertible function based on the cumulative histogram.

12. The method of claim 11, wherein generating the invertible function comprises computing

F_i=( (c_hist_i−

min(c_max(c _hist)−

min(c_hist))*scale,

wherein c_hist_i denotes the cumulative histogram value for the i-th pixel value of the ratio image, min(c_hist) and max (c_hist) denote the minimum and maximum values of the cumulative histogram, and scale denotes a scaling factor.

13. The method of claim 12, wherein scale=2^B−

1, where B denotes the number of bits for representing a pixel of the modified ratio image.

14. A non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing with a processor a method in accordance with claim 1.

15. An apparatus to decode a coded high-dynamic range (HDR) image, the apparatus comprising:

a processor; and

a non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing with the processor a method in accordance with claim 1.