Rate-control-aware reshaping in HDR imaging

Patent metadata

Field	Value
Publication number	US-12177459-B2
Application number	US-202017780895-A
Country	US
Kind code	B2
Filing date	Nov 25, 2020
Priority date	Nov 27, 2019
Publication date	Dec 24, 2024
Grant date	Dec 24, 2024

Abstract

Given an input image in a high dynamic range (HDR) which is mapped to a second image in a second dynamic range using a reshaping function, to improve coding efficiency, a reshaping function generator may adjust the codeword range of the HDR input under certain criteria, such as for noisy HDR images with a relatively-small codeword range. An example of generating a scaler for adjusting the HDR codeword range based on the original codeword range and a metric of the percentage of edge-points in the HDR image is provided. The adjusted reshaping function allows for more efficient rate control during the compression of reshaped images.

First claim

The invention claimed is:

1. A method for generating a reshaping function, the method comprising:
receiving one or more input images ( 120 ) in a first dynamic range;
computing a first minimum luma value and a first maximum luma value in the one or more input images;
computing ( 605 ) a first codeword range for a luma channel of the one or more input images based on the first minimum luma value and the first maximum luma value;
computing ( 610 ) a noise metric for the luma channel of the one or more input images, wherein the noise metric comprises a metric of noisiness of the luma channel;
computing ( 615 ) a scaler based on the first minimum luma value, the first maximum luma value and the noise metric; and
depending on the scaler, generating a forward luma reshaping function mapping luma values in the one or more input images from a source luma codeword range to a target luma codeword range, wherein the forward luma reshaping function is constructed to map a minimum codeword value of the source luma codeword range to a minimum codeword value of the target luma codeword range, and a maximum codeword value of the source luma codeword range to a maximum codeword value of the target luma codeword range, comprising:
if the scaler is bigger than one:
computing a second minimum luma value and a second maximum luma value based on the first minimum luma value, the first maximum luma value and the scaler;
generating ( 620 ) a second codeword range for the luma channel of the one or more input images based on the second minimum luma value and the second maximum luma value, wherein the second codeword range is larger than the first codeword range; and
generating ( 630 ) the forward luma reshaping function using the second codeword range as the source luma codeword range;

else
generating ( 625 ) the forward luma reshaping function using the first codeword range as the source luma codeword range.

2. The method of claim 1 , further comprising:
depending on the scaler, generating, based on a minimum luma value, a maximum luma value, and the forward luma reshaping function, a forward chroma reshaping function mapping chroma values in the one or more input images from a source chroma codeword range to a target chroma codeword range, comprising:
if the scaler is bigger than one:
generating the forward chroma reshaping function using the second minimum luma value as the minimum luma value and the second maximum luma value as the maximum luma value;

else
generating the forward chroma reshaping function using the first minimum luma value as the minimum luma value and the first maximum luma value as the maximum luma value.

3. The method of claim 1 , wherein computing the scaler comprises computing an exponential mapping based on the bit-depth resolution of the one or more input images, the first minimum luma value, the first maximum luma value and the noise metric.

4. The method of claim 1 , wherein computing the scaler (M) comprises computing
M =max(β e −αδ ,1.0),

wherein δ denotes a function of the bit-depth resolution of the one or more input images, the first minimum luma value and the first maximum luma value, β denotes a function of the noise metric, and α is a function of the noise metric and a cut-off parameter C for which if δ≥ C, then M=1.

5. The method of claim 4 , wherein

δ
=

Δ
L
i

2

B
v

,

wherein B v denotes the bit-depth resolution of the one or more input images, and Δ L i denotes a difference of the first minimum luma value from the first maximum luma value,

β
=

min
⁢

(

max
⁢

(

1

P
i

,

P
i

)

,

1
⁢
0
⁢
0

)

⁢

P
i

>
0

,

wherein P i denotes the noise metric, and

α
=

ln
⁢

(
β
)

C

.

6. The method of claim 1 , wherein the second minimum luma value {tilde over (v)} L,min i and the second maximum luma value {tilde over (v)} L,max i are computed as:
{tilde over (v)} L,min i =max(0, v L,avg i −M×Δ L,1 i ),
{tilde over (v)} L,max i =min(2 B v −1, v L,avg i +M×Δ L,2 i ),

wherein B v denotes the bit-depth resolution of the one or more input images, M denotes the scaler, and
Δ L,1 i =v L,avg i −v L,min i ,

Δ L,2 i =v L,max i −v L,avg i ,

wherein v L,min i , v L,max i , and v L,avg i denote the first minimum luma value, the first maximum luma value, and an average luma value in the one or more input images.

7. The method of claim 1 , where computing the noise metric comprises:
normalizing luma values in the one or more input images to [0, 1) to generate one or more normalized images;
determining edge points in the one or more normalized images based on edge-detection operators and one or more thresholds; and
determining a percentage of the determined edge points over the total number of pixels in the one or more normalized images.

8. The method of claim 7 , wherein the edge-detection operators comprise the Sobel operators.

9. The me

Assignees

• Dolby Laboratories Licensing Corp

Inventors

• Kadu Harshad
• Qi Ji
• Su Guan-Ming

Classifications

• H04N19/146

  Data rate or code amount at the encoder output · CPC title

• H04N19/46

  Embedding additional information in the video signal during the compression process (H04N19/517, H04N19/68, H04N19/70 take precedence) · CPC title

• H04N19/186 Primary

  the unit being a colour or a chrominance component · CPC title