Non-linear VDR residual quantizer
In layered VDR coding, inter-layer residuals are quantized by a non-linear quantizer before being coded by a subsequent encoder. Several non-linear quantizers are presented. Such non-linear quantizers may be based on sigmoid-like transfer functions, controlled by one or more free parameters that control their mid-range slope. These functions may also depend on an offset, an output range parameter, and the maximum absolute value of the input data. The quantizer parameters can time-vary and are signaled to a layered decoder. Example non-linear quantizers described herein may be based on the mu-law function, a sigmoid function, and/or a Laplacian distribution.
1. A method for the coding of High Dynamic Range (HDR) images, the method comprising:
accessing an input HDR image and an input Standard Dynamic Range (SDR) image, wherein the input HDR and SDR images represent the same scene but at a different dynamic range;
generating a predicted HDR image based on the input SDR image and the input HDR image;
generating a residual HDR input image based on the input HDR image and the predicted HDR image;
for limiting the dynamic range of the residual HDR input image, applying a non-linear quantization to the residual HDR input image to output a quantized residual image; and
coding the quantized residual image using a residual encoder,
wherein said residual HDR input image has a bit-depth that is higher than the bit-depth supported by the residual encoder, wherein the method step of non-linear quantization comprises:
transforming pixel values of the residual HDR input image to corresponding quantized pixel values according to a non-linear transfer function; said transfer function characterized by one or more function parameters, said function parameters comprising an offset parameter and an output dynamic range parameter representative for a desired maximum dynamic range of the non-linear quantization;
wherein said transfer function has a mid-range slope controlled by one or more of the function parameters, and wherein said parameters of the non-linear transfer function are set by a method comprising:
receiving from the residual encoder quantizer information related to one or more of the function parameters, the offset parameter, and the output dynamic range parameter; and
adjusting said one or more parameters based on said received quantizer information, wherein the non-linear transfer function comprises computing
c
(
x
)
=
O
+
L
max
x
x
max
σ
+
x
sign
(
x
)
,
wherein O denotes the offset parameter, L max denotes the output, dynamic range parameter, x is a residual HDR input pixel value, c(x) is the quantized output value, x max is the maximum absolute value of the residual HDR input pixel values, and σdenotes a function parameter controlling the mid-range slope.
2. The method of claim 1 wherein the residual HDR image has a bit-depth of 12 bits or 16 bits and the residual encoder supports input signals with a bit-depth of 8 bits.
3. The method of claim 1 wherein the offset parameter comprises a value that is equal to 128.
4. The method of claim 1 wherein the output dynamic range parameter comprises a value that is between 0 and 128.
5. The method of claim 1 , wherein said output dynamic range parameter is determined by:
(a) setting the output dynamic range parameter to an initial value;
(b) applying current values of said parameters to the non-linear transfer function to quantize input residual HDR data;
(c) computing an average of block quantization parameter values for a frame applied to the input residual data by the residual encoder; and
(d) lowering a current value of the output dynamic range parameter if said computed average is higher than a first threshold value respectively increasing the current value of the output dynamic range parameter if said computed average is lower than a second threshold value.
6. The method of claim 1 , wherein the parameters of the non-linear transfer function are signaled to a decoder as metadata.
7. The method of claim 1 , wherein the residual encoder is an MPEG encoder.
8. An apparatus comprising a processor and configured to perform the method recited in claim 1 .
9. A non-transitory computer-readable storage medium having stored thereon computer-executable instruction for executing a method using a processor in accordance with claim 1 .
10. A method for the coding of High Dynamic Range (HDR) images, the method comprising:
accessing an input HDR image and an input Standard Dynamic Range (SDR) image, wherein the input HDR and SDR images represent the same scene but at a different dynamic range;
generating a predicted HDR image based on the input SDR image and the input HDR image;
generating a residual HDR input image based on the input HDR image and the predicted HDR image;
applying a non-linear quantization function to the residual HDR input image to output a quantized residual image; and
coding the quantized residual image using a residual encoder,
wherein applying the non-linear quantization function comprises computing:
c
(
x
)
=
O
+
L
max
ln
(
1
+
μ
x
x
max
)
ln
(
1
+
μ
)
sign
(
x
)
,
wherein O denotes an offset parameter, L max denotes an output dynamic range parameter, x is a residual HDR input pixel value, c(x) is the quantized output value, x max is the maximum absolute value of the residual HDR input pixel values , and mu (μ) denotes a function parameter.