Image optimization in mobile capture and editing applications
HDR color patches are sampled throughout an HDR color space parameterized by a parameter. Reference SDR color patches, input HDR color patches and reference HDR color patches are generated from the sampled HDR color patches. An optimization algorithm is executed to generate an optimized forward reshaping mapping and an optimized backward reshaping mapping. The optimized forward reshaping mapping is used to forward reshape input HDR images into forward reshaped SDR images, whereas the optimized backward reshaping mapping is used to backward reshape the forward reshaped SDR images into backward reshaped HDR images.
1 . A method for tensor-product B-Spline (TPB) based image reshaping of high dynamic range (HDR) input images, the method comprising:
building sampled HDR color space points distributed throughout an HDR color space, wherein sampling the HDR color space points comprises sampling each color primary of the color space by a defined number of units to obtain a sampled data set of color patches representing a subset of the HDR color space, wherein the subset of the HDR color space is delineated as a triangle formed by the color primaries in a color coordinate system and is parameterized by a color primary scaling parameter with a candidate value iteratively selected from among a plurality of candidate values, the color primary scaling parameter serving as an input parameter for executing a reshaping operation optimization algorithm for TPB based image reshaping, wherein the color primary scaling parameter is indicative of the size of the subset of the HDR color space;
generating from the sampled HDR color space points in the HDR color space: (a) reference standard dynamic range (SDR) color space points represented in a reference SDR color space, (b) input HDR color space points represented in an input HDR color space, and (c) reference HDR color space points represented in a reference HDR color space; and
executing the reshaping operation optimization algorithm to optimize, in a pipeline of chained reshaping functions comprising a forward reshaping mapping and a corresponding backward reshaping mapping, forward and backward TPB predictors, wherein the reshaping operation optimization algorithm uses the reference SDR color space points, the input HDR color space points and the reference HDR color space points as input to generate an optimized forward reshaping mapping and a corresponding optimized backward reshaping mapping based on a candidate value of the color primary scaling parameter that corresponds to the largest subset of the HDR color space that can achieve minimized HDR prediction errors,
wherein the optimized forward reshaping mapping is used to forward reshape input HDR images in the input HDR color space into forward reshaped SDR images in a forward reshaped SDR color space, wherein the optimized backward reshaping mapping is used to backward reshape the forward reshaped SDR images in the forward reshaped SDR color space into backward reshaped HDR images.
2 . The method of claim 1 , wherein executing the reshaping operation optimization algorithm is iteratively processed to generate a plurality of forward reshaping mappings and corresponding backward reshaping mappings, each forward reshaping mapping and corresponding backward reshaping mapping corresponding to a respective candidate value of the plurality of candidate values for the color primary scaling parameter.
3 . The method of claim 1 , wherein the sampled HDR color space points are mapped to the reference SDR color space points based at least in part on a predefined HDR-to-SDR mapping.
4 . The method of claim 1 , wherein a set of prediction errors is computed for each forward reshaping mapping and the corresponding backward reshaping mapping; wherein the sets of prediction errors are used to select a specific candidate value from among the plurality of candidate values for the color primary scaling parameter.
5 . The method of claim 1 , wherein the HDR color space and the input HDR color space share a common white point.
6 . The method of claim 1 , wherein the reshaping operation optimization algorithm represents a Backward-Error-Subtraction-for-signal-Adjustment (BESA) algorithm with neutral color preservation.
7 . An apparatus comprising a processor and configured to perform the method recited in claim 1 .
8 . A non-transitory computer-readable storage medium having stored thereon computer-executable instruction for executing a method with one or more processors in accordance with the method recited in claim 1 .
9 . A method for tensor-product B-Spline (TPB) based image reshaping of standard dynamic range (SDR) input images, the method comprising:
building sampled high dynamic range (HDR) color space points distributed throughout an HDR color space, wherein sampling the HDR color space points comprises sampling each color primary of the color space by a defined number of units to obtain a sampled data set of color patches representing a subset of the HDR color space, wherein the subset of the HDR color space is delineated as a triangle formed by the color primaries in a color coordinate system and is parameterized by a color primary scaling parameter with a candidate value iteratively selected from among a plurality of candidate values, the color primary scaling parameter serving as an input parameter for executing a reshaping operation optimization algorithm for TPB based image reshaping, wherein the color primary scaling parameter is indicative of the size of the subset of the HDR color space;
generating from the sampled HDR color space points in the HDR color space: (a) input standard dynamic range (SDR) color space points represented in an input SDR color space and (b) reference HDR color space points represented in a reference HDR color space;
executing the reshaping operation optimization algorithm to optimize a backward reshaping mapping, wherein the reshaping operation optimization algorithm receives the input SDR color space points and the reference HDR color space points as input to generate an optimized backward reshaping mapping based on a candidate value of the color primary scaling parameter that corresponds to the largest subset of the HDR color space that can achieve minimized HDR prediction errors;
wherein the optimized backward reshaping mapping is used to backward reshape SDR images in the input SDR color space into backward reshaped HDR images.
10 . The method of claim 9 , wherein executing the reshaping operation optimization algorithm is iteratively processed to generate a plurality of backward reshaping mappings, each backward reshaping mapping corresponding to a respective candidate value of the plurality of candidate values for the color primary scaling parameter.
11 . The method of claim 9 , wherein a set of prediction errors is computed each of backward reshaping mapping; wherein the sets of prediction errors are used to select a specific candidate value from among the plurality of candidate values for the color primary scaling parameter.
12 . The method of claim 9 , wherein the sampled HDR color space points are processed by a programmable image signal processor (ISP) pipeline to the input SDR color space points based at least in part on an optimized value for a programmable configuration parameter of the programmable ISP pipeline.
13 . The method of claim 9 , wherein the optimized value for the programmable configuration parameter of the programmable ISP pipeline is determined by minimizing approximation errors between ISP SDR images generated by the programmable ISP pipeline from HDR images and reference SDR images generated by applying a predefined HDR-to-SDR mapping to the same HDR images.