HDR Video Reconstruction with Split Luma Mapping
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Solution Overview
Problem
Existing HDR video encoding methods using inverse tone mapping (ITM) systems face issues with hard or soft-clipping in luma mapping functions, leading to decoding artefacts and loss of image quality, particularly when high compression is applied.
Innovation Solution
The encoder splits the luma mapping function into two parts: a stretched luma mapping function that maps the clipping point to the maximum output and a scaling factor, ensuring invertibility and reducing decoding artefacts by maintaining the original image grading effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a luma mapping function is used to compress HDR image to LDR proxy for transmission, then bandwidth efficiency is improved, but decoding artifacts and loss of image quality occur due to hard or soft-clipping
Solution Approach 1:
The luma mapping function is segmented into two separate functions: a first luma mapping function that performs the primary compression mapping, and a second luma mapping function that acts as its inverse to restore the image. This segmentation allows the system to maintain bandwidth efficiency while eliminating clipping artifacts through the paired mapping functions.
Solution Approach 2:
The system changes the parameters of the luma mapping function by introducing a scaling factor and offset that are transmitted as metadata. These parameter changes allow the receiver to adjust the mapping function to match the original HDR image characteristics, thereby preserving image quality while maintaining compression efficiency.
2Productivity
If high compression is applied to LDR proxy image, then transmission efficiency is improved, but decoding artifacts increase and image quality deteriorates
Solution Approach 1:
The system uses feedback by transmitting metadata about the luma mapping function parameters (scaling factor and offset) from the encoder to the decoder. This feedback mechanism allows the decoder to reconstruct the original HDR image characteristics accurately, maintaining image quality even with high compression applied to the LDR proxy.
3Device complexity
If a simple clipping mapping is used to map luminances above maximum to maximum value, then device complexity is reduced, but image quality and dynamic range information are lost
Solution Approach 1:
The system performs preliminary action by pre-calculating and storing the inverse luma mapping function parameters during encoding. These pre-computed parameters are then transmitted as metadata, allowing the decoder to efficiently restore the dynamic range information without requiring complex real-time calculations, thus maintaining both low complexity and high image quality.
Data Source
AI summary
To enable better quality decoding of HDR images which are communicated as corresponding LDR images plus luma mapping functions, applicant teaches an encoder for encoding a high dynamic range image (Im_HDR_PSEU), which high dynamic range image is represented firstly by a matrix of pixel colors (Y_SDR, Cb_SDR, Cr_SDR) of an image of lower dynamic range (Im_SDR) than the high dynamic range image, which image of lower dynamic range gets compressed for communication as a compressed lower dynamic range image (Im_C), and also represented secondly by metadata (SEI) of the image which comprises a luma mapping function for calculating high dynamic range pixel lumas (Y_HDR) of the high dynamic range image by applying the function to pixel lumas of the image of lower dynamic range, wherein the encoder comprises an input (997) for receiving the luma mapping function (LMF) from an inverse tone mapping system (200), which is arranged to derive said luma mapping function based on analyzed properties of a master low dynamic range image (Im_LDR_mastr) for constructing the corresponding high dynamic range image (Im_HDR_PSEU), characterized in that the encoder comprises a mapping splitting unit (901) arranged to transform the luma mapping function into a stretched luma mapping function (LMF_HS) which has a shape which maps a maximum normalized input to a maximum normalized output, and arranged to determine a scaling value (SCAL), the encoder having a formatter (104) arranged to output this stretched luma mapping function (LMF_HS) and this scaling value (SCAL) as metadata of the image of lower dynamic range (Im_SDR) which is also output.