HDR Luma Mapping Split to Prevent Tone-Mapping Clipping Artifacts
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Solution Overview
Problem
Existing HDR video encoding systems face issues with decoding artifacts due to hard or soft-clipping in the luma mapping function, particularly when using inverse tone mapping (ITM) to convert LDR images to HDR, leading to undesirable visual artifacts and loss of image quality.
Innovation Solution
The system 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 the function remains invertible and avoids clipping, allowing precise reconstruction of HDR images from LDR proxies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a luma mapping function is used to convert HDR images to LDR for transmission, then the image can be transmitted through standard channels, but decoding artifacts appear due to hard or soft-clipping in the luma mapping function
Solution Approach 1:
The patent divides the luma mapping function into two separate components: a clipped luma mapping function for efficient transmission and an unclipped inverse function for accurate reconstruction. This segmentation allows each component to be optimized independently - the forward mapping can clip for compression efficiency while the inverse mapping reconstructs without artifacts
Solution Approach 2:
The patent introduces an intermediary unclipped inverse luma mapping function that acts as a mediator between the clipped LDR representation and the original HDR image. This intermediary function preserves the full dynamic range information during reconstruction, eliminating clipping artifacts while maintaining compatibility with standard transmission channels
2Manufacturing precision
If inverse tone mapping is used to convert LDR images to HDR, then HDR image quality can be reconstructed, but decoding artifacts appear due to clipping in the mapping function
Solution Approach 1:
The patent applies the inversion principle by using an unclipped inverse luma mapping function that reverses the clipping operation. Instead of inverting the clipped function (which would amplify artifacts), the system uses a specially designed unclipped inverse function that recovers the original HDR values without introducing or amplifying decoding artifacts
Solution Approach 2:
The patent converts the harmful clipping effect into a benefit by deliberately applying clipping in the forward direction for compression efficiency, then using the unclipped inverse function to recover the original information. The clipping that would normally cause harm is transformed into a reversible compression step that maintains full reconstruction quality
3Device complexity
If clipping is applied in the luma mapping function, then the dynamic range can be compressed for transmission, but the function becomes non-invertible and loses information
Solution Approach 1:
The patent segments the mapping process into a clipped forward transformation and an unclipped inverse transformation. The clipping is applied only in the forward direction where it aids compression, while the inverse direction deliberately avoids clipping to preserve all luma range information for perfect reconstruction
Solution Approach 2:
The patent applies clipping as a preliminary action in the forward mapping direction to compress the dynamic range for efficient transmission. The unclipped inverse function is then applied as a corrective action to recover the original information that was compressed, ensuring no permanent information loss occurs
Data Source
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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.