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From 2D Extrapolation to 1D Interpolation: Content-adaptive Image Bit-depth Expansion

From 2D Extrapolation to 1D Interpolation: Content-adaptive Image Bit-depth Expansion

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From 2D Extrapolation to 1D Interpolation: Content-adaptive Image Bit-depth Expansion

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  1. From 2D Extrapolation to 1D Interpolation:Content-adaptive Image Bit-depth Expansion PengfeiWan, Oscar C. Au, KetanTang, YuanfangGuo, Lu Fang Department of Electronic and Computer Engineering The Hong Kong University of Science and Technology

  2. OUTLINE • Introduction to Bit-depth Expansion(BDE) • Existing Methods: A General Framework • Proposed Method: LMM Reconstruction • Experiments • Conclusions

  3. Introduction: What is BDE? ‘original’ image pixel (high bit-depth, HBD) Truncation/quantization contouring artifacts input image pixel (low bit-depth, LBD) 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 ? 0 1 0 ? 0 0 0 0 0 ? 0 ? 1 1 A naive method: zero padding Bit-depth expansion output image pixel (HBD) Estimate the lost LSBs !

  4. Introduction: What is BDE? ground-truth 8-bit image output of proposed method output of zero padding Bit-depth expansion from 4 bit to 8 bit for RGB image.

  5. OUTLINE • Introduction to Bit-depth Expansion(BDE) • Existing Methods: A General Framework • Proposed Method: LMM Reconstruction • Experiments • Conclusions

  6. Existing Methods: A General Framework • Filtering-based methods: e.g. Daly’s de-contouring method[8] • Pros: linear operation, fast implementation • Cons: local-window filtering is unsuitable for removing contours • Non-linear methods: e.g. Cheng’s method[9] • Pros: operation is spatially varying, better performance • Cons: large error in local minima and maxima regions

  7. OUTLINE • Introduction to Bit-depth Expansion(BDE) • Existing Methods: A General Framework • Proposed Method: LMM Reconstruction • Experiments • Conclusions

  8. Proposed Method: Flowchart

  9. Proposed Method S1: Neighborhood Flooding Flooding: exchange locational information with immediate neighbors until convergence. Purpose: 2D interpolation problem in non-LMM regions becomes 1D interpolation.

  10. Proposed Method S2: LMM Regions Detection • Detect LMM regions by thresholdingDM/UM • LMM map refinement by morphological operations

  11. Proposed Method S3: Marking Skeletons for LMM With the help of skeletons, 2D extrapolation problem in LMM regions becomes 1D interpolation.

  12. Proposed Method S3: Marking Skeletons for LMM • LMM map with skeletons

  13. Proposed Method S4: LSBs Reconstruction Where

  14. OUTLINE • Introduction to Bit-depth Expansion(BDE) • Existing Methods: A General Framework • Proposed Method: LMM Reconstruction • Experiments • Conclusions

  15. Experiments • Gains 0.63, 3.33, 4.01 dB in PSNR and 0.004, 0.020, 0.064 in SSIM over the best of remaining methods • for 6-bit, 4-bit, 2-bit to 8-bit expansion respectively. • Compared with zero padding, the gain is 6.49, 9.35, 6.32 dB in PSNR and 0.011, 0.138, 0.282 in SSIM.

  16. Experiments Daly’s Method Zero padding Proposed Method Cheng’s Method Error maps of selected BDE methods

  17. OUTLINE • Introduction to Bit-depth Expansion(BDE) • Existing Methods: A General Framework • Proposed Method: LMM Reconstruction • Experiments • Conclusions

  18. Conclusions • Proposed LMM reconstruction method is general. • it can be applied for any 2D surface reconstruction scenarios. • it can be used for generating HDR images from 8-bit images. • Key part of proposed method: skeleton marking • it can guarantee thin but complete skeletons for arbitrary LMM regions. • Significantly over-performs existing BDE methods. • appealing reconstruction performance even for 2bit-8bit expansion. • Time-efficient parallel implementations are possible.

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  20. Thank you ! Q&A