| JCTVC-B089 [M. U. Demircin, M. Budagavi, M. Zhou, S. Dikbas (TI)] TE2: Compressed reference frame buffers (CRFB)
An in-loop memory access bandwidth reduction technique is proposed. Proposed tool compresses the reference pictures before they are written to the memory and decompresses before they are being read. Fixed compression ratio is targeted for a block of pixels to enable random access. Proposed technique provides 12 bit/pixel to 8 bit/pixel compression when Internal Bit-Depth Increase (IBDI) tool is turned on. 8 bit/pixel to 4 bit/pixel compression is achieved when IBDI is disabled. 2-D integer S-transform on 8x8 blocks, DC prediction, quantization and variable length entropy coding is employed. Performance is tested and verified as a part of the Tool Experiment 2 (TE2). Proposed algorithm results a worst case BD-Rate increase of 1.01% for Class A, B and E bitstreams for IBDI-on and 0.80% for IBDI-off settings.
Investigations: 12 to 8 bit compression for IBDI, 8 bit to 4 bit compression with non-IBDI. Requirement of random access makes this hard (requires small units and fixed length codes).
Algorithm uses transform, quantization, DC prediction (not in IBDI case), EG0 and EG3 entropy coding. Current loss is 0.16% for CS1 and 0.32% for CS2 in IBDI,
8x8 access units, Wavelet transform similar to Haar is used. Fixed number of bits is used per 8x8 block.
Issues raised: This could lead to the case where single blocks of high detailed blocks are looking highly distorted. Visual quality inspection should be made, both at low and even more high QP.
Same conditions should be used in any case. Study must be performed on statistical basis what an allowable / useful access unit size is, e.g. it could happen that 8x8 is too large when four blocks must be accessed to grab one block for MC in cases of heavy overlap. (e.g. assume certain statistical distribution of MC blocks and of MC access positions)
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