============================================= SNOW Video Codec Specification Draft 20070103 ============================================= Definitions: ============ MUST the specific part must be done to conform to this standard SHOULD it is recommended to be done that way, but not strictly required ilog2(x) is the rounded down logarithm of x with basis 2 ilog2(0) = 0 Type definitions: ================= b 1-bit range coded u unsigned scalar value range coded s signed scalar value range coded Bitstream syntax: ================= frame: header prediction residual header: keyframe b MID_STATE if(keyframe || always_reset) reset_contexts if(keyframe){ version u header_state always_reset b header_state temporal_decomposition_type u header_state temporal_decomposition_count u header_state spatial_decomposition_count u header_state colorspace_type u header_state chroma_h_shift u header_state chroma_v_shift u header_state spatial_scalability b header_state max_ref_frames-1 u header_state qlogs } spatial_decomposition_type s header_state qlog s header_state mv_scale s header_state qbias s header_state block_max_depth s header_state qlogs: for(plane=0; plane<2; plane++){ quant_table[plane][0][0] s header_state for(level=0; level < spatial_decomposition_count; level++){ quant_table[plane][level][1]s header_state quant_table[plane][level][3]s header_state } } reset_contexts *_state[*]= MID_STATE prediction: for(y=0; ylevel + 2*top->level + topleft->level + topright->level leaf b block_state[4 + s_context] } if(level==max_block_depth || leaf){ intra b block_state[1 + left->intra + top->intra] if(intra){ y_diff s block_state[32] cb_diff s block_state[64] cr_diff s block_state[96] }else{ ref_context= ilog2(2*left->ref) + ilog2(2*top->ref) if(ref_frames > 1) ref u block_state[128 + 1024 + 32*ref_context] mx_context= ilog2(2*abs(left->mx - top->mx)) my_context= ilog2(2*abs(left->my - top->my)) mvx_diff s block_state[128 + 32*(mx_context + 16*!!ref)] mvy_diff s block_state[128 + 32*(my_context + 16*!!ref)] } }else{ block(level+1) block(level+1) block(level+1) block(level+1) } } residual: FIXME Tag description: ---------------- version 0 this MUST NOT change within a bitstream always_reset if 1 then the range coder contexts will be reset after each frame temporal_decomposition_type 0 temporal_decomposition_count 0 spatial_decomposition_count FIXME colorspace_type 0 this MUST NOT change within a bitstream chroma_h_shift log2(luma.width / chroma.width) this MUST NOT change within a bitstream chroma_v_shift log2(luma.height / chroma.height) this MUST NOT change within a bitstream spatial_scalability 0 max_ref_frames maximum number of reference frames this MUST NOT change within a bitstream ref_frames minimum of the number of available reference frames and max_ref_frames for example the first frame after a key frame always has ref_frames=1 spatial_decomposition_type wavelet type 0 is a 9/7 symmetric compact integer wavelet 1 is a 5/3 symmetric compact integer wavelet others are reserved stored as delta from last, last is reset to 0 if always_reset || keyframe qlog quality (logarthmic quantizer scale) stored as delta from last, last is reset to 0 if always_reset || keyframe mv_scale stored as delta from last, last is reset to 0 if always_reset || keyframe FIXME check that everything works fine if this chanes between frames qbias dequantization bias stored as delta from last, last is reset to 0 if always_reset || keyframe block_max_depth maximum depth of the block tree stored as delta from last, last is reset to 0 if always_reset || keyframe quant_table quantiztation table Range Coder: ============ FIXME Neighboring Blocks: =================== left and top are set to the respective blocks unless they are outside of the image in which case they are set to the Null block top-left is set to the top left block unless its outside of the image in which case it is set to the left block if this block has no larger parent block or its at the left side of its parent block and the top right block is not outside of the image then the top right block is used for top-right else the top-left block is used Null block y,cb,cr are 128 level, ref, mx and my are 0 Motion Vector Prediction: ========================= 1. the motion vectors of all the neighboring blocks are scaled to compensate for the difference of reference frames scaled_mv= (mv * (256 * (current_reference+1) / (mv.reference+1)) + 128)>>8 2. the median of the scaled left, top and top-right vectors is used as motion vector prediction 3. the used motion vector is the sum of the predictor and (mvx_diff, mvy_diff)*mv_scale Intra DC Predicton: ====================== the luma and chroma values of the left block are used as predictors the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff Motion Compensation: ==================== FIXME LL band prediction: =================== FIXME Dequantizaton: ============== FIXME Wavelet Transform: ================== FIXME TODO: ===== Important: finetune initial contexts spatial_decomposition_count per frame? flip wavelet? try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients try the MV length as context for coding the residual coefficients use extradata for stuff which is in the keyframes now? the MV median predictor is patented IIRC Not Important: spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later) Credits: ======== Michael Niedermayer Loren Merritt Copyright: ========== GPL + GFDL + whatever is needed to make this a RFC