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diff --git a/doc/snow.txt b/doc/snow.txt new file mode 100644 index 0000000000..b9ffe9f8c7 --- /dev/null +++ b/doc/snow.txt @@ -0,0 +1,259 @@ +============================================= +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; y<block_count_vertical; y++) + for(x=0; x<block_count_horizontal; x++) + block(0) + +block(level): + if(keyframe){ + intra=1 + y_diff=cb_diff=cr_diff=0 + }else{ + if(level!=max_block_depth){ + s_context= 2*left->level + 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 |