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