snow specification

(this is just quickly written and probably isnt error free, its also incomplete)
all types of help are very welcome (fixing FIXMEs, fixing errors adding missing stuff,
correcting spelling grammmer, ...)

Originally committed as revision 7406 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/snow.txt

+=============================================
+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