ffmpeg-0.6

============================================= Snow Video Codec Specification Draft 20080110 ============================================= Introduction: ============= This specification describes the Snow bitstream syntax and semantics as well as the formal Snow decoding process. The decoding process is described precisely and any compliant decoder MUST produce the exact same output for a spec-conformant Snow stream. For encoding, though, any process which generates a stream compliant to the syntactical and semantic requirements and which is decodable by the process described in this spec shall be considered a conformant Snow encoder. 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 } if(!keyframe){ update_mc b header_state if(update_mc){ for(plane=0; plane<2; plane++){ diag_mc b header_state htaps/2-1 u header_state for(i= p->htaps/2; i; i--) |hcoeff[i]| u header_state } } update_qlogs b header_state if(update_qlogs){ spatial_decomposition_count 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): mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0 if(keyframe){ intra=1 }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: residual2(luma) residual2(chroma_cr) residual2(chroma_cb) residual2: for(level=0; level<spatial_decomposition_count; level++){ if(level==0) subband(LL, 0) subband(HL, level) subband(LH, level) subband(HH, level) } subband: 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 update_mc indicates that motion compensation filter parameters are stored in the header diag_mc flag to enable faster diagonal interpolation this SHOULD be 1 unless it turns out to be covered by a valid patent htaps number of half pel interpolation filter taps, MUST be even, >0 and <10 hcoeff half pel interpolation filter coefficients, hcoeff[0] are the 2 middle coefficients [1] are the next outer ones and so on, resulting in a filter like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ... the sign of the coefficients is not explicitly stored but alternates after each coeff and coeff[0] is positive, so ...,+,-,+,-,+,+,-,+,-,+,... hcoeff[0] is not explicitly stored but found by subtracting the sum of all stored coefficients with signs from 32 hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ... a good choice for hcoeff and htaps is htaps= 6 hcoeff={40,-10,2} an alternative which requires more computations at both encoder and decoder side and may or may not be better is htaps= 8 hcoeff={42,-14,6,-2} 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 changes 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 Highlevel bitstream structure: ============================= -------------------------------------------- | Header | -------------------------------------------- | ------------------------------------ | | | Block0 | | | | split? | | | | yes no | | | | ......... intra? | | | | : Block01 : yes no | | | | : Bl