Hi Philipp, On 11/28/18 3:01 PM, Philipp Zabel wrote: > Hi, > > the coda driver handles the H.264 loop filter alpha/beta offset controls > incorrectly. When trying to fix them, I noticed that the documentation > is not clear about what these values actually are. > > From the value range of -6 to +6 used in the existing drivers (s5p-mfc, > venus), it looks like they currently correspond directly to the values > stored into the slice headers: slice_alpha_c0_offset_div2 and > slice_beta_offset_div2. These are only half of the actual alpha/beta > filter offsets. > > The ITU-T Rec. H.264 (02/2016) states: > > slice_alpha_c0_offset_div2 specifies the offset used in accessing the > α [...] deblocking filter tables for filtering operations controlled > by the macroblocks within the slice. From this value, the offset that > shall be applied when addressing these tables shall be computed as > > FilterOffsetA = slice_alpha_c0_offset_div2 << 1 (7-32) > > The value of slice_alpha_c0_offset_div2 shall be in the range of −6 to > +6, inclusive. When slice_alpha_c0_offset_div2 is not present in the > slice header, the value of slice_alpha_c0_offset_div2 shall be inferred > to be equal to 0. > > And the same for slice_beta_offset_div2 / FilterOffsetB. > > Do the s5p-mfc and venus drivers use the controls > V4L2_MPEG_VIDEO_H264_LOOP_FILTER_ALPHA and _BETA directly as slice > header fields, and thus their values are to be interpreted as half of > FilterOffsetA/B defined in the H.264 spec, respectively? That is correct for Venus encoder, it uses slice header fields directly [-6, 6]. -- regards, Stan
