forked from mirror/libbpg
libbpg-0.9.5
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@ -1,8 +1,8 @@
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BPG Specification
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version 0.9.4
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version 0.9.5
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Copyright (c) 2014 Fabrice Bellard
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Copyright (c) 2014-2015 Fabrice Bellard
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1) Introduction
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---------------
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@ -26,6 +26,9 @@ alpha and color data.
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Arbitrary metadata (such as EXIF, ICC profile, XMP) are supported.
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Animations are supported as an optional feature. Decoders not
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supporting animation display the first frame of the animation.
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2) Bitstream conventions
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------------------------
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@ -69,7 +72,7 @@ heic_file() {
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extension_present_flag u(1)
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alpha2_flag u(1)
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limited_range_flag u(1)
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reserved_zero u(1)
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animation_flag u(1)
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picture_width ue7(32)
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picture_height ue7(32)
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@ -90,12 +93,26 @@ extension_data()
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while (more_bytes()) {
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extension_tag ue7(32)
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extension_tag_length ue7(32)
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for(j = 0; j < extension_tag_length; j++) {
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extension_tag_data_byte b(8)
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if (extension_tag == 5) {
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animation_control_extension(extension_tag_length)
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} else {
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for(j = 0; j < extension_tag_length; j++) {
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extension_tag_data_byte b(8)
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}
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}
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}
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}
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animation_control_extension(payload_length)
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{
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loop_count ue7(16)
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frame_period_num ue7(16)
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frame_period_den ue7(16)
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while (more_bytes()) {
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dummy_byte b(8)
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}
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}
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hevc_header_and_data()
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{
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if (alpha1_flag || alpha2_flag) {
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@ -148,7 +165,11 @@ hevc_data()
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hevc_data_byte b(8)
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}
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}
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frame_duration_sei(payloadSize)
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{
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frame_duration u(16)
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}
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3.2) Semantics
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--------------
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@ -166,9 +187,6 @@ hevc_data()
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The other values are reserved.
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The chroma samples in the 4:2:0 and 4:2:2 formats are sampled
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at the same position as JPEG [2].
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'alpha1_flag' and 'alpha2_flag' give information about the alpha plane:
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alpha1_flag=0 alpha2_flag=0: no alpha plane.
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@ -243,7 +261,11 @@ hevc_data()
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The alpha (or W) plane always uses the full range.
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'reserved_zero' must be 0 in this version.
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'animation_flag'. The value '1' indicates that more than one
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frame are encoded in the hevc data. The animation control
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extension must be present. If the decoder does not support
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animations, it shall decode the first frame only and ignore the
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animation information.
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'picture_width' is the picture width in pixels. The value 0 is
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not allowed.
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@ -270,10 +292,20 @@ hevc_data()
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4: Thumbnail (the thumbnail shall be a lower resolution version
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of the image and stored in BPG format).
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5: Animation control data.
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The decoder shall ignore the tags it does not support.
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'extension_tag_length' is the length in bytes of the extension tag.
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'loop_count' gives the number of times the animation shall be
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played. The value of 0 means infinite.
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'frame_period_num' and 'frame_period_den' encode the default
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delay between each frame as frame_period_num/frame_period_den
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seconds. The value of 0 for 'frame_period_num' or
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'frame_period_den' is forbidden.
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'hevc_header_length' is the length in bytes of the following data
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up to and including 'trailing_bits'.
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@ -324,7 +356,12 @@ hevc_data()
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with cb_size = 1 << log2_min_luma_coding_block_size
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- bit_depth_luma_minus8 = bit_depth_minus_8
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- bit_depth_chroma_minus8 = bit_depth_minus_8
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- max_transform_hierarchy_depth_inter = max_transform_hierarchy_depth_intra
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- scaling_list_enabled_flag = 0
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- log2_max_pic_order_cnt_lsb_minus4 = 4
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- amp_enabled_flag = 1
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- sps_temporal_mvp_enabled_flag = 1
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Alpha data encoding:
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@ -338,6 +375,16 @@ hevc_data()
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slices and should be interleaved with color slices. alpha NALs
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shall come before the corresponding color NALs.
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Animation encoding:
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- The optional prefix SEI with payloadType = 257 (defined in
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frame_duration_sei()) specifies that the image must be repeated
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'frame_duration' times. 'frame_duration' shall not be zero. If
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the frame duration SEI is not present for a given frame,
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frame_duration = 1 shall be assumed by the decoder. If alpha
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data is present, the frame duration SEI shall be present only
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for the color data.
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3.3) HEVC Profile
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-----------------
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@ -366,6 +413,14 @@ two is done for the chroma and that the width (resp. height) is n
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pixels, ceil(n/2) pixels must be kept as the resulting chroma
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information.
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When animations are present, the next frames shall be encoded with the
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following changes:
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- P slices are allowed (but B slices are not allowed).
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- Only the previous picture can be used as reference (hence a DPB size
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of 2 pictures).
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4) Design choices
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-----------------
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@ -434,6 +489,20 @@ information.
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easily skip all the metadata because their length is explicitly
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stored in the image header.
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- Animations: they are interesting compared to WebM or MP4 short
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videos for the following reasons:
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* transparency is supported
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* lossless encoding is supported
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* the decoding resources are smaller than with a generic video
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player because only two frames need to be stored (DPB size = 2).
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* the animations are expected to be small so the decoder can cache
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all the decoded frames in memory.
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* the animation can be decoded as a still image if the decoder
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does not support animations.
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Compared to the other animated image formats (GIF, APNG, WebP), the
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compression ratio is usually much higher because of the HEVC inter
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frame prediction.
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5) References
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-------------
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