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Detecting video frames in AVC/H.264 compressed bitstreams

Presentation describes high level structore of AVC(H.264) video conpressed bitstream by using Virinext Bitstream Analyzer. Report is focused on explaining how video frames are stored within an AVC/H.264 bitstream.

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Detecting video frames in AVC/H.264 compressed bitstreams

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  1. Detecting video frames in AVC/H.264 compressed bitstreams

  2. AVC/H.264 video overview At a high level, video consists of a sequence of individual images or video frames; Uncompressed video requires a substantial amount of storage space; 1 second of 1920×1080/50p video requires 1920 * 1080 * 1.5 (bytes per pixel in I420) * 50 = 155.52 MB; Video compression is used to reduce storage and bitrate demands; AVC/H.264, initially released in 2003, remains one of the most popular video compression standards worldwide; We will use Virinext Bitstream Analyzer for analyzing AVC/H.264 files. ● ● ● ● ● ●

  3. High level AVC/H.264 bitstream overview At a high level, AVC/H.264 video is a sequence of NAL Units (Network Abstraction Layer Units)

  4. AVC/H.264 Slice NAL Unit Encoded video frames are stored within Slice NAL units; Slice can contain either a full frame or just part of a frame. A special slice type, called an IDR-slice (Instantaneous Decoding Refresh), signals a reset in frame referencing

  5. AVC/H.264 Slice Types The following slice types are used: I-slice: Contains only intra-predicted data (spatial prediction within the slice); P-slice: Contains inter-predicted data referencing one prior slice; B-slice: Contains inter-predicted data referencing up to two different frames. ● ● ● I-slice only exploit spatial redundancy only. P- and B-slices use both intra-prediction and inter-prediction.

  6. AVC/H.264 Slice Types

  7. AVC/H.264 Slice and error propagation Slice NAL units are stored in bitstream independently; Error in a bitstream within one slice(data loss or corruption) do not affect directly other slices; Errors in decoding in some slice can lead in errors in another frames due to to incorrect predictions. ● ● ●

  8. AVC/H.264 Slice and multithreading Slices are stored independently on bitstream level; They allows for multithreaded processing on both the encoding and decoding sides; When frame consists of multiple independent slices, this slices can be encoded and decoded in parallel. ● ● ●

  9. AVC/H.264 frame boundary detection: AUD Access Unit Delimiter (AUD) NAL Unit is used to indicate the start of a new video frame; AUD is an optional, and it may not appear in the bitstream at all. ● ●

  10. AVC/H.264 frame boundary detection If any of these conditions are met, the new slice belongs to a new frame; otherwise, both the last and new slices are part of the same frame. 1. lastSlice::pic_parameter_set_id != newSlice::pic_parameter_set_id 2. lastSlice::frame_num != newSlice::frame_num 3. lastSlice::field_pic_flag != newSlice::field_pic_flag 4. lastSlice::field_pic_flag == 1 && newSlice::field_pic_flag == 1 && lastSlice::bottom_field_flag != newSlice::bottom_field_flag 5. lastSlice::nal_ref_idc != newSlice::nal_ref_idc and (lastSlice::nal_ref_idc == 0 || newSlice::nal_ref_idc == 0) 6. (lastSlice::m_nalUnitType == AVC::NAL_SLICE_IDR) != (newSlice::m_nalUnitType == AVC::NAL_SLICE_IDR)

  11. AVC/H.264 frame boundary detection If any of these conditions are met, the new slice belongs to a new frame; otherwise, both the last and new slices are part of the same frame. 7. (lastSlice::m_nalUnitType == AVC::NAL_SLICE_IDR) && (newSlice::m_nalUnitType == AVC::NAL_SLICE_IDR) && (lastSlice::idr_pic_id != newSlice::idr_pic_id) For cases when SPS::pic_order_cnt_type == 0: 8.1. lastSlice::pic_order_cnt_lsb != newSlice::pic_order_cnt_lsb 8.2. lastSlice::delta_pic_order_cnt_bottom != newSlice::delta_pic_order_cnt_bottom For cases when SPS::pic_order_cnt_type == 1: 9.1. lastSlice::delta_pic_order_cnt[0] != newSlice::delta_pic_order_cnt[0] 9.2. lastSlice::delta_pic_order_cnt[1] != newSlice::delta_pic_order_cnt[1]

  12. AV1 video analysis with Virinext Bitstream Analyzer Screenshot of frame preview, syntax viewer and GOP widgets of AVC video file opened in Virinext Bitstream Analyzer:

  13. References Virinext Bitsteam Analyzer - GUI tool for displaying and analyzing coded video and audio streams Detecting video frames in AVC/H.264 compressed bitstreams The structure of AVC (H264) video H.264 : Advanced video coding for generic audiovisual services Advanced Video Coding ● ● ● ● ●

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