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The Brightness and Darkness of the VENC-Encoded Streams Are Different from Those of the Original YUV

Symptom

When a user uses a third-party player to play a stream encoded by VENC, the brightness and darkness of the encoded stream differs from those of the original YUV.

Possible Cause

Due to the uncontrollable rendering effect of the player, when decoding and displaying the stream content, the player's target pixel value range is inconsistent with the video_full_range_flag set in VENC encoding. This results in pixel value range mapping and causes inconsistency in brightness and darkness between the encoded stream and the original YUV.

For details about the full_range, see See Also.

Solution

You are not advised using a player to check the brightness and darkness effect, because the rendering effect of the player is uncontrollable. It is recommended that the VENC stream be decoded into a YUV file and compare it with the original YUV. At the same time, ensure that the video_full_range_flag is used consistently during encoding and decoding.

  1. Specify the value of the video_full_range_flag during VENC encoding.

    If the default value does not meet the requirements, you can call hi_mpi_venc_set_h264_vui or hi_mpi_venc_set_h265_vui to change the value of video_full_range_flag.

  2. Check the video_full_range_flag of the stream encoded by VENC.

    Use a stream analysis tool to check the value of video_full_range_flag in the SPS field. If the value of flag is 1, the original YUV is full_range. If the value of flag is 0, the original YUV is limited_range.

  3. Specify video_full_range_flag during decoding, and then compare the decoded YUV file with the original YUV file in terms of brightness and darkness.

    The video_full_range_flag usage varies depending on the decoder. The following uses FFmpeg as an example. You can use the -vf option to specify the target output video_full_range_flag. The default output is limited_range.

    FFmpeg is used as an example. The example command is as follows:

    ffmpeg -i ${instream} -pix_fmt nv12 -vf scale=out_range=full/limited -y ${outyuv}

    The following uses examples to describe the brightness/darkness contrast. Currently, the YUV format is full-range. Therefore, the scenario where the source YUV format is limited-range is excluded. Considering the four combinations of VENC and FFmpeg parameters, vui indicates video_full_range_flag set during VENC encoding, and ffmpeg indicates video_full_range_flag set during FFmpeg decoding.

    • The results of vui0_ffmpeg0 and vui1_ffmpeg1 are the same, and those of the source YUV are also the same. This is because the vui and ffmpeg parameters are consistent and the YUV data is directly decompressed and output.
    • The vui0_ffmpeg1 image appears darker because ffmpeg requires a conversion from limited_range to full_range, which results in the stretching of the pixel value distribution towards both ends. This is exacerbated by the fact that the scene itself is already dark, causing a greater number of pixel values to shift towards 0. Consequently, the image appears darker than it actually is.
    • The vui1_ffmpeg0 image appears brighter because the ffmpeg value range is compressed from 0–255 to 16–235. This compression causes a significant number of low pixel values to move into the middle range, resulting in a brighter image.

See Also

Generally, a TV supports 240 color levels ranging from 16 to 255, that is, limited_range. The corresponding YUV value ranges are Y [16, 235] and UV [16, 240].

A modern computer supports 255 color levels, ranging from 0 to 255, that is, full_range. The corresponding YUV value range is YUV [0, 255].

The following flags are equivalent in different software or modules:

  • "full range" = "jpeg" = "pc" = "cg" = "high rgb"
  • "limited range" = "mpeg" = "tv" = "broadcast" = "low rgb"

In the VUI field of H.265 streams, video_full_range_flag = 0 indicates that the original YUV is limited_range, and video_full_range_flag = 1 indicates that the original YUV is full_range. Note that this flag bit does not affect the VENC encoding process. The stream data produced during encoding is solely determined by the actual pixel values of the input YUV. Pixel value mapping does not take place during the encoding process.

For example, during FFmpeg decoding, the system checks whether the original YUV format matches the output YUV format. Pixel value remapping is only initiated when there is a mismatch between the two YUV formats. If the original YUV format is full_range, video_full_range_flag is set to 1 for VENC encoding. If the output YUV format after FFmpeg decoding is limited_range, the YUV format is degraded from full_range to limited_range. Therefore, pixel values are mapped from [0, 255] to [16, 235] after decoding. This results in a difference in brightness and darkness between the decoded YUV and the original YUV.

Parent topic: JPEGE and VENC