274 lines
4.6 KiB
ReStructuredText
274 lines
4.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. _yuv-formats:
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***********
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YUV Formats
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***********
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YUV is the format native to TV broadcast and composite video signals. It
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separates the brightness information (Y) from the color information (U
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and V or Cb and Cr). The color information consists of red and blue
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*color difference* signals, this way the green component can be
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reconstructed by subtracting from the brightness component. See
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:ref:`colorspaces` for conversion examples. YUV was chosen because
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early television would only transmit brightness information. To add
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color in a way compatible with existing receivers a new signal carrier
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was added to transmit the color difference signals.
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Subsampling
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===========
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YUV formats commonly encode images with a lower resolution for the chroma
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components than for the luma component. This compression technique, taking
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advantage of the human eye being more sensitive to luminance than color
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differences, is called chroma subsampling.
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While many combinations of subsampling factors in the horizontal and vertical
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direction are possible, common factors are 1 (no subsampling), 2 and 4, with
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horizontal subsampling always larger than or equal to vertical subsampling.
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Common combinations are named as follows.
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- `4:4:4`: No subsampling
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- `4:2:2`: Horizontal subsampling by 2, no vertical subsampling
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- `4:2:0`: Horizontal subsampling by 2, vertical subsampling by 2
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- `4:1:1`: Horizontal subsampling by 4, no vertical subsampling
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- `4:1:0`: Horizontal subsampling by 4, vertical subsampling by 4
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Subsampling the chroma component effectively creates chroma values that can be
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located in different spatial locations:
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- .. _yuv-chroma-centered:
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The subsampled chroma value may be calculated by simply averaging the chroma
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value of two consecutive pixels. It effectively models the chroma of a pixel
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sited between the two original pixels. This is referred to as centered or
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interstitially sited chroma.
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- .. _yuv-chroma-cosited:
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The other option is to subsample chroma values in a way that place them in
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the same spatial sites as the pixels. This may be performed by skipping every
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other chroma sample (creating aliasing artifacts), or with filters using an
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odd number of taps. This is referred to as co-sited chroma.
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The following examples show different combination of chroma siting in a 4x4
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image.
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.. flat-table:: 4:2:2 subsampling, interstitially sited
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:header-rows: 1
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:stub-columns: 1
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* -
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- 0
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-
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- 1
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-
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- 2
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-
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- 3
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* - 0
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- Y
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- C
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- Y
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-
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- Y
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- C
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- Y
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* - 1
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- Y
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- C
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- Y
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-
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- Y
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- C
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- Y
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* - 2
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- Y
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- C
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- Y
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-
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- Y
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- C
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- Y
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* - 3
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- Y
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- C
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- Y
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-
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- Y
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- C
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- Y
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.. flat-table:: 4:2:2 subsampling, co-sited
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:header-rows: 1
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:stub-columns: 1
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* -
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- 0
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- 1
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-
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* - 0
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- Y/C
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- Y
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- Y/C
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- Y
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* - 1
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- Y/C
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- Y
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- Y/C
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- Y
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* - 2
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- Y/C
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- Y
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- Y/C
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- Y
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* - 3
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- Y/C
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- Y
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- Y/C
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- Y
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.. flat-table:: 4:2:0 subsampling, horizontally interstitially sited, vertically co-sited
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:header-rows: 1
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:stub-columns: 1
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* -
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* - 0
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- Y
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- C
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- Y
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- Y
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- C
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- Y
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* - 1
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- Y
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- Y
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- Y
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- Y
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* - 2
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- Y
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- C
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- Y
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- Y
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* - 3
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- Y
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- Y
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- Y
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- Y
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.. flat-table:: 4:1:0 subsampling, horizontally and vertically interstitially sited
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:header-rows: 1
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:stub-columns: 1
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* -
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- 0
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* - 0
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- Y
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- Y
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- Y
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- Y
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* -
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* - 1
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- Y
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- Y
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- Y
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- Y
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* -
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- C
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* - 2
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- Y
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- Y
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- Y
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- Y
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* -
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* - 3
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- Y
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- Y
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- Y
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- Y
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.. toctree::
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:maxdepth: 1
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pixfmt-packed-yuv
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pixfmt-yuv-planar
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pixfmt-yuv-luma
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pixfmt-y8i
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pixfmt-y12i
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pixfmt-uv8
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pixfmt-m420
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