236 lines
7.3 KiB
ReStructuredText
236 lines
7.3 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0+
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===================================
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Arm Framebuffer Compression (AFBC)
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===================================
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AFBC is a proprietary lossless image compression protocol and format.
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It provides fine-grained random access and minimizes the amount of
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data transferred between IP blocks.
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AFBC can be enabled on drivers which support it via use of the AFBC
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format modifiers defined in drm_fourcc.h. See DRM_FORMAT_MOD_ARM_AFBC(*).
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All users of the AFBC modifiers must follow the usage guidelines laid
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out in this document, to ensure compatibility across different AFBC
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producers and consumers.
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Components and Ordering
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=======================
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AFBC streams can contain several components - where a component
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corresponds to a color channel (i.e. R, G, B, X, A, Y, Cb, Cr).
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The assignment of input/output color channels must be consistent
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between the encoder and the decoder for correct operation, otherwise
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the consumer will interpret the decoded data incorrectly.
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Furthermore, when the lossless colorspace transform is used
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(AFBC_FORMAT_MOD_YTR, which should be enabled for RGB buffers for
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maximum compression efficiency), the component order must be:
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* Component 0: R
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* Component 1: G
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* Component 2: B
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The component ordering is communicated via the fourcc code in the
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fourcc:modifier pair. In general, component '0' is considered to
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reside in the least-significant bits of the corresponding linear
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format. For example, COMP(bits):
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* DRM_FORMAT_ABGR8888
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* Component 0: R(8)
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* Component 1: G(8)
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* Component 2: B(8)
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* Component 3: A(8)
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* DRM_FORMAT_BGR888
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* Component 0: R(8)
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* Component 1: G(8)
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* Component 2: B(8)
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* DRM_FORMAT_YUYV
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* Component 0: Y(8)
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* Component 1: Cb(8, 2x1 subsampled)
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* Component 2: Cr(8, 2x1 subsampled)
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In AFBC, 'X' components are not treated any differently from any other
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component. Therefore, an AFBC buffer with fourcc DRM_FORMAT_XBGR8888
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encodes with 4 components, like so:
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* DRM_FORMAT_XBGR8888
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* Component 0: R(8)
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* Component 1: G(8)
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* Component 2: B(8)
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* Component 3: X(8)
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Please note, however, that the inclusion of a "wasted" 'X' channel is
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bad for compression efficiency, and so it's recommended to avoid
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formats containing 'X' bits. If a fourth component is
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required/expected by the encoder/decoder, then it is recommended to
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instead use an equivalent format with alpha, setting all alpha bits to
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'1'. If there is no requirement for a fourth component, then a format
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which doesn't include alpha can be used, e.g. DRM_FORMAT_BGR888.
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Number of Planes
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================
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Formats which are typically multi-planar in linear layouts (e.g. YUV
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420), can be encoded into one, or multiple, AFBC planes. As with
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component order, the encoder and decoder must agree about the number
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of planes in order to correctly decode the buffer. The fourcc code is
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used to determine the number of encoded planes in an AFBC buffer,
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matching the number of planes for the linear (unmodified) format.
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Within each plane, the component ordering also follows the fourcc
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code:
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For example:
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* DRM_FORMAT_YUYV: nplanes = 1
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* Plane 0:
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* Component 0: Y(8)
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* Component 1: Cb(8, 2x1 subsampled)
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* Component 2: Cr(8, 2x1 subsampled)
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* DRM_FORMAT_NV12: nplanes = 2
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* Plane 0:
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* Component 0: Y(8)
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* Plane 1:
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* Component 0: Cb(8, 2x1 subsampled)
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* Component 1: Cr(8, 2x1 subsampled)
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Cross-device interoperability
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=============================
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For maximum compatibility across devices, the table below defines
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canonical formats for use between AFBC-enabled devices. Formats which
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are listed here must be used exactly as specified when using the AFBC
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modifiers. Formats which are not listed should be avoided.
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.. flat-table:: AFBC formats
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* - Fourcc code
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- Description
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- Planes/Components
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* - DRM_FORMAT_ABGR2101010
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- 10-bit per component RGB, with 2-bit alpha
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- Plane 0: 4 components
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* Component 0: R(10)
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* Component 1: G(10)
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* Component 2: B(10)
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* Component 3: A(2)
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* - DRM_FORMAT_ABGR8888
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- 8-bit per component RGB, with 8-bit alpha
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- Plane 0: 4 components
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* Component 0: R(8)
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* Component 1: G(8)
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* Component 2: B(8)
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* Component 3: A(8)
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* - DRM_FORMAT_BGR888
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- 8-bit per component RGB
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- Plane 0: 3 components
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* Component 0: R(8)
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* Component 1: G(8)
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* Component 2: B(8)
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* - DRM_FORMAT_BGR565
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- 5/6-bit per component RGB
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- Plane 0: 3 components
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* Component 0: R(5)
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* Component 1: G(6)
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* Component 2: B(5)
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* - DRM_FORMAT_ABGR1555
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- 5-bit per component RGB, with 1-bit alpha
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- Plane 0: 4 components
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* Component 0: R(5)
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* Component 1: G(5)
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* Component 2: B(5)
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* Component 3: A(1)
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* - DRM_FORMAT_VUY888
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- 8-bit per component YCbCr 444, single plane
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- Plane 0: 3 components
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* Component 0: Y(8)
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* Component 1: Cb(8)
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* Component 2: Cr(8)
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* - DRM_FORMAT_VUY101010
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- 10-bit per component YCbCr 444, single plane
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- Plane 0: 3 components
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* Component 0: Y(10)
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* Component 1: Cb(10)
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* Component 2: Cr(10)
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* - DRM_FORMAT_YUYV
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- 8-bit per component YCbCr 422, single plane
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- Plane 0: 3 components
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* Component 0: Y(8)
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* Component 1: Cb(8, 2x1 subsampled)
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* Component 2: Cr(8, 2x1 subsampled)
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* - DRM_FORMAT_NV16
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- 8-bit per component YCbCr 422, two plane
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- Plane 0: 1 component
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* Component 0: Y(8)
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Plane 1: 2 components
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* Component 0: Cb(8, 2x1 subsampled)
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* Component 1: Cr(8, 2x1 subsampled)
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* - DRM_FORMAT_Y210
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- 10-bit per component YCbCr 422, single plane
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- Plane 0: 3 components
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* Component 0: Y(10)
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* Component 1: Cb(10, 2x1 subsampled)
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* Component 2: Cr(10, 2x1 subsampled)
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* - DRM_FORMAT_P210
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- 10-bit per component YCbCr 422, two plane
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- Plane 0: 1 component
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* Component 0: Y(10)
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Plane 1: 2 components
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* Component 0: Cb(10, 2x1 subsampled)
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* Component 1: Cr(10, 2x1 subsampled)
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* - DRM_FORMAT_YUV420_8BIT
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- 8-bit per component YCbCr 420, single plane
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- Plane 0: 3 components
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* Component 0: Y(8)
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* Component 1: Cb(8, 2x2 subsampled)
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* Component 2: Cr(8, 2x2 subsampled)
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* - DRM_FORMAT_YUV420_10BIT
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- 10-bit per component YCbCr 420, single plane
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- Plane 0: 3 components
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* Component 0: Y(10)
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* Component 1: Cb(10, 2x2 subsampled)
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* Component 2: Cr(10, 2x2 subsampled)
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* - DRM_FORMAT_NV12
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- 8-bit per component YCbCr 420, two plane
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- Plane 0: 1 component
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* Component 0: Y(8)
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Plane 1: 2 components
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* Component 0: Cb(8, 2x2 subsampled)
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* Component 1: Cr(8, 2x2 subsampled)
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* - DRM_FORMAT_P010
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- 10-bit per component YCbCr 420, two plane
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- Plane 0: 1 component
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* Component 0: Y(10)
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Plane 1: 2 components
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* Component 0: Cb(10, 2x2 subsampled)
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* Component 1: Cr(10, 2x2 subsampled)
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