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235 lines
7.3 KiB
235 lines
7.3 KiB
.. 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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