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diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-jpeg.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-jpeg.rst new file mode 100644 index 0000000000..522095c084 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-jpeg.rst @@ -0,0 +1,105 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later + +.. _jpeg-controls: + +********************** +JPEG Control Reference +********************** + +The JPEG class includes controls for common features of JPEG encoders +and decoders. Currently it includes features for codecs implementing +progressive baseline DCT compression process with Huffman entropy +coding. + + +.. _jpeg-control-id: + +JPEG Control IDs +================ + +``V4L2_CID_JPEG_CLASS (class)`` + The JPEG class descriptor. Calling + :ref:`VIDIOC_QUERYCTRL` for this control will + return a description of this control class. + +``V4L2_CID_JPEG_CHROMA_SUBSAMPLING (menu)`` + The chroma subsampling factors describe how each component of an + input image is sampled, in respect to maximum sample rate in each + spatial dimension. See :ref:`itu-t81`, clause A.1.1. for more + details. The ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING`` control determines + how Cb and Cr components are downsampled after converting an input + image from RGB to Y'CbCr color space. + +.. tabularcolumns:: |p{7.5cm}|p{10.0cm}| + +.. flat-table:: + :header-rows: 0 + :stub-columns: 0 + + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_444`` + - No chroma subsampling, each pixel has Y, Cr and Cb values. + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_422`` + - Horizontally subsample Cr, Cb components by a factor of 2. + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_420`` + - Subsample Cr, Cb components horizontally and vertically by 2. + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_411`` + - Horizontally subsample Cr, Cb components by a factor of 4. + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_410`` + - Subsample Cr, Cb components horizontally by 4 and vertically by 2. + * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY`` + - Use only luminance component. + + + +``V4L2_CID_JPEG_RESTART_INTERVAL (integer)`` + The restart interval determines an interval of inserting RSTm + markers (m = 0..7). The purpose of these markers is to additionally + reinitialize the encoder process, in order to process blocks of an + image independently. For the lossy compression processes the restart + interval unit is MCU (Minimum Coded Unit) and its value is contained + in DRI (Define Restart Interval) marker. If + ``V4L2_CID_JPEG_RESTART_INTERVAL`` control is set to 0, DRI and RSTm + markers will not be inserted. + +.. _jpeg-quality-control: + +``V4L2_CID_JPEG_COMPRESSION_QUALITY (integer)`` + Determines trade-off between image quality and size. + It provides simpler method for applications to control image quality, + without a need for direct reconfiguration of luminance and chrominance + quantization tables. In cases where a driver uses quantization tables + configured directly by an application, using interfaces defined + elsewhere, ``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control should be set by + driver to 0. + + The value range of this control is driver-specific. Only positive, + non-zero values are meaningful. The recommended range is 1 - 100, + where larger values correspond to better image quality. + +.. _jpeg-active-marker-control: + +``V4L2_CID_JPEG_ACTIVE_MARKER (bitmask)`` + Specify which JPEG markers are included in compressed stream. This + control is valid only for encoders. + + + +.. flat-table:: + :header-rows: 0 + :stub-columns: 0 + + * - ``V4L2_JPEG_ACTIVE_MARKER_APP0`` + - Application data segment APP\ :sub:`0`. + * - ``V4L2_JPEG_ACTIVE_MARKER_APP1`` + - Application data segment APP\ :sub:`1`. + * - ``V4L2_JPEG_ACTIVE_MARKER_COM`` + - Comment segment. + * - ``V4L2_JPEG_ACTIVE_MARKER_DQT`` + - Quantization tables segment. + * - ``V4L2_JPEG_ACTIVE_MARKER_DHT`` + - Huffman tables segment. + + + +For more details about JPEG specification, refer to :ref:`itu-t81`, +:ref:`jfif`, :ref:`w3c-jpeg-jfif`. |