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+.. 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 entrophy
+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`.