1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _jpeg-controls: 5 6********************** 7JPEG Control Reference 8********************** 9 10The JPEG class includes controls for common features of JPEG encoders 11and decoders. Currently it includes features for codecs implementing 12progressive baseline DCT compression process with Huffman entropy 13coding. 14 15 16.. _jpeg-control-id: 17 18JPEG Control IDs 19================ 20 21``V4L2_CID_JPEG_CLASS (class)`` 22 The JPEG class descriptor. Calling 23 :ref:`VIDIOC_QUERYCTRL` for this control will 24 return a description of this control class. 25 26``V4L2_CID_JPEG_CHROMA_SUBSAMPLING (menu)`` 27 The chroma subsampling factors describe how each component of an 28 input image is sampled, in respect to maximum sample rate in each 29 spatial dimension. See :ref:`itu-t81`, clause A.1.1. for more 30 details. The ``V4L2_CID_JPEG_CHROMA_SUBSAMPLING`` control determines 31 how Cb and Cr components are downsampled after converting an input 32 image from RGB to Y'CbCr color space. 33 34.. tabularcolumns:: |p{7.5cm}|p{10.0cm}| 35 36.. flat-table:: 37 :header-rows: 0 38 :stub-columns: 0 39 40 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_444`` 41 - No chroma subsampling, each pixel has Y, Cr and Cb values. 42 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_422`` 43 - Horizontally subsample Cr, Cb components by a factor of 2. 44 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_420`` 45 - Subsample Cr, Cb components horizontally and vertically by 2. 46 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_411`` 47 - Horizontally subsample Cr, Cb components by a factor of 4. 48 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_410`` 49 - Subsample Cr, Cb components horizontally by 4 and vertically by 2. 50 * - ``V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY`` 51 - Use only luminance component. 52 53 54 55``V4L2_CID_JPEG_RESTART_INTERVAL (integer)`` 56 The restart interval determines an interval of inserting RSTm 57 markers (m = 0..7). The purpose of these markers is to additionally 58 reinitialize the encoder process, in order to process blocks of an 59 image independently. For the lossy compression processes the restart 60 interval unit is MCU (Minimum Coded Unit) and its value is contained 61 in DRI (Define Restart Interval) marker. If 62 ``V4L2_CID_JPEG_RESTART_INTERVAL`` control is set to 0, DRI and RSTm 63 markers will not be inserted. 64 65.. _jpeg-quality-control: 66 67``V4L2_CID_JPEG_COMPRESSION_QUALITY (integer)`` 68 Determines trade-off between image quality and size. 69 It provides simpler method for applications to control image quality, 70 without a need for direct reconfiguration of luminance and chrominance 71 quantization tables. In cases where a driver uses quantization tables 72 configured directly by an application, using interfaces defined 73 elsewhere, ``V4L2_CID_JPEG_COMPRESSION_QUALITY`` control should be set by 74 driver to 0. 75 76 The value range of this control is driver-specific. Only positive, 77 non-zero values are meaningful. The recommended range is 1 - 100, 78 where larger values correspond to better image quality. 79 80.. _jpeg-active-marker-control: 81 82``V4L2_CID_JPEG_ACTIVE_MARKER (bitmask)`` 83 Specify which JPEG markers are included in compressed stream. This 84 control is valid only for encoders. 85 86 87 88.. flat-table:: 89 :header-rows: 0 90 :stub-columns: 0 91 92 * - ``V4L2_JPEG_ACTIVE_MARKER_APP0`` 93 - Application data segment APP\ :sub:`0`. 94 * - ``V4L2_JPEG_ACTIVE_MARKER_APP1`` 95 - Application data segment APP\ :sub:`1`. 96 * - ``V4L2_JPEG_ACTIVE_MARKER_COM`` 97 - Comment segment. 98 * - ``V4L2_JPEG_ACTIVE_MARKER_DQT`` 99 - Quantization tables segment. 100 * - ``V4L2_JPEG_ACTIVE_MARKER_DHT`` 101 - Huffman tables segment. 102 103 104 105For more details about JPEG specification, refer to :ref:`itu-t81`, 106:ref:`jfif`, :ref:`w3c-jpeg-jfif`. 107