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