1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3********************** 4Standard Image Formats 5********************** 6 7In order to exchange images between drivers and applications, it is 8necessary to have standard image data formats which both sides will 9interpret the same way. V4L2 includes several such formats, and this 10section is intended to be an unambiguous specification of the standard 11image data formats in V4L2. 12 13V4L2 drivers are not limited to these formats, however. Driver-specific 14formats are possible. In that case the application may depend on a codec 15to convert images to one of the standard formats when needed. But the 16data can still be stored and retrieved in the proprietary format. For 17example, a device may support a proprietary compressed format. 18Applications can still capture and save the data in the compressed 19format, saving much disk space, and later use a codec to convert the 20images to the X Windows screen format when the video is to be displayed. 21 22Even so, ultimately, some standard formats are needed, so the V4L2 23specification would not be complete without well-defined standard 24formats. 25 26The V4L2 standard formats are mainly uncompressed formats. The pixels 27are always arranged in memory from left to right, and from top to 28bottom. The first byte of data in the image buffer is always for the 29leftmost pixel of the topmost row. Following that is the pixel 30immediately to its right, and so on until the end of the top row of 31pixels. Following the rightmost pixel of the row there may be zero or 32more bytes of padding to guarantee that each row of pixel data has a 33certain alignment. Following the pad bytes, if any, is data for the 34leftmost pixel of the second row from the top, and so on. The last row 35has just as many pad bytes after it as the other rows. 36 37In V4L2 each format has an identifier which looks like ``PIX_FMT_XXX``, 38defined in the :ref:`videodev2.h <videodev>` header file. These 39identifiers represent 40:ref:`four character (FourCC) codes <v4l2-fourcc>` which are also 41listed below, however they are not the same as those used in the Windows 42world. 43 44For some formats, data is stored in separate, discontiguous memory 45buffers. Those formats are identified by a separate set of FourCC codes 46and are referred to as "multi-planar formats". For example, a 47:ref:`YUV422 <V4L2-PIX-FMT-YUV422M>` frame is normally stored in one 48memory buffer, but it can also be placed in two or three separate 49buffers, with Y component in one buffer and CbCr components in another 50in the 2-planar version or with each component in its own buffer in the 513-planar case. Those sub-buffers are referred to as "*planes*". 52