52 lines
2.6 KiB
ReStructuredText
52 lines
2.6 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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**********************
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Standard Image Formats
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**********************
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In order to exchange images between drivers and applications, it is
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necessary to have standard image data formats which both sides will
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interpret the same way. V4L2 includes several such formats, and this
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section is intended to be an unambiguous specification of the standard
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image data formats in V4L2.
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V4L2 drivers are not limited to these formats, however. Driver-specific
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formats are possible. In that case the application may depend on a codec
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to convert images to one of the standard formats when needed. But the
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data can still be stored and retrieved in the proprietary format. For
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example, a device may support a proprietary compressed format.
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Applications can still capture and save the data in the compressed
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format, saving much disk space, and later use a codec to convert the
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images to the X Windows screen format when the video is to be displayed.
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Even so, ultimately, some standard formats are needed, so the V4L2
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specification would not be complete without well-defined standard
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formats.
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The V4L2 standard formats are mainly uncompressed formats. The pixels
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are always arranged in memory from left to right, and from top to
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bottom. The first byte of data in the image buffer is always for the
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leftmost pixel of the topmost row. Following that is the pixel
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immediately to its right, and so on until the end of the top row of
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pixels. Following the rightmost pixel of the row there may be zero or
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more bytes of padding to guarantee that each row of pixel data has a
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certain alignment. Following the pad bytes, if any, is data for the
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leftmost pixel of the second row from the top, and so on. The last row
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has just as many pad bytes after it as the other rows.
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In V4L2 each format has an identifier which looks like ``PIX_FMT_XXX``,
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defined in the :ref:`videodev2.h <videodev>` header file. These
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identifiers represent
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:ref:`four character (FourCC) codes <v4l2-fourcc>` which are also
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listed below, however they are not the same as those used in the Windows
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world.
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For some formats, data is stored in separate, discontiguous memory
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buffers. Those formats are identified by a separate set of FourCC codes
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and are referred to as "multi-planar formats". For example, a
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:ref:`YUV422 <V4L2-PIX-FMT-YUV422M>` frame is normally stored in one
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memory buffer, but it can also be placed in two or three separate
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buffers, with Y component in one buffer and CbCr components in another
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in the 2-planar version or with each component in its own buffer in the
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3-planar case. Those sub-buffers are referred to as "*planes*".
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