228 lines
7.4 KiB
XML
228 lines
7.4 KiB
XML
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<?xml version="1.0"?>
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<!-- vim: set ts=2 sw=2 expandtab: -->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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]>
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<refentry id="file-format">
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<refmeta>
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<refentrytitle>The VIPS file format</refentrytitle>
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<manvolnum>3</manvolnum>
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<refmiscinfo>VIPS Library</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>File format</refname>
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<refpurpose>The VIPS file format</refpurpose>
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</refnamediv>
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<refsect1 id="vips-format">
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<title>Introduction</title>
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<para>
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VIPS has a simple, native file format. It's very fast, there is no image
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size limit, and it supports
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arbitrary metadata. Although few other programs can read these images
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(though recent versions of ImageMagick do support <code>.vips</code>
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format), it can be useful as an intermediate format for command-line
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processing. For example:
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<programlisting language="bash">
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$ vips invert input.tif t.v
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$ vips gamma t.v output.tif
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</programlisting>
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is faster than using <code>.tif</code> for the temporary intermediate
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image. This section documents the VIPS file format.
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</para>
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<para>
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VIPS comes with a command-line program called
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<command>vipsedit</command> which is useful for destructively changing
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fields in a vips image. The <command>vipsheader</command> program can be
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used to extract any metadata.
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</para>
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<para>
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VIPS files come in three parts. First, there is a 64-byte header,
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containing an identifying magic number and a set of very basic fields,
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such as image width in pixels. Next, the image data is stored as a set
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of band-interleaved scanlines, from the top of the image to the bottom.
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Finally, after the pixel data comes an optional block of XML containing
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any extra metadata, such as ICC profiles.
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</para>
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</refsect1>
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<refsect1 id="vips-format-header">
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<title>The header</title>
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<para>
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The fields in the VIPS header are always stored least-significant byte
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first (Intel ordering). Only the most basic information about the image
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is in the header: most metadata is stored in the XML extension block
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after the pixel data.
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<table>
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<title>The VIPS header</title>
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<tgroup cols='4' align='left' colsep='1' rowsep='1'>
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<thead>
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<row>
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<entry>Bytes</entry>
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<entry>Type</entry>
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<entry>VIPS name</entry>
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<entry>Meaning</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>0 -- 3</entry>
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<entry></entry>
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<entry></entry>
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<entry>VIPS magic number, 08 f2 f6 b6</entry>
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</row>
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<row>
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<entry>4 -- 7</entry>
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<entry>%gint</entry>
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<entry><code>width</code></entry>
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<entry>Width of image, in pixels</entry>
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</row>
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<row>
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<entry>8 -- 11</entry>
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<entry>%gint</entry>
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<entry><code>height</code></entry>
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<entry>Height of image, in pixels</entry>
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</row>
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<row>
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<entry>12 -- 15</entry>
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<entry>%gint</entry>
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<entry><code>bands</code></entry>
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<entry>Number of image bands</entry>
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</row>
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<row>
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<entry>16 -- 19</entry>
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<entry></entry>
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<entry></entry>
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<entry>Unused</entry>
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</row>
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<row>
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<entry>20 -- 23</entry>
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<entry>#VipsBandFormat</entry>
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<entry><code>format</code></entry>
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<entry>Band format</entry>
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</row>
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<row>
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<entry>24 -- 27</entry>
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<entry>#VipsCoding</entry>
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<entry><code>coding</code></entry>
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<entry>Image coding</entry>
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</row>
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<row>
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<entry>28 -- 31</entry>
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<entry>#VipsInterpretation</entry>
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<entry><code>interpretation</code></entry>
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<entry>Pixel interpretation</entry>
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</row>
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<row>
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<entry>32 -- 35</entry>
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<entry>%gfloat</entry>
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<entry><code>xres</code></entry>
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<entry>Horizontal resolution, in pixels per millimetre</entry>
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</row>
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<row>
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<entry>36 -- 39</entry>
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<entry>%gfloat</entry>
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<entry><code>yres</code></entry>
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<entry>Vertical resolution, in pixels per millimetre</entry>
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</row>
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<row>
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<entry>40 -- 47</entry>
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<entry></entry>
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<entry></entry>
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<entry>Unused</entry>
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</row>
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<row>
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<entry>48 -- 51</entry>
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<entry>%gint</entry>
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<entry><code>xoffset</code></entry>
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<entry>Horizontal offset of origin, in pixels</entry>
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</row>
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<row>
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<entry>52 -- 55</entry>
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<entry>%gint</entry>
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<entry><code>yoffset</code></entry>
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<entry>Vertical offset of origin, in pixels</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</para>
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</refsect1>
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<refsect1 id="vips-format-data">
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<title>The image data</title>
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<para>
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If <code>coding</code> is set to #VIPS_CODING_NONE, pixels are stored in
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native C format, that is, the native format of the
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machine that wrote the data. If you open a big-endian image on a
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little-endian machine, VIPS will automatically byte-swap for you.
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VIPS has 10 band formats, see #VipsBandFormat.
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Image data is stored as a simple list of scanlines, from the top of the
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image to the bottom. Pixels are band-interleaved, so RGBRGBRGBRGB,
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for example. There is no padding at the end of scanlines.
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</para>
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<para>
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If <code>coding</code> is set to #VIPS_CODING_LABQ, each pixel is four
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bytes, with 10 bits for L* and 11 bits for each of a* and b*. These
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32 bits are packed into 4 bytes, with the most significant 8 bits of each
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value in the first 3 bytes, and the left-over bits packed into the final
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byte as 2:3:3.
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</para>
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<para>
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If <code>coding</code> is set to #VIPS_CODING_RAD, each pixel is
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RGB or XYZ float, with 8 bytes of mantissa
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and then 8 bytes of exponent, shared between the three channels. This
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coding style is used by the Radiance family of programs (and the HDR
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format) commonly used for HDR imaging.
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</para>
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<para>
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Other values of <code>coding</code> can set other coding styles. Use
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VIPS_IMAGE_SIZEOF_IMAGE() to calculate the size of the image data
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section.
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</para>
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</refsect1>
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<refsect1 id="vips-format-metadata">
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<title>The metadata</title>
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<para>
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Following the image data is a chunk of XML holding a simple list of
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name-value pairs. Binary data is encoded with base64. Use
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vips_image_set() and friends to set and get image metadata.
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</para>
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<para>
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You can use <command>vipsheader -f getext x.v</command> to get the
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XML from a VIPS image, and
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<command>vipsedit --setext x.v < file.xml</command> to replace the
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XML.
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</para>
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</refsect1>
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</refentry>
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