228 lines
8.9 KiB
XML
228 lines
8.9 KiB
XML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE article 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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<refentry id="binding">
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<para>
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<refmeta> <refentrytitle>How to write bindings</refentrytitle> <manvolnum>3</manvolnum> <refmiscinfo>libvips</refmiscinfo> </refmeta>
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</para>
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<para>
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<refnamediv> <refname>Binding</refname> <refpurpose>Writing bindings for libvips</refpurpose> </refnamediv>
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</para>
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<para>
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There are full libvips bindings for quite a few environments now: C, C++, command-line, Ruby, PHP, Lua, Python and JavaScript (node).
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</para>
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<para>
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This chapter runs through the four main styles that have been found to work well. If you want to write a new binding, one of these should be close to what you need.
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</para>
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<refsect3 xml:id="dont-bind-the-top-level-c-api">
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<title>Don’t bind the top-level C API</title>
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<para>
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The libvips C API (vips_add() and so on) was designed to be easy for humans to write. It is inconvenient and dangerous to use from other languages due to its heavy use of varargs.
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</para>
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<para>
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It’s much better to use the layer below. This lower layer is structured as:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Create operator. You can use vips_operation_new() to make a new <literal>VipsOperation</literal> object from an operator nickname, like <literal>"add"</literal>.
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</para>
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</listitem>
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<listitem>
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<para>
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Set parameters. You can loop over the operation with vips_argument_map() to get the name and type of each input argument. For each argument, you need to get the value from your language, convert to a <literal>GValue</literal>, then use g_object_set_property() to set that value on the operator.
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</para>
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</listitem>
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<listitem>
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<para>
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Execute with vips_cache_operation_build().
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</para>
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</listitem>
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<listitem>
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<para>
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Extract results. Again, you loop over the operator arguments with vips_argument_map(), but instead of inputs, this time you look for output arguments. You extract their value with g_object_get_property(), and pass the value back to your language.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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For example, you can execute vips_invert() like this:
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</para>
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<programlisting language="c">
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/* compile with
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*
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* gcc -g -Wall callvips.c `pkg-config vips --cflags --libs`
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*
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*/
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#include <vips/vips.h>
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int
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main( int argc, char **argv )
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{
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VipsImage *in;
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VipsImage *out;
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VipsOperation *op;
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VipsOperation *new_op;
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GValue gvalue = { 0 };
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if( VIPS_INIT( argv[0] ) )
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/* This shows the vips error buffer and quits with a fail exit
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* code.
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*/
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vips_error_exit( NULL );
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/* This will print a table of any ref leaks on exit, very handy for
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* development.
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*/
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vips_leak_set( TRUE );
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if( argc != 3 )
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vips_error_exit( "usage: %s input-filename output-filename",
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argv[0] );
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if( !(in = vips_image_new_from_file( argv[1], NULL )) )
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vips_error_exit( NULL );
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/* Create a new operator from a nickname. NULL for unknown operator.
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*/
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op = vips_operation_new( "invert" );
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/* Init a gvalue as an image, set it to in, use the gvalue to set the
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* operator property.
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*/
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g_value_init( &gvalue, VIPS_TYPE_IMAGE );
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g_value_set_object( &gvalue, in );
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g_object_set_property( G_OBJECT( op ), "in", &gvalue );
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g_value_unset( &gvalue );
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/* We no longer need in: op will hold a ref to it as long as it needs
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* it.
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*/
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g_object_unref( in );
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/* Call the operation. This will look up the operation+args in the vips
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* operation cache and either return a previous operation, or build
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* this one. In either case, we have a new ref we must release.
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*/
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if( !(new_op = vips_cache_operation_build( op )) ) {
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g_object_unref( op );
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vips_error_exit( NULL );
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}
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g_object_unref( op );
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op = new_op;
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/* Now get the result from op. g_value_get_object() does not ref the
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* object, so we need to make a ref for out to hold.
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*/
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g_value_init( &gvalue, VIPS_TYPE_IMAGE );
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g_object_get_property( G_OBJECT( op ), "out", &gvalue );
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out = VIPS_IMAGE( g_value_get_object( &gvalue ) );
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g_object_ref( out );
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g_value_unset( &gvalue );
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/* All done: we can unref op. The output objects from op actually hold
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* refs back to it, so before we can unref op, we must unref them.
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*/
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vips_object_unref_outputs( VIPS_OBJECT( op ) );
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g_object_unref( op );
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if( vips_image_write_to_file( out, argv[2], NULL ) )
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vips_error_exit( NULL );
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g_object_unref( out );
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return( 0 );
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}
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</programlisting>
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</refsect3>
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<refsect3 xml:id="compiled-language-which-can-call-c">
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<title>Compiled language which can call C</title>
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<para>
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The C++ binding uses this lower layer to define a function called <literal>VImage::call()</literal> which can call any libvips operator with a set of variable arguments.
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</para>
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<para>
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A small Python program walks the set of all libvips operators and generates a set of static bindings. For example:
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</para>
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<programlisting language="c++">
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VImage VImage::invert( VOption *options )
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{
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VImage out;
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call( "invert", (options ? options : VImage::option()) ->
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set( "in", *this ) ->
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set( "out", &out ) );
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return( out );
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}
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</programlisting>
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<para>
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So from C++ you can call any libvips operator (though without static typechecking) with <literal>VImage::call()</literal>, or use the member functions on <literal>VImage</literal> to get type-checked calls for at least the required operator arguments.
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</para>
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<para>
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The <literal>VImage</literal> class also adds automatic reference counting, constant expansion, operator overloads, and various other useful features.
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</para>
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</refsect3>
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<refsect3 xml:id="dynamic-language-with-ffi">
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<title>Dynamic language with FFI</title>
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<para>
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Languages like Ruby, Python, JavaScript and LuaJIT can’t call C directly, but they do support FFI. The bindings for these languages work rather like C++, but use FFI to call into libvips and run operations.
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</para>
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<para>
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Since these languages are dynamic, they can add another trick: they intercept the method-missing hook and attempt to run any method calls not implemented by the <literal>Image</literal> class as libvips operators. In effect, the binding is generated at runtime.
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</para>
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</refsect3>
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<refsect3 xml:id="dynamic-language-without-ffi">
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<title>Dynamic language without FFI</title>
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<para>
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PHP does not have a useful FFI, unfortunately, so for this language a small C module implements the general <literal>vips_call()</literal> function for PHP language types, and a larger pure PHP layer makes it convenient to use.
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</para>
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</refsect3>
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<refsect3 xml:id="gobject-introspection">
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<title>gobject-introspection</title>
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<para>
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The C source code to libvips has been marked up with special comments describing the interface in a standard way. These comments are read by the <literal>gobject-introspection</literal> package when libvips is compiled and used to generate a typelib, a description of how to call the library. Many languages have gobject-introspection packages: all you need to do to call libvips from your favorite language is to start g-o-i, load the libvips typelib, and you should have the whole library available. For example, from Python it’s as simple as:
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</para>
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<programlisting language="python">
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from gi.repository import Vips
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</programlisting>
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<para>
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You can now use all of the libvips introspection machinery, as noted above.
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</para>
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<para>
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Unfortunately g-o-i has some strong disadvantages. It is not very portable, since you will need a g-o-i layer for whatever platform you are targetting; it does not cross-compile well, since typelibs include a lot of very-low level data (such as exact structure layouts); and installation for your users is likely to be tricky.
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</para>
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<para>
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If you have a choice, I would recommend simply using FFI.
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</para>
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</refsect3>
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<refsect3 xml:id="documentation">
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<title>Documentation</title>
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<para>
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You can generate searchable docs from a <code>.gir</code> (the thing that is built from scanning libvips and which in turn turn the typelib is made from) with <command>g-ir-doc-tool</command>, for example:
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</para>
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<programlisting language="bash">
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$ g-ir-doc-tool --language=Python -o ~/mydocs Vips-8.0.gir
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</programlisting>
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<para>
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Then to view them, either:
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</para>
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<programlisting language="bash">
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$ yelp ~/mydocs
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</programlisting>
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<para>
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Or perhaps:
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</para>
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<programlisting language="bash">
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$ cd ~/mydocs
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$ yelp-build html .
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</programlisting>
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<para>
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To make HTML docs. This is an easy way to see what you can call in the library.
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</para>
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</refsect3>
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</refentry>
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