228 lines
6.9 KiB
Markdown
228 lines
6.9 KiB
Markdown
<refmeta>
|
|
<refentrytitle>How to write bindings</refentrytitle>
|
|
<manvolnum>3</manvolnum>
|
|
<refmiscinfo>libvips</refmiscinfo>
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>Binding</refname>
|
|
<refpurpose>Writing bindings for libvips</refpurpose>
|
|
</refnamediv>
|
|
|
|
There are full libvips bindings for quite a few environments now: C, C++,
|
|
command-line, Ruby, PHP, Lua, Python and JavaScript (node).
|
|
|
|
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.
|
|
|
|
# Don't bind the top-level C API
|
|
|
|
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.
|
|
|
|
It's much better to use the layer below. This lower layer is structured as:
|
|
|
|
- Create operator. You can use vips_operation_new() to make a new
|
|
`VipsOperation` object from an operator nickname, like `"add"`.
|
|
|
|
- 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 `GValue`, then
|
|
use g_object_set_property() to set that value on the operator.
|
|
|
|
- Execute with vips_cache_operation_build().
|
|
|
|
- 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.
|
|
|
|
For example, you can execute vips_invert() like this:
|
|
|
|
```C
|
|
/* compile with
|
|
*
|
|
* gcc -g -Wall callvips.c `pkg-config vips --cflags --libs`
|
|
*
|
|
*/
|
|
|
|
#include <vips/vips.h>
|
|
|
|
int
|
|
main( int argc, char **argv )
|
|
{
|
|
VipsImage *in;
|
|
VipsImage *out;
|
|
VipsOperation *op;
|
|
VipsOperation *new_op;
|
|
GValue gvalue = { 0 };
|
|
|
|
if( VIPS_INIT( argv[0] ) )
|
|
/* This shows the vips error buffer and quits with a fail exit
|
|
* code.
|
|
*/
|
|
vips_error_exit( NULL );
|
|
|
|
/* This will print a table of any ref leaks on exit, very handy for
|
|
* development.
|
|
*/
|
|
vips_leak_set( TRUE );
|
|
|
|
if( argc != 3 )
|
|
vips_error_exit( "usage: %s input-filename output-filename",
|
|
argv[0] );
|
|
|
|
if( !(in = vips_image_new_from_file( argv[1], NULL )) )
|
|
vips_error_exit( NULL );
|
|
|
|
/* Create a new operator from a nickname. NULL for unknown operator.
|
|
*/
|
|
op = vips_operation_new( "invert" );
|
|
|
|
/* Init a gvalue as an image, set it to in, use the gvalue to set the
|
|
* operator property.
|
|
*/
|
|
g_value_init( &gvalue, VIPS_TYPE_IMAGE );
|
|
g_value_set_object( &gvalue, in );
|
|
g_object_set_property( G_OBJECT( op ), "in", &gvalue );
|
|
g_value_unset( &gvalue );
|
|
|
|
/* We no longer need in: op will hold a ref to it as long as it needs
|
|
* it.
|
|
*/
|
|
g_object_unref( in );
|
|
|
|
/* Call the operation. This will look up the operation+args in the vips
|
|
* operation cache and either return a previous operation, or build
|
|
* this one. In either case, we have a new ref we must release.
|
|
*/
|
|
if( !(new_op = vips_cache_operation_build( op )) ) {
|
|
g_object_unref( op );
|
|
vips_error_exit( NULL );
|
|
}
|
|
g_object_unref( op );
|
|
op = new_op;
|
|
|
|
/* Now get the result from op. g_value_get_object() does not ref the
|
|
* object, so we need to make a ref for out to hold.
|
|
*/
|
|
g_value_init( &gvalue, VIPS_TYPE_IMAGE );
|
|
g_object_get_property( G_OBJECT( op ), "out", &gvalue );
|
|
out = VIPS_IMAGE( g_value_get_object( &gvalue ) );
|
|
g_object_ref( out );
|
|
g_value_unset( &gvalue );
|
|
|
|
/* All done: we can unref op. The output objects from op actually hold
|
|
* refs back to it, so before we can unref op, we must unref them.
|
|
*/
|
|
vips_object_unref_outputs( VIPS_OBJECT( op ) );
|
|
g_object_unref( op );
|
|
|
|
if( vips_image_write_to_file( out, argv[2], NULL ) )
|
|
vips_error_exit( NULL );
|
|
|
|
g_object_unref( out );
|
|
|
|
return( 0 );
|
|
}
|
|
```
|
|
|
|
# Compiled language which can call C
|
|
|
|
The C++ binding uses this lower layer to define a function called
|
|
`VImage::call()` which can call any libvips operator with a set of variable
|
|
arguments.
|
|
|
|
A small Python program walks the set of all libvips operators and generates a
|
|
set of static bindings. For example:
|
|
|
|
```c++
|
|
VImage VImage::invert( VOption *options )
|
|
{
|
|
VImage out;
|
|
|
|
call( "invert", (options ? options : VImage::option()) ->
|
|
set( "in", *this ) ->
|
|
set( "out", &out ) );
|
|
|
|
return( out );
|
|
}
|
|
```
|
|
|
|
So from C++ you can call any libvips operator (though without static
|
|
typechecking) with `VImage::call()`, or use the member functions on `VImage`
|
|
to get type-checked calls for at least the required operator arguments.
|
|
|
|
The `VImage` class also adds automatic reference counting, constant expansion,
|
|
operator overloads, and various other useful features.
|
|
|
|
# Dynamic language with FFI
|
|
|
|
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.
|
|
|
|
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 `Image` class as libvips operators. In effect, the binding is generated at
|
|
runtime.
|
|
|
|
# Dynamic language without FFI
|
|
|
|
PHP does not have a useful FFI, unfortunately, so for this language a small
|
|
C module implements the general `vips_call()` function for PHP language
|
|
types, and a larger pure PHP layer makes it convenient to use.
|
|
|
|
# gobject-introspection
|
|
|
|
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 `gobject-introspection` 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:
|
|
|
|
```python
|
|
from gi.repository import Vips
|
|
```
|
|
|
|
You can now use all of the libvips introspection machinery, as noted above.
|
|
|
|
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.
|
|
|
|
If you have a choice, I would recommend simply using FFI.
|
|
|
|
# Documentation
|
|
|
|
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:
|
|
|
|
```bash
|
|
$ g-ir-doc-tool --language=Python -o ~/mydocs Vips-8.0.gir
|
|
```
|
|
|
|
Then to view them, either:
|
|
|
|
```bash
|
|
$ yelp ~/mydocs
|
|
```
|
|
|
|
Or perhaps:
|
|
|
|
```bash
|
|
$ cd ~/mydocs
|
|
$ yelp-build html .
|
|
```
|
|
|
|
To make HTML docs. This is an easy way to see what you can call in the
|
|
library.
|