libvips/doc/using-cpp.xml

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<?xml version="1.0"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
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<!-- vim: set ts=2 sw=2 expandtab: -->
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<refentry id="using-from-cpp">
<refmeta>
<refentrytitle>VIPS from C++</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>VIPS Library</refmiscinfo>
</refmeta>
<refnamediv>
<refname>Using VIPS</refname>
<refpurpose>How to use the VIPS library from C++</refpurpose>
</refnamediv>
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<refsect3 id="using-cpp">
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<title>Introduction</title>
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<para>
VIPS comes with a convenient C++ API. It is a very thin wrapper over the
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C API and adds automatic reference counting, exceptions, operator
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overloads, and automatic constant expansion. You can drop down to the C
API at any point, so all the C API docs also work for C++.
</para>
<programlisting language="C++">
/* compile with:
* g++ -g -Wall try.cc `pkg-config vips-cpp --cflags --libs`
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*/
#include &lt;vips/vips8&gt;
using namespace vips;
int
main( int argc, char **argv )
{
GOptionContext *context;
GOptionGroup *main_group;
GError *error = NULL;
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if( VIPS_INIT( argv[0] ) )
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vips_error_exit( NULL );
context = g_option_context_new( "" );
main_group = g_option_group_new( NULL, NULL, NULL, NULL, NULL );
g_option_context_set_main_group( context, main_group );
g_option_context_add_group( context, vips_get_option_group() );
if( !g_option_context_parse( context, &amp;argc, &amp;argv, &amp;error ) ) {
if( error ) {
fprintf( stderr, "%s\n", error->message );
g_error_free( error );
}
vips_error_exit( NULL );
}
VImage in = VImage::new_from_file( argv[1],
VImage::option()->
set( "access", VIPS_ACCESS_SEQUENTIAL_UNBUFFERED ) );
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double avg = in.avg();
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printf( "avg = %g\n", avg );
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printf( "width = %d\n", in.width() );
VImage in = VImage::new_from_file( argv[1],
VImage::option()->
set( "access", VIPS_ACCESS_SEQUENTIAL_UNBUFFERED ) );
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VImage out = in.embed( 10, 10, 1000, 1000,
VImage::option()->
set( "extend", "background" )->
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set( "background", 128 ) );
out.write_to_file( argv[2] );
vips_shutdown();
return( 0 );
}
</programlisting>
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<para>
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Everything before <code>VImage in = VImage::..</code> is exactly
as the C API. This boilerplate gives the example a set of standard
command-line flags.
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</para>
<para>
This line is the C++ equivalent of vips_image_new_from_file(). It works
in the same way, the differences being:
<itemizedlist>
<listitem>
<para>
<code>VImage</code> lifetime is managed automatically, like a smart
pointer. You don't need to call g_object_unref().
</para>
</listitem>
<listitem>
<para>
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Instead of using varargs and a %NULL-terminated option list, this
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function takes an optional <code>VOption</code> pointer. This
gives a list of name / value pairs for optional arguments to the
function.
</para>
<para>
In this case we request unbuffered IO for the image, meaning, we
expect to do a single top-to-bottom scan of the image and do not
need it to be decompressed entirely. You can use the C enum name,
as is done in this case, or use a string and have the string
looked up. See below.
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</para>
<para>
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The function will delete the <code>VOption</code> pointer for
us when it's finished with it.
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</para>
</listitem>
<listitem>
<para>
Instead of returning %NULL on error, this constructor will
raise a <code>VError</code> exception.
</para>
</listitem>
</itemizedlist>
There are a series of similar constructors which parallel the other
constructors in the C API, see VImage::new_from_memory(),
VImage::new_from_buffer(), and VImage::new_matrix(). There's also
VImage::new_memory() and VImage::new_temp_file(), which when written to
with VImage::write() will create whole images on memory or on disc.
</para>
<para>
The next line finds the average pixel value, it's the equivalent of the
vips_avg() function. The differences from the C API are:
<itemizedlist>
<listitem>
<para>
VImage::avg() is a member function: the <code>this</code>
parameter is the first (the only, in this case) input image.
</para>
<para>
The function returns the first output parameter, in this case the
average pixel value. Other return values are via pointer arguments,
as in the C API.
</para>
<para>
Like VImage::new_from_file(), function raises the
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<code>VError</code> exception on error.
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</para>
<para>
Like VImage::new_from_file(), extra arguments are passed
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via an optional <code>VOption</code> parameter. There are none
in this case, so the function brackets can be left empty.
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</para>
</listitem>
</itemizedlist>
All other operations follow the same pattern, for example the C API call
vips_add():
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<programlisting language="C++">
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int vips_add( VipsImage *left, VipsImage *right, VipsImage **out, ... );
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</programlisting>
appears in C++ as:
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<programlisting language="C++">
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VImage VImage::add( VImage right, VOption *options = 0 );
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</programlisting>
</para>
<para>
The next line uses VImage::width() to get the image width in pixels.
There are similar functions paralleling vips_image_get_format() and
friends. Use VImage::set() to set metadata fields, VImage::get_int() and
c. to fetch metadata.
</para>
<para>
Next we reload the image. The VImage::avg() will have scanned the image
and reached the end of the file, we need to scan again for the next
operation. If we'd selected random access mode (the default) in the
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original VImage::new_from_file(), we would not need to reload.
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</para>
<para>
The next line runs vips_embed() with two optional parameters. The first
sets the value to an enum (here we use a string to set the value, it'll
be looked up in the list of possible enum values, or you can use the
symbols from the C API), the
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second sets the value to an <code>int</code>. The
<code>"background"</code>
parameter is actually a #VipsArrayDouble: if you pass an
<code>int</code> instead, it will be automatically converted to a
one-element array for you. You can pass a
<code>std::vector&lt;double&gt;</code> too: the utility function
VImage::to_vectorv() is a convenient way to make one.
</para>
<para>
Finally, VImage::write_to_file() will write the new image to the
filesystem. You can add a #VOption as a final parameter and set options
for the writer if you wish. Again, the operation will throw a #VError
exception on error. The other writers from the C API are also present:
you can write to a memory array, to a formatted image in memory, or to
another image.
</para>
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</refsect3>
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<refsect3 id="cpp-expansion">
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<title>Automatic constant expansion</title>
<para>
The C++ API will automatically turn constants into images in some cases.
For example, you can join two images together bandwise (the
bandwise join of two RGB images would be a six-band image) with:
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<programlisting language="C++">
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VImage rgb = ...;
VImage six_band = rgb.bandjoin( rgb );
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</programlisting>
You can also bandjoin a constant, for example:
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<programlisting language="C++">
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VImage rgb_with_alpha = rgb.bandjoin( 255 );
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</programlisting>
Will add an extra band to an image, with every element in the new band
having the value 255. This is quite a general feature. You can use a
constant in most places where you can use an image and it will be
converted. For example:
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<programlisting language="C++">
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VImage a = (a &lt; 128).ifthenelse( 128, a );
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</programlisting>
Will set every band element of <code>a</code> less than 128 to 128.
</para>
<para>
The C++ API includes the usual range of arithmetic operator overloads.
You can mix constants, vectors and images freely.
</para>
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</refsect3>
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<refsect3 id="cpp-enum">
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<title>Enum expansion</title>
<para>
VIPS operations which implement several functions with a controlling
enum, such as vips_math(), are expanded to a set of member functions
named after the enum. For example, the C function:
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<programlisting language="C++">
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int vips_math( VipsImage *in, VipsImage **out, VipsOperationMath math, ... );
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</programlisting>
where #VipsOperationMath has the member #VIPS_OPERATION_MATH_SIN, has a
C convenience function vips_sin():
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<programlisting language="C++">
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int vips_sin( VipsImage *in, VipsImage **out, ... );
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</programlisting>
and a C++ member function VImage::sin():
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<programlisting language="C++">
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VImage VImage::sin( VOption *options = 0 );
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</programlisting>
</para>
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</refsect3>
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<refsect3 id="cpp-extend">
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<title>Extending the C++ interface</title>
<para>
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The C++ interface comes in two parts. First, <code>VImage8.h</code>
defines a simple layer over #GObject for automatic reference counting,
then a generic way to call any vips8 operation with VImage::call(),
then a few convenience functions, then a set of overloads.
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</para>
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<para>
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The member function for each operation, for example VImage::add(), is
generated by a small Python program called <code>gen-operators.py</code>,
and its companion, <code>gen-operators-h.py</code> to generate the
headers. If you write a new VIPS operator, you'll need to rerun these
programs to make the new member function.
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</para>
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<para>
You can write the wrapper yourself, of course, they are very simple.
The one for VImage::add() looks like this:
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<programlisting language="C++">
VImage VImage::add(VImage right, VOption *options)
throw VError
{
VImage out;
call("add" ,
(options ? options : VImage::option()) ->
set("out", &amp;out) ->
set("left", *this) ->
set("right", right));
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return out;
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}
</programlisting>
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Where VImage::call() is the generic call-a-vips8-operation function.
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</para>
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</refsect3>
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</refentry>