VIPS from C: libvips Reference Manual

## VIPS from C Using VIPS — How to use the VIPS library from C |

#### Introduction

VIPS comes with a convenient, high-level C API. You should read the API docs for full details, but this section will try to give a brief overview.

#### Library startup

When your program starts, use `VIPS_INIT()` to start up the VIPS library. You should pass it the name of your program, usually `argv[0]`. If you call `vips_shutdown()` just before you exit, libvips will attempt to free all resources. This can help leak checking, but is not required.

`VIPS_INIT()` is a macro to let it check that the libvips library you have linked to matches the libvips headers you included.

You can add the VIPS flags to your `GObject` command-line processing with `vips_add_option_entries()`.

#### The VipsImage class

The basic data object is the VipsImage. You can create an image from a file on disc or from an area of memory, either as a C-style array, or as a formatted object, like JPEG. See `vips_image_new_from_file()` and friends. Loading an image is fast: VIPS read just enough of the image to be able to get the various properties, such as width, but no decoding occurs until pixel values are really needed.

Once you have an image, you can get properties from it in the usual way. You can use projection functions, like `vips_image_get_width()` or `g_object_get()`, to get `GObject` properties. All VIPS objects are immutable, meaning you can only get properties, you can't set them. See VIPS Header to read about image properties.

#### Reference counting

VIPS is based on the `GObject` library and is therefore reference counted. `vips_image_new_from_file()` returns an object with a count of 1. When you are done with an image, use `g_object_unref()` to dispose of it. If you pass an image to an operation and that operation needs to keep a copy of the image, it will ref it. So you can unref an image as soon as you no longer need it, you don't need to hang on to it in case anyone else is still using it.

See VipsOperation for more detail on VIPS reference counting conventions. See the Reference pools section below for a way to automate reference counting in C.

#### VIPS operations

Use things like `vips_embed()` to manipulate your images. You use it from C like this:

    const char *filename;
    VipsImage *in = vips_image_new_from_file (filename, NULL);
    const int x = 10;
    const int y = 10;
    const int width = 1000;
    const int height = 1000;
    VipsImage *out;

    if (vips_embed (in, &out, x, y, width, height, NULL))
      error_handling();

Now `out` will hold a reference to a 1000 by 1000 pixel image, with `in` pasted 10 right and 10 down from the top left-hand corner. The remainder of the image will be black. If `in` is too large, it will be clipped at the image edges.

Operations can take optional arguments. You give these as a set of NULL-terminated name-value pairs at the end of the call. For example, you can write:

    if (vips_embed (in, &out, x, y, width, height,
      "extend", VIPS_EXTEND_COPY,
      NULL))
      error_handling();

And now the new edge pixels, which were black, will be filled with a copy of the edge pixels of `in`. Operation options are listed at the top of each operation's entry in the docs. Alternatively, the vips program is handy for getting a summary of an operation's parameters. For example:

    $ vips embed
    embed an image in a larger image
    usage:
       embed in out x y width height
    where:
       in           - Input image, input VipsImage
       out          - Output image, output VipsImage
       x            - Left edge of input in output, input gint
                          default: 0
                          min: -1000000000, max: 1000000000
       y            - Top edge of input in output, input gint
                          default: 0
                          min: -1000000000, max: 1000000000
       width        - Image width in pixels, input gint
                          default: 1
                          min: 1, max: 1000000000
       height       - Image height in pixels, input gint
                          default: 1
                          min: 1, max: 1000000000
    optional arguments:
       extend       - How to generate the extra pixels, input VipsExtend
                          default: black
                          allowed: black, copy, repeat, mirror, white, background
       background   - Colour for background pixels, input VipsArrayDouble
    operation flags: sequential-unbuffered

See VipsOperation for more information on running operations on images.

The API docs have a handy table of all vips operations, if you want to find out how to do something, try searching that.

When you are done, you can write the final image to a disc file, to a formatted memory buffer, or to C-style memory array. See `vips_image_write_to_file()` and friends.

#### Getting pixels

Use VipsRegion to read pixels out of images. You can use `VIPS_IMAGE_ADDR()` as well, but this can need a large amount of memory to work. See extending for an introduction to writing your own operations.

#### Error handling

VIPS keeps a log of error message, see VIPS Error to find out how to get and clear the error log.

#### Example

On *nix systems, you can compile the example code with something like:

    $ gcc -g -Wall myprog.c `pkg-config vips --cflags --libs`

On Windows, you'll need to set the compiler flags by hand, perhaps:

    x86_64-w64-mingw32-gcc-win32 -mms-bitfields \
      -Ic:/vips-8.6/include \
      -Ic:/vips-8.6/include/glib-2.0 \
      -Ic:/vips-8.6/lib/glib-2.0/include \
      myprog.c \
      -Lc:/vips-8.6/lib \
      -lvips -lz -ljpeg -lstdc++ -lxml2 -lfftw3 -lm -lMagickWand -llcms2 \
      -lopenslide -lcfitsio -lpangoft2-1.0 -ltiff -lpng14 -lexif \
      -lMagickCore -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 \
      -lgmodule-2.0 -lgthread-2.0 -lglib-2.0 -lintl \
      -o myprog.exe

Example 1. VIPS from C example

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    #include <stdio.h>
    #include <vips/vips.h>

    int
    main( int argc, char **argv )
    {
      VipsImage *in;
      double mean;
      VipsImage *out;

      if( VIPS_INIT( argv[0] ) )
        vips_error_exit( NULL );

      if( argc != 3 )
        vips_error_exit( "usage: %s infile outfile", argv[0] );

      if( !(in = vips_image_new_from_file( argv[1], NULL )) )
        vips_error_exit( NULL );

      printf( "image width = %d\n", vips_image_get_width( in ) );

      if( vips_avg( in, &mean, NULL ) )
        vips_error_exit( NULL );

      printf( "mean pixel value = %g\n", mean );

      if( vips_invert( in, &out, NULL ) )
        vips_error_exit( NULL );

      g_object_unref( in );

      if( vips_image_write_to_file( out, argv[2], NULL ) )
        vips_error_exit( NULL );

      g_object_unref( out );

      return( 0 );
    }

#### Reference pools

libvips has a simple system to automate at least some reference counting issues. Reference pools are arrays of object pointers which will be released automatically when some other object is finalized.

The code below crops a many-page image (perhaps a GIF or PDF). It splits the image into separate pages, crops each page, reassembles the cropped areas, and saves again. It creates a `context` object representing the state of processing, and `crop_animation` allocates two reference pools off that using `vips_object_local_array`, one to hold the cropped frames, and one to assemble and copy the result.

All unreffing is handled by `main`, and it doesn't need to know anything about `crop_animation`.

Example 2. Reference pool example

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    #include <vips/vips.h>

    static int
    crop_animation( VipsObject *context, VipsImage *image, VipsImage **out,
      int left, int top, int width, int height )
    {
      int page_height = vips_image_get_page_height( image );
      int n_pages = image->Ysize / page_height;
      VipsImage **page = (VipsImage **) vips_object_local_array( context, n_pages );
      VipsImage **copy = (VipsImage **) vips_object_local_array( context, 1 );

      int i;

      /* Split the image into cropped frames.
       */
      for( i = 0; i < n_pages; i++ )
        if( vips_crop( image, &page[i],
          left, page_height * i + top, width, height, NULL ) )
          return( -1 );

      /* Reassemble the frames and set the page height. You must copy before
       * modifying metadata.
       */
      if( vips_arrayjoin( page, &copy[0], n_pages, "across", 1, NULL ) ||
        vips_copy( copy[0], out, NULL ) )
        return( -1 );
      vips_image_set_int( *out, "page-height", height );

      return( 0 );
    }

    int
    main( int argc, char **argv )
    {
      VipsImage *image;
      VipsObject *context;
      VipsImage *x;

      if( VIPS_INIT( NULL ) )
        vips_error_exit( NULL );

      if( !(image = vips_image_new_from_file( argv[1],
        "access", VIPS_ACCESS_SEQUENTIAL,
        NULL )) )
        vips_error_exit( NULL );

      context = VIPS_OBJECT( vips_image_new() );
      if( crop_animation( context, image, &x, 10, 10, 500, 500 ) ) {
        g_object_unref( image );
        g_object_unref( context );
        vips_error_exit( NULL );
      }
      g_object_unref( image );
      g_object_unref( context );
      image = x;

      if( vips_image_write_to_file( image, argv[2], NULL ) ) {
        g_object_unref( image );
        vips_error_exit( NULL );
      }

      g_object_unref( image );

      return( 0 );
    }