use libvips to compose two images with alpha channels

composite2.c

/* compile with:
 *
 *      gcc -g -Wall composite2.c `pkg-config vips --cflags --libs`
 */

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

/* Composite images a and b.
 */
int
composite( VipsObject *context, VipsImage *a, VipsImage *b, VipsImage **out )
{
	/* These images will all be unreffed when context is unreffed.
	 */
	VipsImage **t = (VipsImage **) vips_object_local_array( context, 16 ); 

	/* Extract the alpha channels.
	 */
	if( vips_extract_band( a, &t[0], 3, NULL ) ||
		vips_extract_band( b, &t[1], 3, NULL ) )
		return( -1 );

	/* Extract the RGB bands.
	 */
	if( vips_extract_band( a, &t[2], 0, "n", 3, NULL ) ||
		vips_extract_band( b, &t[3], 0, "n", 3, NULL ) )
		return( -1 );

	/* Go to scrgb. This is a simple linear colourspace. 
	 */
	if( vips_colourspace( t[2], &t[4], VIPS_INTERPRETATION_scRGB, NULL ) ||
		vips_colourspace( t[3], &t[5], VIPS_INTERPRETATION_scRGB, NULL ) )
		return( -1 );

	/* Compute normalized input alpha channels.
	 */
	if( vips_linear1( t[0], &t[6], 1.0 / 255.0, 0.0, NULL ) ||
		vips_linear1( t[1], &t[7], 1.0 / 255.0, 0.0, NULL ) )
		return( -1 ); 

	// Compute normalized output alpha channel:
	//
	// References:
	// - http://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
	// - https://github.com/jcupitt/ruby-vips/issues/28#issuecomment-9014826
	//
	// out_a = src_a + dst_a * (1 - src_a)

	if( vips_linear1( t[6], &t[8], -1.0, 1.0, NULL ) ||
		vips_multiply( t[7], t[8], &t[9], NULL ) ||
		vips_add( t[6], t[9], &t[10], NULL ) )
		return( -1 ); 

	// Compute output RGB channels:
	//
	// Wikipedia:
	// out_rgb = (src_rgb * src_a + dst_rgb * dst_a * (1 - src_a)) / out_a
	//
	// `vips_ifthenelse` with `blend=TRUE`: http://bit.ly/1KoSsga
	// out = (cond / 255) * in1 + (1 - cond / 255) * in2
	//
	// Substitutions:
	//
	//     cond --> src_a
	//     in1 --> src_rgb
	//     in2 --> dst_rgb * dst_a (premultiplied destination RGB)
	//
	// Division by `out_a` is not required as `vips_ifthenelse` doesn’t require
	// normalized inputs.

	if( vips_multiply( t[5], t[7], &t[11], NULL ) ||
		vips_ifthenelse( t[0], t[4], t[11], &t[12], 
			"blend", TRUE, 
			NULL ) )
		return( -1 ); 

	// back to the start colourspace, unnormalise the alpha, reattach
	if( vips_colourspace( t[12], &t[13], t[1]->Type, NULL ) ||
		vips_linear1( t[10], &t[14], 255.0, 0.0, 
			"uchar", TRUE,
			NULL ) ||
		vips_bandjoin2( t[13], t[14], &t[15], NULL ) )
		return( -1 ); 

	/* Return a reference to the output image.
	 */
	g_object_ref( t[15] ); 
	*out = t[15];

	return( 0 );
}

int
main( int argc, char **argv )
{
  GOptionContext *context;
  GOptionGroup *main_group;
  GError *error = NULL;

  // Main
  char* programName = argv[0];
  char* srcPath = argv[1];
  char* dstPath = argv[2];
  char* outPath = argv[3];

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

  context = g_option_context_new( "src dst out" );

  main_group = g_option_group_new( NULL, NULL, NULL, NULL, NULL );
  g_option_context_set_main_group( context, main_group );
  vips_add_option_entries( main_group );

  if( !g_option_context_parse( context, &argc, &argv, &error ) ) {
    if( error ) {
      fprintf( stderr, "%s\n", error->message );
      g_error_free( error );
    }

    vips_error_exit( NULL );
  }

  if( argc != 4 )
    vips_error_exit( "usage: %s src dst out", programName );

  // Load input images:
  VipsImage *srcInput;
  VipsImage *dstInput;

  if( !(srcInput = vips_image_new_from_file( srcPath, NULL )) ||
    !(dstInput = vips_image_new_from_file( dstPath, NULL )) )
    vips_error_exit( NULL );

  // the controlling object for this processing: all temp objects get unreffed
  // when this is unreffed
  VipsObject *handle = VIPS_OBJECT( vips_image_new() ); 

  VipsImage *out;

  if( composite( handle, srcInput, dstInput, &out ) ) {
    g_object_unref( handle );
    vips_error_exit( NULL );
  }
  g_object_unref( handle );

  // Output
  if( vips_image_write_to_file( out, outPath, NULL ) )
    vips_error_exit( NULL );

  g_object_unref( out ); 
  g_object_unref( srcInput ); 
  g_object_unref( dstInput ); 

  return( 0 );
}