combine two alpha channels with vips

trans.cpp

/* compile with
 *
 * g++ trans.cpp `pkg-config vips-cpp --cflags --libs`
 */

#include <vips/vips8>

using namespace vips;

/* Return the image alpha maximum. Useful for combining alpha bands. scRGB
 * images are 0 - 1 for image data, but the alpha is 0 - 255.
 */
int
image_alpha_max( VipsInterpretation interpretation )
{
	if( interpretation == VIPS_INTERPRETATION_RGB16 ||
		interpretation == VIPS_INTERPRETATION_GREY16 )
		return( 65535 );
	else
		return( 255 );
}

/* Split an image to pixels plus an optional alpha. An image can be mono, rgb
 * or cmyk, each with an optional alpha as the final band.
 */
VImage 
image_split( VImage in, VImage &alpha, bool &has_alpha )
{
	VImage image;

	if( in.bands() == 2 || 
		(in.bands() == 4 && 
		 in.interpretation() != VIPS_INTERPRETATION_CMYK ) ||
		(in.bands() == 5 && 
		 in.interpretation() == VIPS_INTERPRETATION_CMYK ) ) {
		has_alpha = TRUE;
		image = in.extract_band( 0, 
			VImage::option()->set( "n", in.bands() - 1 ) );
		alpha = in.extract_band( in.bands() - 1,
			VImage::option()->set( "n", 1 ) );
	}
	else {
		has_alpha = FALSE;
		image = in;
	}

	return( image );
}

int
main( int argc, char **argv )
{
	if( VIPS_INIT( argv[0] ) )
		vips_error_exit( NULL ); 

	// load the main image ... it can optionally have an alpha
	VImage in = VImage::new_from_file( argv[1] );

	VImage image_alpha;
	bool image_has_alpha;
	VImage image = image_split( in, image_alpha, image_has_alpha );

	// load the mask image
	VImage in2 = VImage::new_from_file( argv[2] );
	VImage mask_alpha;
	bool mask_has_alpha;
	VImage mask = image_split( in2, mask_alpha, mask_has_alpha );

	// we use the mask alpha, or if the mask only has one band, use that
	if( mask_has_alpha )
		mask = mask_alpha;
	else if( mask.bands() > 1 )
		vips_error_exit( "mask has no alpha or more than one band" );

	// the range of the mask and the image need to match .. one could be
	// 16-bit, one 8-bit
	int image_max = image_alpha_max( image.interpretation() );
	int mask_max = image_alpha_max( mask.interpretation() ); 

	if( image_has_alpha ) 
		// combine the new mask and the existing alpha ... there are 
		// many ways of doing this, mult is the simplest
		mask = image_max * 
			((mask / mask_max) * (image_alpha / image_max));
	else if( image_max != mask_max ) 
		// adjust the range of the mask to match the image
		mask = image_max * (mask / mask_max);

	// append the mask to the image data ... the mask might be float now,
	// we must cast the format down to match the image data
	VImage out = image.bandjoin( mask.cast( image.format() ) );
	
	out.write_to_file( argv[3] );

	return( 0 );
}