works!

but only OVER mode at the moment, and it's not especially quick

libvips/conversion/Makefile.am

@@ -1,6 +1,7 @@
 noinst_LTLIBRARIES = libconversion.la
 
 libconversion_la_SOURCES = \
+	composite.c \
 	smartcrop.c \
 	conversion.c \
 	pconversion.h \

libvips/conversion/composite.c

@@ -49,6 +49,16 @@
 #include <vips/internal.h>
 #include <vips/debug.h>
 
+#include "pconversion.h"
+
+/* Maximum number of input images -- why not?
+ */
+#define MAX_INPUT_IMAGES (64)
+
+/* Maximum number of image bands. 
+ */
+#define MAX_BANDS (64)
+
 /**
  * VipsBlendMode:
  * VIPS_BLEND_MODE_OVER: 
@@ -61,6 +71,13 @@
  * @gasi's composite example https://gist.github.com/jcupitt/abacc012e2991f332e8b
  *
  * https://en.wikipedia.org/wiki/Alpha_compositing
+ *
+ * composite -compose over PNG_transparency_demonstration_1.png \
+ * 	Opera-icon-high-res.png x.png
+ *
+ * vips composite "PNG_transparency_demonstration_1.png \
+ * 	Opera-icon-high-res.png" x.png 0
+ *
  */
 
 typedef struct _VipsComposite {
@@ -79,44 +96,112 @@ typedef struct _VipsComposite {
 	 */
 	VipsInterpretation compositing_space;
 
+	/* Set if the input images have already been premultiplied.
+	 */
+	gboolean premultiplied;
+
 	/* The number of inputs. This can be less than the number of images in
 	 * @in.
 	 */
 	int n;
+
+	/* The number of bands we are blending.
+	 */
+	int bands;
+
 } VipsComposite;
 
 typedef VipsConversionClass VipsCompositeClass;
 
 G_DEFINE_TYPE( VipsComposite, vips_composite, VIPS_TYPE_CONVERSION );
 
-#define COMPOSE_INT( TYPE ) { \
-	TYPE
+#define BLEND( OUT, AOUT, SRC1, A1, SRC2, A2 ) { \
+	if( AOUT == 0 ) \
+		OUT = 0; \
+	else \
+		switch( mode ) { \
+		case VIPS_BLEND_MODE_OVER: \
+			OUT = (SRC1 + SRC2 * (1 - A1)) / AOUT; \
+			break; \
+		\
+		default: \
+			 g_assert_not_reached(); \
+		} \
 }
 
+#define COMBINE( TYPE ) { \
+	TYPE **tp = (TYPE **) p; \
+	TYPE *tq = (TYPE *) q; \
+	\
+	for( x = 0; x < width; x++ ) { \
+		int o = x * (bands + 1); \
+		\
+		for( b = 0; b < bands; b++ ) \
+			pixel[b] = tp[n - 1][o + b]; \
+		alpha = tp[n - 1][o + bands] / 255.0; \
+		\
+		for( i = n - 2; i >= 0; i-- ) { \
+			TYPE *src1 = tp[i] + o; \
+			double a1 = src1[bands] / 255.0; \
+			double aout = a1 + alpha * (1 - a1); \
+			VipsBlendMode mode = ((VipsBlendMode *) \
+				composite->mode->area.data)[i]; \
+			\
+			for( b = 0; b < bands; b++ ) \
+				BLEND( pixel[b], aout, \
+					src1[b], a1, pixel[b], alpha ); \
+			\
+			alpha = aout; \
+		} \
+		\
+		for( b = 0; b < bands; b++ ) \
+			tq[o + b] = pixel[b]; \
+		tq[o + bands] = alpha * 255; \
+	} \
+}
 
 static void
 vips_composite_process_line( VipsComposite *composite, VipsBandFormat format,
 	VipsPel *q, VipsPel **p, int width )
 {
 	int n = composite->n;
-	int i;
+	int bands = composite->bands;
+
+	double pixel[MAX_BANDS];
+	double alpha;
+	int x, i, b;
 
 	switch( format ) {
 	case VIPS_FORMAT_UCHAR:
-		COMPOSE_INT( unsigned char );
+		COMBINE( unsigned char );
 		break;
 
 	case VIPS_FORMAT_CHAR:
-		COMPOSE_INT( unsigned char );
+		COMBINE( signed char );
 		break;
 
 	case VIPS_FORMAT_USHORT:
+		COMBINE( unsigned short );
+		break;
+
 	case VIPS_FORMAT_SHORT:
+		COMBINE( signed short );
+		break;
+
 	case VIPS_FORMAT_UINT:
+		COMBINE( unsigned int );
+		break;
+
 	case VIPS_FORMAT_INT:
+		COMBINE( signed int );
+		break;
+
 	case VIPS_FORMAT_FLOAT:
+		COMBINE( float );
+		break;
+
 	case VIPS_FORMAT_DOUBLE:
-		g_assert_not_reached();
+		COMBINE( double );
 		break;
 
 	case VIPS_FORMAT_COMPLEX:
@@ -138,7 +223,7 @@ vips_composite_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop
 
 	if( vips_reorder_prepare_many( or->im, ir, r ) )
 		return( -1 );
-	for( i = 0; i < bandary->n; i++ ) 
+	for( i = 0; i < composite->n; i++ ) 
 		p[i] = VIPS_REGION_ADDR( ir[i], r->left, r->top );
 	p[i] = NULL;
 	q = VIPS_REGION_ADDR( or, r->left, r->top );
@@ -149,7 +234,7 @@ vips_composite_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop
 		vips_composite_process_line( composite, ir[0]->im->BandFmt, 
 			q, p, r->width );
 
-		for( i = 0; i < bandary->n; i++ )
+		for( i = 0; i < composite->n; i++ )
 			p[i] += VIPS_REGION_LSKIP( ir[i] );
 		q += VIPS_REGION_LSKIP( or );
 	}
@@ -164,8 +249,8 @@ vips_composite_build( VipsObject *object )
 {
 	VipsObjectClass *class = VIPS_OBJECT_GET_CLASS( object );
 	VipsConversion *conversion = VIPS_CONVERSION( object );
-	VipsBandary *bandary = (VipsBandary *) object;
 	VipsComposite *composite = (VipsComposite *) object;
+	VipsImage **t = (VipsImage **) vips_object_local_array( object, 5 );
 
 	int i;
 	VipsImage **in;
@@ -177,13 +262,49 @@ vips_composite_build( VipsObject *object )
 	VipsInterpretation compositing_space;
 	int max_bands;
 	VipsInterpretation max_interpretation;
+	VipsBlendMode *mode;
+	VipsImage *out;
 
 	if( VIPS_OBJECT_CLASS( vips_composite_parent_class )->build( object ) )
 		return( -1 );
 
-	composite->n = composite->in->area->n;
+	composite->n = composite->in->area.n;
+
+	if( composite->n <= 0 ) { 
+		vips_error( class->nickname, "%s", _( "no input images" ) ); 
+		return( -1 );
+	}
+	if( composite->mode->area.n != composite->n - 1 ) {
+		vips_error( class->nickname, 
+			_( "for %d input images there must be %d blend modes" ),
+			composite->n, composite->n - 1 ); 
+		return( -1 );
+	}
+	mode = (VipsBlendMode *) composite->mode->area.data;
+	for( i = 0; i < composite->n - 1; i++ ) {
+		if( mode[i] < 0 || 
+			mode[i] >= VIPS_BLEND_MODE_LAST ) {
+			vips_error( class->nickname, 
+				_( "blend mode index %d (%d) invalid" ),
+				i, mode[i] ); 
+			return( -1 );
+		}
+	}
+
+	in = (VipsImage **) composite->in->area.data;
+
+	/* Premultiply alpha, if it hasn't been.
+	 */
+	if( !composite->premultiplied ) { 
+		VipsImage **premultiply = (VipsImage **) 
+			vips_object_local_array( object, composite->n );
+
+		for( i = 0; i < composite->n; i++ )
+			if( vips_premultiply( in[i], &premultiply[i], NULL ) )
+				return( -1 );
+		in = premultiply;
+	}
 
-	in = (VipsImage **) composite->in->data;
 	decode = (VipsImage **) vips_object_local_array( object, composite->n );
 	for( i = 0; i < composite->n; i++ )
 		if( vips_image_decode( in[i], &decode[i] ) )
@@ -197,7 +318,7 @@ vips_composite_build( VipsObject *object )
 		if( !vips_image_hasalpha( in[i] ) ) { 
 			VipsImage *x;
 
-			if( vips_bandjoin( in[i], &x, 255 ) )
+			if( vips_bandjoin_const1( in[i], &x, 255, NULL ) )
 				return( -1 );
 			g_object_unref( in[i] );
 			in[i] = x;
@@ -205,6 +326,12 @@ vips_composite_build( VipsObject *object )
 			break;
 		}
 
+	if( composite->n > MAX_INPUT_IMAGES ) {
+		vips_error( class->nickname, 
+			"%s", _( "too many input images" ) ); 
+		return( -1 );
+	}
+
 	/* Transform to compositing space. It defaults to sRGB or B_W. 
 	 */
 	if( !vips_object_argument_isset( object, "compositing_space" ) ) {
@@ -249,15 +376,37 @@ vips_composite_build( VipsObject *object )
 			return( -1 );
 		}
 
-	if( vips_image_pipeline_array( conversion->out, 
+	if( in[0]->Bands > MAX_BANDS ) {
+		vips_error( class->nickname, 
+			"%s", _( "too many input bands" ) ); 
+		return( -1 );
+	}
+
+	composite->bands = in[0]->Bands - 1;
+
+	t[0] = vips_image_new();
+	out = t[0];
+
+	if( vips_image_pipeline_array( out, 
 		VIPS_DEMAND_STYLE_THINSTRIP, in ) )
 		return( -1 );
 
-	if( vips_image_generate( conversion->out,
+	if( vips_image_generate( out,
 		vips_start_many, vips_composite_gen, vips_stop_many, 
 		in, composite ) )
 		return( -1 );
 
+	/* And unpremultiply alpha, if we need to.
+	 */
+	if( !composite->premultiplied ) { 
+		if( vips_unpremultiply( out, &t[1], NULL ) )
+			return( -1 );
+		out = t[1];
+	}
+
+	if( vips_image_write( out, conversion->out ) )
+		return( -1 ); 
+
 	return( 0 );
 }
 
@@ -286,7 +435,7 @@ vips_composite_class_init( VipsCompositeClass *class )
 		G_STRUCT_OFFSET( VipsComposite, in ),
 		VIPS_TYPE_ARRAY_IMAGE );
 
-	VIPS_ARG_BOXED( class, "mode", 1, 
+	VIPS_ARG_BOXED( class, "mode", 3, 
 		_( "Blend modes" ), 
 		_( "Array of VipsBlendMode to join with" ),
 		VIPS_ARGUMENT_REQUIRED_INPUT,
@@ -294,12 +443,19 @@ vips_composite_class_init( VipsCompositeClass *class )
 		VIPS_TYPE_ARRAY_INT );
 
 	VIPS_ARG_ENUM( class, "compositing_space", 10, 
-		_( "Interpretation" ), 
-		_( "Pixel interpretation" ),
+		_( "Compositing space" ), 
+		_( "Composite images in this colour space" ),
 		VIPS_ARGUMENT_OPTIONAL_INPUT,
 		G_STRUCT_OFFSET( VipsComposite, compositing_space ),
 		VIPS_TYPE_INTERPRETATION, VIPS_INTERPRETATION_sRGB ); 
 
+	VIPS_ARG_BOOL( class, "premultiplied", 11, 
+		_( "Premultiplied" ), 
+		_( "Images have premultiplied alpha" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsComposite, premultiplied ),
+		FALSE ); 
+
 }
 
 static void
@@ -309,14 +465,18 @@ vips_composite_init( VipsComposite *composite )
 }
 
 static int
-vips_compositev( VipsImage **in, VipsImage **out, int n, va_list ap )
+vips_compositev( VipsImage **in, VipsImage **out, int n, int *mode, va_list ap )
 {
-	VipsArrayImage *array; 
+	VipsArrayImage *image_array; 
+	VipsArrayInt *mode_array; 
 	int result;
 
-	array = vips_array_image_new( in, n ); 
-	result = vips_call_split( "composite", ap, array, out );
-	vips_area_unref( VIPS_AREA( array ) );
+	image_array = vips_array_image_new( in, n ); 
+	mode_array = vips_array_int_new( mode, n - 1 ); 
+	result = vips_call_split( "composite", ap, 
+		image_array, out, mode_array );
+	vips_area_unref( VIPS_AREA( image_array ) );
+	vips_area_unref( VIPS_AREA( mode_array ) );
 
 	return( result );
 }
@@ -326,34 +486,23 @@ vips_compositev( VipsImage **in, VipsImage **out, int n, va_list ap )
  * @in: (array length=n) (transfer none): array of input images
  * @out: output image
  * @n: number of input images
+ * @mode: array of (@n - 1) #VipsBlendMode
  * @...: %NULL-terminated list of optional named arguments
  *
- * Join a set of images together, bandwise. 
- *
- * If the images
- * have n and m bands, then the output image will have n + m
- * bands, with the first n coming from the first image and the last m
- * from the second. 
- *
- * If the images differ in size, the smaller images are enlarged to match the
- * larger by adding zero pixels along the bottom and right.
- *
- * The input images are cast up to the smallest common type (see table 
- * Smallest common format in 
- * <link linkend="libvips-arithmetic">arithmetic</link>).
+ * Composite an array of images together. 
  *
  * See also: vips_insert().
  *
  * Returns: 0 on success, -1 on error
  */
 int
-vips_composite( VipsImage **in, VipsImage **out, int n, ... )
+vips_composite( VipsImage **in, VipsImage **out, int n, int *mode, ... )
 {
 	va_list ap;
 	int result;
 
-	va_start( ap, n );
-	result = vips_compositev( in, out, n, ap );
+	va_start( ap, mode );
+	result = vips_compositev( in, out, n, mode, ap );
 	va_end( ap );
 
 	return( result );
@@ -364,24 +513,28 @@ vips_composite( VipsImage **in, VipsImage **out, int n, ... )
  * @in1: first input image
  * @in2: second input image
  * @out: output image
+ * @mode: composite with this blend mode
  * @...: %NULL-terminated list of optional named arguments
  *
- * Join a pair of images together, bandwise. See vips_composite().
+ * Composite a pair of images together. See vips_composite().
  *
  * Returns: 0 on success, -1 on error
  */
 int
-vips_composite2( VipsImage *in1, VipsImage *in2, VipsImage **out, ... )
+vips_composite2( VipsImage *in1, VipsImage *in2, VipsImage **out, 
+	VipsBlendMode mode1, ... )
 {
 	va_list ap;
 	int result;
 	VipsImage *in[2];
+	int mode[1];
 
 	in[0] = in1;
 	in[1] = in2;
+	mode[0] = mode1;
 
-	va_start( ap, out );
-	result = vips_compositev( in, out, 2, ap );
+	va_start( ap, mode1 );
+	result = vips_compositev( in, out, 2, mode, ap );
 	va_end( ap );
 
 	return( result );

libvips/conversion/conversion.c

@@ -274,6 +274,7 @@ vips_conversion_operation_init( void )
 	extern GType vips_xyz_get_type( void ); 
 	extern GType vips_falsecolour_get_type( void ); 
 	extern GType vips_gamma_get_type( void ); 
+	extern GType vips_composite_get_type( void ); 
 
 	vips_copy_get_type();
 	vips_tile_cache_get_type(); 
@@ -321,4 +322,5 @@ vips_conversion_operation_init( void )
 	vips_xyz_get_type(); 
 	vips_falsecolour_get_type(); 
 	vips_gamma_get_type(); 
+	vips_composite_get_type(); 
 }

libvips/include/vips/conversion.h

@@ -89,6 +89,11 @@ typedef enum {
 	VIPS_INTERESTING_LAST
 } VipsInteresting;
 
+typedef enum {
+	VIPS_BLEND_MODE_OVER,
+	VIPS_BLEND_MODE_LAST
+} VipsBlendMode;
+
 int vips_copy( VipsImage *in, VipsImage **out, ... )
 	__attribute__((sentinel));
 int vips_tilecache( VipsImage *in, VipsImage **out, ... )

libvips/include/vips/enumtypes.h

@@ -44,6 +44,8 @@ GType vips_angle45_get_type (void) G_GNUC_CONST;
 #define VIPS_TYPE_ANGLE45 (vips_angle45_get_type())
 GType vips_interesting_get_type (void) G_GNUC_CONST;
 #define VIPS_TYPE_INTERESTING (vips_interesting_get_type())
+GType vips_blend_mode_get_type (void) G_GNUC_CONST;
+#define VIPS_TYPE_BLEND_MODE (vips_blend_mode_get_type())
 /* enumerations from "../../../libvips/include/vips/convolution.h" */
 GType vips_combine_get_type (void) G_GNUC_CONST;
 #define VIPS_TYPE_COMBINE (vips_combine_get_type())

libvips/iofuncs/enumtypes.c

@@ -348,6 +348,23 @@ vips_interesting_get_type( void )
 
 	return( etype );
 }
+GType
+vips_blend_mode_get_type( void )
+{
+	static GType etype = 0;
+
+	if( etype == 0 ) {
+		static const GEnumValue values[] = {
+			{VIPS_BLEND_MODE_OVER, "VIPS_BLEND_MODE_OVER", "over"},
+			{VIPS_BLEND_MODE_LAST, "VIPS_BLEND_MODE_LAST", "last"},
+			{0, NULL, NULL}
+		};
+		
+		etype = g_enum_register_static( "VipsBlendMode", values );
+	}
+
+	return( etype );
+}
 /* enumerations from "../../libvips/include/vips/convolution.h" */
 GType
 vips_combine_get_type( void )