quantise: optional thresholding of alpha channel (#2425)

Moving this logic from gifsave to quantise improves
the performance of writing RGBA GIFs by ~15%

libvips/foreign/cgifsave.c

@@ -86,7 +86,7 @@ vips_foreign_save_cgif_build( VipsObject *object )
 	VipsForeignSave *save = (VipsForeignSave *) object;
 	VipsForeignSaveCgif *cgif = (VipsForeignSaveCgif *) object;
 	VipsImage **t = (VipsImage **) 
-		vips_object_local_array( VIPS_OBJECT( cgif ), 6 );
+		vips_object_local_array( VIPS_OBJECT( cgif ), 2 );
 
 	int rgb;
 	int rgba;
@@ -117,31 +117,17 @@ vips_foreign_save_cgif_build( VipsObject *object )
 	if( vips_image_get_typeof( save->ready, "loop" ) )
 		vips_image_get_int( save->ready, "loop", &loop );
 
-	/* Threshold alpha, if any, to fully transparent or opaque
+	/* Generate indexed image (t[0]) and palette (t[1])
 	 */
-	if( vips_image_hasalpha( save->ready ) ) {
-		if( vips_extract_band( save->ready, &t[0], 0, "n", 3, NULL ) ||
-			vips_extract_band( save->ready, &t[1], 3, NULL ) ||
-			vips_moreeq_const1( t[1], &t[2], 128, NULL ) ||
-			vips_bandjoin2( t[0], t[2], &t[3], NULL ) ) 
-			return( -1 );
-
-		VIPS_UNREF( save->ready );
-		save->ready = t[3];
-		g_object_ref( save->ready );
-	}
-
-	/* Generate indexed image (t[4]) and palette (t[5])
-	 */
-	if( vips__quantise_image( save->ready, &t[4], &t[5],
-		255, 100, cgif->dither, cgif->effort ) )
+	if( vips__quantise_image( save->ready, &t[0], &t[1],
+		255, 100, cgif->dither, cgif->effort, TRUE ) )
 		return( -1 );
 
 	/* Convert palette to RGB
 	 */
-	paletteRgba = (uint8_t *) VIPS_IMAGE_ADDR( t[5], 0, 0 );
-	paletteRgb = g_malloc0( t[5]->Xsize * 3 );
-	for( rgb = 0, rgba = 0; rgb < t[5]->Xsize * 3; rgb += 3 ) {
+	paletteRgba = (uint8_t *) VIPS_IMAGE_ADDR( t[1], 0, 0 );
+	paletteRgb = g_malloc0( t[1]->Xsize * 3 );
+	for( rgb = 0, rgba = 0; rgb < t[1]->Xsize * 3; rgb += 3 ) {
 		paletteRgb[rgb] = paletteRgba[rgba];
 		paletteRgb[rgb + 1] = paletteRgba[rgba + 1];
 		paletteRgb[rgb + 2] = paletteRgba[rgba + 2];
@@ -156,10 +142,10 @@ vips_foreign_save_cgif_build( VipsObject *object )
 	/* Initiialise cgif
 	 */
 	memset( &cgif_config, 0, sizeof( CGIF_Config ) );
-	cgif_config.width = t[4]->Xsize;
+	cgif_config.width = t[0]->Xsize;
 	cgif_config.height = page_height;
 	cgif_config.pGlobalPalette = paletteRgb;
-	cgif_config.numGlobalPaletteEntries = t[5]->Xsize;
+	cgif_config.numGlobalPaletteEntries = t[1]->Xsize;
 	cgif_config.numLoops = loop;
 	cgif_config.attrFlags = CGIF_ATTR_IS_ANIMATED;
 	if( has_transparency ) 
@@ -171,12 +157,12 @@ vips_foreign_save_cgif_build( VipsObject *object )
 
 	/* Add each vips page as a cgif frame
 	 */
-	for( top = 0; top < t[4]->Ysize; top += page_height ) {
+	for( top = 0; top < t[0]->Ysize; top += page_height ) {
 		int page_index = top / page_height;
 
 		memset( &cgif_frame_config, 0, sizeof( CGIF_FrameConfig ) );
 		cgif_frame_config.pImageData = (uint8_t *)
-			VIPS_IMAGE_ADDR( t[4], 0, top );
+			VIPS_IMAGE_ADDR( t[0], 0, top );
 		if( delay &&
 			page_index < delay_length )
 			cgif_frame_config.delay =

libvips/foreign/pforeign.h

@@ -219,7 +219,8 @@ int vips__webp_write_target( VipsImage *image, VipsTarget *target,
 
 int vips__quantise_image( VipsImage *in, 
 	VipsImage **index_out, VipsImage **palette_out,
-	int colours, int Q, double dither, int effort );
+	int colours, int Q, double dither, int effort,
+	gboolean threshold_alpha );
 
 extern const char *vips_foreign_nifti_suffs[];
 

libvips/foreign/quantise.c

@@ -106,13 +106,15 @@ vips__quantise_new( VipsImage *in,
 int
 vips__quantise_image( VipsImage *in, 
 	VipsImage **index_out, VipsImage **palette_out,
-	int colours, int Q, double dither, int effort )
+	int colours, int Q, double dither, int effort,
+	gboolean threshold_alpha )
 {
 	Quantise *quantise;
 	VipsImage *index;
 	VipsImage *palette;
 	const liq_palette *lp;
 	int i;
+	gboolean added_alpha = FALSE;
 
 	quantise = vips__quantise_new( in, index_out, palette_out, 
 		colours, Q, dither, effort );
@@ -135,6 +137,7 @@ vips__quantise_image( VipsImage *in,
 			vips__quantise_free( quantise ); 
 			return( -1 );
 		}
+		added_alpha = TRUE;
 		in = quantise->t[1];
 	}
 
@@ -144,6 +147,19 @@ vips__quantise_image( VipsImage *in,
 	}
 	in = quantise->t[2];
 
+	/* Threshold alpha channel.
+	 */
+	if( threshold_alpha && !added_alpha ) {
+		VipsPel * restrict p = VIPS_IMAGE_ADDR( in, 0, 0 );
+		const guint64 n_pels = VIPS_IMAGE_N_PELS( in );
+		guint64 i;
+
+		for( i = 0; i < n_pels; i++ ) {
+			p[3] = p[3] > 128 ? 255 : 0;
+			p += 4;
+		}
+	}
+
 	quantise->attr = liq_attr_create();
 	liq_set_max_colors( quantise->attr, colours );
 	liq_set_quality( quantise->attr, 0, Q );
@@ -215,7 +231,8 @@ vips__quantise_image( VipsImage *in,
 int
 vips__quantise_image( VipsImage *in, 
 	VipsImage **index_out, VipsImage **palette_out,
-	int colours, int Q, double dither )
+	int colours, int Q, double dither, int effort,
+	gboolean threshold_alpha )
 {
   vips_error( "vips__quantise_image", 
       "%s", _( "libvips not built with quantisation support" ) ); 

libvips/foreign/vipspng.c

@@ -1190,7 +1190,7 @@ write_vips( Write *write,
 		int trans_count;
 
 		if( vips__quantise_image( in, &im_index, &im_palette, 
-			1 << bitdepth, Q, dither, effort ) )
+			1 << bitdepth, Q, dither, effort, FALSE ) )
 			return( -1 );
 
 		palette_count = im_palette->Xsize;