Add alpha_q(uality) webpsave option, fix Q init

This allows to control the fidelity of the alpha channel by allowing
webp to reduce the number of palette entries from 256 at alpha_q=100
to 2 at alpha_q=0. This is most useful with cutout pictures where
something like alpha_q=30 or around 8 levels is usually sufficient
for smooth outlines.

For some reason webp->Q was intiailized to 80 even though the Q option
and libwebp use 75 by default. This was never used, so it's just to
avoid confusion about what default is used.

libvips/foreign/foreign.c

@@ -2462,6 +2462,7 @@ vips_webpload_buffer( void *buf, size_t len, VipsImage **out, ... )
  * @preset: #VipsForeignWebpPreset choose lossy compression preset
  * @smart_subsample: enables high quality chroma subsampling
  * @near_lossless: use preprocessing in lossless mode (controlled by Q)
+ * @alpha_q: set alpha quality in lossless mode
  *
  * See also: vips_webpload(), vips_image_write_to_file().
  *
@@ -2494,6 +2495,7 @@ vips_webpsave( VipsImage *in, const char *filename, ... )
  * @preset: #VipsForeignWebpPreset choose lossy compression preset
  * @smart_subsample: enables high quality chroma subsampling
  * @near_lossless: use preprocessing in lossless mode (controlled by Q)
+ * @alpha_q: set alpha quality in lossless mode
  *
  * See also: vips_webpsave().
  *

libvips/foreign/vips2webp.c

@@ -85,7 +85,8 @@ get_preset( VipsForeignWebpPreset preset )
 static int
 write_webp( WebPPicture *pic, VipsImage *in,
 	int Q, gboolean lossless, VipsForeignWebpPreset preset,
-	gboolean smart_subsample, gboolean near_lossless )
+	gboolean smart_subsample, gboolean near_lossless,
+	int alpha_q )
 {
 	VipsImage *memory;
 	WebPConfig config;
@@ -102,6 +103,7 @@ write_webp( WebPPicture *pic, VipsImage *in,
 		config.preprocessing |= 4;
 	if( near_lossless )
 		config.near_lossless = Q;
+	config.alpha_quality = alpha_q;
 
 	if( !WebPValidateConfig(&config) ) {
 		vips_error( "vips2webp",
@@ -147,7 +149,8 @@ write_webp( WebPPicture *pic, VipsImage *in,
 int
 vips__webp_write_file( VipsImage *in, const char *filename, 
 	int Q, gboolean lossless, VipsForeignWebpPreset preset,
-	gboolean smart_subsample, gboolean near_lossless )
+	gboolean smart_subsample, gboolean near_lossless,
+	int alpha_q )
 {
 	WebPPicture pic;
 	WebPMemoryWriter writer;
@@ -164,7 +167,7 @@ vips__webp_write_file( VipsImage *in, const char *filename,
 	pic.custom_ptr = &writer;
 
 	if( write_webp( &pic, in, Q, lossless, preset, smart_subsample,
-		near_lossless ) ) {
+		near_lossless, alpha_q ) ) {
 		WebPPictureFree( &pic );
 		WebPMemoryWriterClear( &writer );
 		return -1;
@@ -192,7 +195,8 @@ vips__webp_write_file( VipsImage *in, const char *filename,
 int
 vips__webp_write_buffer( VipsImage *in, void **obuf, size_t *olen, 
 	int Q, gboolean lossless, VipsForeignWebpPreset preset,
-	gboolean smart_subsample, gboolean near_lossless )
+	gboolean smart_subsample, gboolean near_lossless,
+	int alpha_q )
 {
 	WebPPicture pic;
 	WebPMemoryWriter writer;
@@ -209,7 +213,7 @@ vips__webp_write_buffer( VipsImage *in, void **obuf, size_t *olen,
 	pic.custom_ptr = &writer;
 
 	if( write_webp( &pic, in, Q, lossless, preset, smart_subsample,
-		near_lossless) ) {
+		near_lossless, alpha_q ) ) {
 		WebPPictureFree( &pic );
 		WebPMemoryWriterClear( &writer );
 		return -1;

libvips/foreign/webp.h

@@ -50,10 +50,12 @@ int vips__webp_read_buffer( const void *buf, size_t len,
 
 int vips__webp_write_file( VipsImage *out, const char *filename, 
 	int Q, gboolean lossless, VipsForeignWebpPreset preset,
-	gboolean smart_subsample, gboolean near_lossless );
+	gboolean smart_subsample, gboolean near_lossless,
+	int alpha_q );
 int vips__webp_write_buffer( VipsImage *out, void **buf, size_t *len, 
 	int Q, gboolean lossless, VipsForeignWebpPreset preset,
-	gboolean smart_subsample, gboolean near_lossless );
+	gboolean smart_subsample, gboolean near_lossless,
+	int alpha_q );
 
 #ifdef __cplusplus
 }

libvips/foreign/webpsave.c

@@ -72,6 +72,10 @@ typedef struct _VipsForeignSaveWebp {
 	 */
 	gboolean near_lossless;
 
+	/* Alpha quality.
+	 */
+	int alpha_q;
+
 } VipsForeignSaveWebp;
 
 typedef VipsForeignSaveClass VipsForeignSaveWebpClass;
@@ -143,12 +147,19 @@ vips_foreign_save_webp_class_init( VipsForeignSaveWebpClass *class )
 		G_STRUCT_OFFSET( VipsForeignSaveWebp, near_lossless ),
 		FALSE );
 
+	VIPS_ARG_INT( class, "alpha_q", 15,
+		_( "Alpha quality" ),
+		_( "Change alpha plane fidelity for lossy compression" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsForeignSaveWebp, alpha_q ),
+		0, 100, 100 );
+
 }
 
 static void
 vips_foreign_save_webp_init( VipsForeignSaveWebp *webp )
 {
-	webp->Q = 80;
+	webp->Q = 75;
 }
 
 typedef struct _VipsForeignSaveWebpFile {
@@ -178,7 +189,8 @@ vips_foreign_save_webp_file_build( VipsObject *object )
 
 	if( vips__webp_write_file( save->ready, file->filename, 
 		webp->Q, webp->lossless, webp->preset,
-		webp->smart_subsample, webp->near_lossless ) )
+		webp->smart_subsample, webp->near_lossless,
+		webp->alpha_q ) )
 		return( -1 );
 
 	return( 0 );
@@ -241,7 +253,8 @@ vips_foreign_save_webp_buffer_build( VipsObject *object )
 
 	if( vips__webp_write_buffer( save->ready, &obuf, &olen, 
 		webp->Q, webp->lossless, webp->preset,
-		webp->smart_subsample, webp->near_lossless ) )
+		webp->smart_subsample, webp->near_lossless,
+		webp->alpha_q ) )
 		return( -1 );
 
 	blob = vips_blob_new( (VipsCallbackFn) vips_free, obuf, olen );
@@ -303,7 +316,8 @@ vips_foreign_save_webp_mime_build( VipsObject *object )
 
 	if( vips__webp_write_buffer( save->ready, &obuf, &olen, 
 		webp->Q, webp->lossless, webp->preset,
-		webp->smart_subsample, webp->near_lossless ) )
+		webp->smart_subsample, webp->near_lossless,
+		webp->alpha_q ) )
 		return( -1 );
 
 	printf( "Content-length: %zu\r\n", olen );