add skip-blanks option to dzsave

Skip-blanks was a feature of google layout. This patch makes it into an
option you can control which defaults on in gm mode.

See https://github.com/libvips/libvips/issues/1185

ChangeLog

@@ -12,6 +12,7 @@
 - don't stop composite on first non-transparent image [felixbuenemann, GDmac]
 - add vips_rect_overlapsrect()
 - composite is much faster at positioning subimages
+- dzsave has a new skip_blanks option
 
 21/11/18 started 8.7.3
 - fix infinite loop for autofit with non-scaleable font 

libvips/foreign/dzsave.c

@@ -76,6 +76,8 @@
  * 24/11/17
  * 	- output overlap-only tiles on edges for better deepzoom spec
  * 	  compliance
+ * 19/12/18
+ * 	- add @skip_blanks
  */
 
 /*
@@ -448,6 +450,7 @@ struct _VipsForeignSaveDz {
 	VipsForeignDzContainer container; 
 	int compression;
 	VipsRegionShrink region_shrink;
+	gboolean skip_blanks;
 
 	/* Tile and overlap geometry. The members above are the parameters we
 	 * accept, this next set are the derived values which are actually 
@@ -1166,10 +1169,7 @@ strip_work( VipsThreadState *state, void *a )
 		state->pos.width, state->pos.height, NULL ) ) 
 		return( -1 );
 
-	/* If we are writing a google map pyramid and the tile is equal to the 
+	if( dz->skip_blanks &&
-	 * background, don't save. The viewer will display blank.png for us.
-	 */
-	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_GOOGLE &&
 		tile_equal( x, dz->ink ) ) { 
 		g_object_unref( x );
 
@@ -1631,6 +1631,12 @@ vips_foreign_save_dz_build( VipsObject *object )
 			dz->tile_size = 256;
 	}
 
+	/* skip_blanks defaults TRUE in google mode.
+	 */
+	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_GOOGLE &&
+		!vips_object_argument_isset( object, "skip_blanks" ) )
+		dz->skip_blanks = TRUE;
+
 	/* Our tile layout.
 	 */
 	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_DZ ) { 
@@ -1650,8 +1656,7 @@ vips_foreign_save_dz_build( VipsObject *object )
 	/* Default to white background. vips_foreign_save_init() defaults to
 	 * black. 
 	 */
-	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_GOOGLE &&
+	if( !vips_object_argument_isset( object, "background" ) ) {
-		!vips_object_argument_isset( object, "background" ) ) {
 		VipsArrayDouble *background; 
 
 		/* Using g_object_set() to set an input param in build will
@@ -1690,9 +1695,9 @@ vips_foreign_save_dz_build( VipsObject *object )
 	save->ready = z;
 }
 
-	/* We use ink in google mode to check for blank tiles.
+	/* We use ink to check for blank tiles.
 	 */
-	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_GOOGLE ) {
+	if( dz->skip_blanks ) {
 		if( !(dz->ink = vips__vector_to_ink( 
 			class->nickname, save->ready,
 			VIPS_AREA( save->background )->data, NULL, 
@@ -2105,6 +2110,13 @@ vips_foreign_save_dz_class_init( VipsForeignSaveDzClass *class )
 		G_STRUCT_OFFSET( VipsForeignSaveDz, region_shrink ),
 		VIPS_TYPE_REGION_SHRINK, VIPS_REGION_SHRINK_MEAN ); 
 
+	VIPS_ARG_BOOL( class, "skip_blanks", 19, 
+		_( "Skip blanks" ), 
+		_( "Skip tiles which are nearly equal to the background" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsForeignSaveDz, skip_blanks ),
+		FALSE );
+
 	/* How annoying. We stupidly had these in earlier versions.
 	 */
 
@@ -2340,7 +2352,8 @@ vips_foreign_save_dz_buffer_init( VipsForeignSaveDzBuffer *buffer )
  * * @container: #VipsForeignDzContainer set container type
  * * @properties: %gboolean write a properties file
  * * @compression: %gint zip deflate compression level
- * * @shrink_region: #VipsRegionShrink How to shrink each 2x2 region.
+ * * @region_shrink: #VipsRegionShrink How to shrink each 2x2 region
+ * * @skip_blanks: %gboolean Skip tiles which are equal to the background
  *
  * Save an image as a set of tiles at various resolutions. By default dzsave
  * uses DeepZoom layout -- use @layout to pick other conventions.
@@ -2387,6 +2400,10 @@ vips_foreign_save_dz_buffer_init( VipsForeignSaveDzBuffer *buffer )
  * You can use @region_shrink to control the method for shrinking each 2x2
  * region. This defaults to using the average of the 4 input pixels but you can
  * also use the median in cases where you want to preserve the range of values.
+ *
+ * If you set @skip_blanks, tiles which are almost equal to the background are
+ * skipped. This can save a lot of space for some image types. This option
+ * defaults %TRUE in Google layout mode.
  * 
  * See also: vips_tiffsave().
  *
@@ -2426,7 +2443,8 @@ vips_dzsave( VipsImage *in, const char *name, ... )
  * * @container: #VipsForeignDzContainer set container type
  * * @properties: %gboolean write a properties file
  * * @compression: %gint zip deflate compression level
- * * @shrink_region: #VipsRegionShrink How to shrink each 2x2 region.
+ * * @region_shrink: #VipsRegionShrink How to shrink each 2x2 region.
+ * * @skip_blanks: %gboolean Skip tiles which are equal to the background
  *
  * As vips_dzsave(), but save to a memory buffer. 
  *