better max_alpha default

it used to default to 255, now it sniff the interpretation and defaults to
65535 for RGB16 and GREY16

see https://github.com/jcupitt/libvips/issues/449

ChangeLog

@@ -7,6 +7,7 @@
 - allow nested [] in CLI args
 - restore BandFmt on unpremultiply in vipsthumbnail
 - better python detection and build [Felix Bünemann]
+- max-alpha defaults to 65535 for RGB16/GREY16
 
 18/5/16 started 8.3.2
 - more robust vips image reading

libvips/conversion/flatten.c

@@ -7,6 +7,8 @@
  * 	- better rounding 
  * 9/5/15
  * 	- add max_alpha to match vips_premultiply() etc.
+ * 25/5/16
+ * 	- max_alpha defaults to 65535 for RGB16/GREY16
  */
 
 /*
@@ -320,6 +322,14 @@ vips_flatten_build( VipsObject *object )
 
 	conversion->out->Bands -= 1;
 
+	/* Is max-alpha unset? Default to the correct value for this
+	 * interpretation.
+	 */
+	if( !vips_object_argument_isset( object, "max_alpha" ) ) 
+		if( in->Type == VIPS_INTERPRETATION_GREY16 ||
+			in->Type == VIPS_INTERPRETATION_RGB16 )
+			flatten->max_alpha = 65535;
+
 	/* Is the background black? We have a special path for this.
 	 */
 	black = TRUE;
@@ -421,9 +431,9 @@ vips_flatten_init( VipsFlatten *flatten )
  * Non-complex images only.
  * @background defaults to zero (black). 
  *
- * @max_alpha has the default value 255. You will need to set this to 65535
+ * @max_alpha has the default value 255, or 65535 for images tagged as
- * for images with a 16-bit alpha, or perhaps 1.0 for images with a float
+ * #VIPS_INTERPRETATION_RGB16 or
- * alpha. 
+ * #VIPS_INTERPRETATION_GREY16. 
  *
  * Useful for flattening PNG images to RGB.
  *

libvips/conversion/premultiply.c

@@ -5,6 +5,8 @@
  *
  * 11/4/16 Lovell
  * 	- fix RGBA path
+ * 25/5/16
+ * 	- max_alpha defaults to 65535 for RGB16/GREY16
  */
 
 /*
@@ -214,6 +216,14 @@ vips_premultiply_build( VipsObject *object )
 		VIPS_DEMAND_STYLE_THINSTRIP, in, NULL ) )
 		return( -1 );
 
+	/* Is max-alpha unset? Default to the correct value for this
+	 * interpretation.
+	 */
+	if( !vips_object_argument_isset( object, "max_alpha" ) ) 
+		if( in->Type == VIPS_INTERPRETATION_GREY16 ||
+			in->Type == VIPS_INTERPRETATION_RGB16 )
+			premultiply->max_alpha = 65535;
+
 	if( in->BandFmt == VIPS_FORMAT_DOUBLE )
 		conversion->out->BandFmt = VIPS_FORMAT_DOUBLE;
 	else
@@ -294,9 +304,9 @@ vips_premultiply_init( VipsPremultiply *premultiply )
  * #VIPS_FORMAT_FLOAT unless the input format is #VIPS_FORMAT_DOUBLE, in which
  * case the output is double as well.
  *
- * @max_alpha has the default value 255. You will need to set this to 65535
+ * @max_alpha has the default value 255, or 65535 for images tagged as
- * for images with a 16-bit alpha, or perhaps 1.0 for images with a float
+ * #VIPS_INTERPRETATION_RGB16 or
- * alpha. 
+ * #VIPS_INTERPRETATION_GREY16. 
  *
  * Non-complex images only.
  *

libvips/conversion/unpremultiply.c

@@ -3,6 +3,8 @@
  * Author: John Cupitt
  * Written on: 7/5/15
  *
+ * 25/5/16
+ * 	- max_alpha defaults to 65535 for RGB16/GREY16
  */
 
 /*
@@ -223,6 +225,14 @@ vips_unpremultiply_build( VipsObject *object )
 		VIPS_DEMAND_STYLE_THINSTRIP, in, NULL ) )
 		return( -1 );
 
+	/* Is max-alpha unset? Default to the correct value for this
+	 * interpretation.
+	 */
+	if( !vips_object_argument_isset( object, "max_alpha" ) ) 
+		if( in->Type == VIPS_INTERPRETATION_GREY16 ||
+			in->Type == VIPS_INTERPRETATION_RGB16 )
+			unpremultiply->max_alpha = 65535;
+
 	if( in->BandFmt == VIPS_FORMAT_DOUBLE )
 		conversion->out->BandFmt = VIPS_FORMAT_DOUBLE;
 	else
@@ -307,9 +317,9 @@ vips_unpremultiply_init( VipsUnpremultiply *unpremultiply )
  * #VIPS_FORMAT_FLOAT unless the input format is #VIPS_FORMAT_DOUBLE, in which
  * case the output is double as well.
  *
- * @max_alpha has the default value 255. You will need to set this to 65535
+ * @max_alpha has the default value 255, or 65535 for images tagged as
- * for images with a 16-bit alpha, or perhaps 1.0 for images with a float
+ * #VIPS_INTERPRETATION_RGB16 or
- * alpha. 
+ * #VIPS_INTERPRETATION_GREY16. 
  *
  * Non-complex images only.
  *

libvips/foreign/foreign.c

@@ -12,6 +12,8 @@
  * 	- fix conversion to 16-bit RGB, thanks John
  * 18/6/15
  * 	- forward progress signals from load
+ * 23/5/16
+ * 	- remove max-alpha stuff, this is now automatic
  */
 
 /*
@@ -1214,9 +1216,6 @@ vips_foreign_convert_saveable( VipsForeignSave *save )
 
 			if( vips_flatten( in, &out, 
 				"background", save->background,
-				"max_alpha", 
-					in->BandFmt == VIPS_FORMAT_USHORT ?
-						65535.0 : 255.0, 
 				NULL ) ) {
 				g_object_unref( in );
 				return( -1 );

tools/vipsthumbnail.c

@@ -79,6 +79,8 @@
  * 	- deprecate sharpen and interpolate
  * 6/5/16
  * 	- restore BandFmt after unpremultiply
+ * 23/5/16
+ * 	- no need to guess max-alpha now premultiply does this for us
  */
 
 #ifdef HAVE_CONFIG_H
@@ -367,10 +369,6 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 	gboolean have_premultiplied;
 	VipsBandFormat unpremultiplied_format;
 
-	/* Sniff the incoming image and try to guess what the alpha max is.
-	 */
-	double max_alpha;
-
 	double shrink; 
 
 	/* RAD needs special unpacking.
@@ -385,12 +383,6 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 		in = t[0];
 	}
 
-	/* Try to guess what the maximum alpha might be.
-	 */
-	max_alpha = 255;
-	if( in->BandFmt == VIPS_FORMAT_USHORT )
-		max_alpha = 65535;
-
 	/* In linear mode, we import right at the start. 
 	 *
 	 * We also have to import the whole image if it's CMYK, since
@@ -443,9 +435,7 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 		(in->Bands == 4 && in->Type != VIPS_INTERPRETATION_CMYK) ||
 		in->Bands == 5 ) {
 		vips_info( "vipsthumbnail", "premultiplying alpha" ); 
-		if( vips_premultiply( in, &t[3], 
+		if( vips_premultiply( in, &t[3], NULL ) ) 
-			"max_alpha", max_alpha,
-			NULL ) ) 
 			return( NULL );
 		have_premultiplied = TRUE;
 
@@ -465,9 +455,7 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 
 	if( have_premultiplied ) {
 		vips_info( "vipsthumbnail", "unpremultiplying alpha" ); 
-		if( vips_unpremultiply( in, &t[5], 
+		if( vips_unpremultiply( in, &t[5], NULL ) || 
-			"max_alpha", max_alpha,
-			NULL ) || 
 			vips_cast( t[5], &t[6], unpremultiplied_format, NULL ) )
 			return( NULL );
 		in = t[6];