add VIPS_COMBINE_MIN combining mode

for vips_compass()

ChangeLog

@@ -33,6 +33,7 @@
 - "--size Nx" to vipsthumbnail was broken, thanks jrochkind 
 - fix build with gcc 7
 - add vips_fill_nearest() ... fill pixels with nearest colour
+- add VIPS_COMBINE_MIN, a new combining mode for vips_compass()
 
 29/8/17 started 8.5.9
 - make --fail stop jpeg read on any libjpeg warning, thanks @mceachen

libvips/convolution/compass.c

@@ -5,6 +5,8 @@
  * 8/5/17
  *      - default to float ... int will often lose precision and should not be
  *        the default
+ * 2/11/17
+ * 	- add MIN mode
  */
 
 /*
@@ -116,6 +118,14 @@ vips_compass_build( VipsObject *object )
 		x = combine[0];
 		break;
 
+	case VIPS_COMBINE_MIN:
+		if( vips_bandrank( abs, &combine[0], compass->times,
+			"index", 0, 
+			NULL ) )
+			return( -1 ); 
+		x = combine[0];
+		break;
+
 	case VIPS_COMBINE_SUM:
 		if( vips_sum( abs, &combine[0], compass->times, NULL ) )
 			return( -1 );

libvips/convolution/convolution.c

@@ -75,8 +75,9 @@
  * VipsCombine:
  * @VIPS_COMBINE_MAX: take the maximum of the possible values
  * @VIPS_COMBINE_SUM: sum all the values
+ * @VIPS_COMBINE_MIN: take the minimum value
  *
- * How to combine values. 
+ * How to combine values. See vips_compass(), for example.
  */
 
 G_DEFINE_ABSTRACT_TYPE( VipsConvolution, vips_convolution, 

libvips/include/vips/convolution.h

@@ -41,6 +41,7 @@ extern "C" {
 typedef enum {
 	VIPS_COMBINE_MAX,
 	VIPS_COMBINE_SUM,
+	VIPS_COMBINE_MIN,
 	VIPS_COMBINE_LAST
 } VipsCombine;
 

libvips/iofuncs/enumtypes.c

@@ -836,6 +836,7 @@ vips_combine_get_type( void )
 		static const GEnumValue values[] = {
 			{VIPS_COMBINE_MAX, "VIPS_COMBINE_MAX", "max"},
 			{VIPS_COMBINE_SUM, "VIPS_COMBINE_SUM", "sum"},
+			{VIPS_COMBINE_MIN, "VIPS_COMBINE_MIN", "min"},
 			{VIPS_COMBINE_LAST, "VIPS_COMBINE_LAST", "last"},
 			{0, NULL, NULL}
 		};