better alpha handling for tiff save

the tiff saver was writing all five-band images as CMYKA, even if they
were tagged as srgb ... it now follows the interpretation tag and will
write many alpha channels instead

see https://github.com/jcupitt/libvips/issues/348

thanks sadaqatullahn

ChangeLog

@@ -7,6 +7,7 @@
 - only allow [] for filename options
 - add memory.h to Python API .. makes tracked highwater visible
 - added bandjoin_const to add constant bands to an image
+- better alpha handling for tiff write, thanks sadaqatullahn
 
 7/5/15 started 8.1.1
 - oop, vips-8.0 wrapper script should be vips-8.1, thanks Danilo

libvips/foreign/vips2tiff.c

@@ -156,6 +156,8 @@
  * 29/9/15
  * 	- try to write IPCT metadata
  * 	- try to write photoshop metadata
+ * 11/11/15
+ * 	- better alpha handling, thanks sadaqatullahn
  */
 
 /*
@@ -211,6 +213,10 @@
 
 #include "tiff.h"
 
+/* Max number of alpha channels we allow.
+ */
+#define MAX_ALPHA (64)
+
 typedef struct _Layer Layer;
 typedef struct _Write Write;
 
@@ -510,7 +516,6 @@ write_tiff_header( Write *write, Layer *layer )
 {
 	TIFF *tif = layer->tif;
 
-	uint16 v[1];
 	int format; 
 
 	/* Output base header fields.
@@ -559,34 +564,39 @@ write_tiff_header( Write *write, Layer *layer )
 	else {
 		int photometric;
 
+		/* Number of bands that have colour in .. other bands are saved
+		 * as alpha.
+		 */
+		int colour_bands;
+
+		int alpha_bands;
+
 		TIFFSetField( tif, TIFFTAG_SAMPLESPERPIXEL, write->im->Bands );
 		TIFFSetField( tif, TIFFTAG_BITSPERSAMPLE, 
 			vips_format_sizeof( write->im->BandFmt ) << 3 );
 
-		switch( write->im->Bands ) {
+		if( write->im->Bands < 3 ) {
-		case 1:
+			/* Mono or mono + alpha.
-		case 2:
+			 */
 			photometric = write->miniswhite ? 
 				PHOTOMETRIC_MINISWHITE :  
 				PHOTOMETRIC_MINISBLACK;
-			if( write->im->Bands == 2 ) {
+			colour_bands = 1;
-				v[0] = EXTRASAMPLE_ASSOCALPHA;
+		}
-				TIFFSetField( tif, TIFFTAG_EXTRASAMPLES, 1, v );
+		else {
-			}
+			/* Could be: RGB, CMYK, LAB, perhaps with extra alpha.
-			break;
-
-		case 3:
-		case 4:
-			/* could be: RGB, RGBA, CMYK, LAB, LABA, generic
-			 * multi-band image.
 			 */
 			if( write->im->Type == VIPS_INTERPRETATION_LAB || 
-				write->im->Type == VIPS_INTERPRETATION_LABS ) 
+				write->im->Type == VIPS_INTERPRETATION_LABS ) {
 				photometric = PHOTOMETRIC_CIELAB;
-			else if( write->im->Type == VIPS_INTERPRETATION_CMYK ) {
+				colour_bands = 3;
+			}
+			else if( write->im->Type == VIPS_INTERPRETATION_CMYK &&
+				write->im->Bands >= 4 ) {
 				photometric = PHOTOMETRIC_SEPARATED;
 				TIFFSetField( tif, 
 					TIFFTAG_INKSET, INKSET_CMYK );
+				colour_bands = 4;
 			}
 			else if( write->compression == COMPRESSION_JPEG &&
 				write->im->Bands == 3 &&
@@ -599,32 +609,27 @@ write_tiff_header( Write *write, Layer *layer )
 				photometric = PHOTOMETRIC_YCBCR;
 				TIFFSetField( tif, TIFFTAG_JPEGCOLORMODE, 
 					JPEGCOLORMODE_RGB );
+				colour_bands = 3;
 			}
-			else
+			else {
+				/* Some kind of generic multi-band image ..
+				 * save the first three bands as RGB, the rest
+				 * as alpha.
+				 */
 				photometric = PHOTOMETRIC_RGB;
-
+				colour_bands = 3;
-			if( write->im->Type != VIPS_INTERPRETATION_CMYK && 
-				write->im->Bands == 4 ) {
-				v[0] = EXTRASAMPLE_ASSOCALPHA;
-				TIFFSetField( tif, TIFFTAG_EXTRASAMPLES, 1, v );
 			}
+		}
 
-			break;
+		alpha_bands = write->im->Bands - colour_bands;
+		if( alpha_bands > 0 ) {
+			uint16 v[MAX_ALPHA];
+			int i;
 
-		case 5:
+			for( i = 0; i < alpha_bands; i++ )
-			/* Only CMYKA
+				v[i] = EXTRASAMPLE_ASSOCALPHA;
-			 */
+			TIFFSetField( tif, 
-			photometric = PHOTOMETRIC_SEPARATED;
+				TIFFTAG_EXTRASAMPLES, alpha_bands, v );
-			TIFFSetField( tif, TIFFTAG_INKSET, INKSET_CMYK );
-			v[0] = EXTRASAMPLE_ASSOCALPHA;
-			TIFFSetField( tif, TIFFTAG_EXTRASAMPLES, 1, v );
-			break;
-
-		default:
-			/* Who knows. Just call it RGB.
-			 */
-			photometric = PHOTOMETRIC_RGB;
-			break; 
 		}
 
 		TIFFSetField( tif, TIFFTAG_PHOTOMETRIC, photometric );