load and save TIFFTAG_IMAGEDESCRIPTION

this often has useful metadata in, for example the OME spec has all the
metadata in there as an XML document

see https://github.com/jcupitt/libvips/issues/358

ChangeLog

@@ -23,6 +23,7 @@
 - oop, removed a DEBUG from buffer.c, vips is 30% faster
 - faster and lower-mem TIFF read
 - faster bilinear interpolator
+- TIFF loads and saves IMAGEDESCRIPTION
 
 7/5/15 started 8.1.1
 - oop, vips-8.0 wrapper script should be vips-8.1, thanks Danilo

libvips/foreign/tiff2vips.c

@@ -160,6 +160,8 @@
  * 29/9/15
  * 	- load IPCT metadata
  * 	- load photoshop metadata
+ * 21/12/15
+ * 	- load TIFFTAG_IMAGEDESCRIPTION
  */
 
 /*
@@ -1224,6 +1226,16 @@ parse_header( ReadTiff *rtiff, VipsImage *out )
 			(VipsCallbackFn) vips_free, data_copy, data_length );
 	}
 
+	/* IMAGEDESCRIPTION often has useful metadata.
+	 */
+	if( TIFFGetField( rtiff->tiff, TIFFTAG_IMAGEDESCRIPTION, &data ) ) {
+		/* libtiff makes sure that data is null-terminated and contains
+		 * no embedded null characters.
+		 */
+		vips_image_set_string( out, 
+			VIPS_META_IMAGEDESCRIPTION, (char *) data ); 
+	}
+
 	return( 0 );
 }
 

libvips/foreign/vips2tiff.c

@@ -158,6 +158,8 @@
  * 	- try to write photoshop metadata
  * 11/11/15
  * 	- better alpha handling, thanks sadaqatullahn
+ * 21/12/15
+ * 	- write TIFFTAG_IMAGEDESCRIPTION
  */
 
 /*
@@ -496,8 +498,8 @@ write_embed_photoshop( Write *write, TIFF *tif )
 
 	if( !vips_image_get_typeof( write->im, VIPS_META_PHOTOSHOP_NAME ) )
 		return( 0 );
-	if( vips_image_get_blob( write->im, VIPS_META_PHOTOSHOP_NAME, 
+	if( vips_image_get_blob( write->im, 
-		&data, &data_length ) )
+		VIPS_META_PHOTOSHOP_NAME, &data, &data_length ) )
 		return( -1 );
 	TIFFSetField( tif, TIFFTAG_PHOTOSHOP, data_length, data );
 
@@ -508,6 +510,27 @@ write_embed_photoshop( Write *write, TIFF *tif )
 	return( 0 );
 }
 
+/* Set IMAGEDESCRIPTION, if it's there.  
+ */
+static int
+write_embed_imagedescription( Write *write, TIFF *tif )
+{
+	const char *imagedescription;
+
+	if( !vips_image_get_typeof( write->im, VIPS_META_IMAGEDESCRIPTION ) )
+		return( 0 );
+	if( vips_image_get_string( write->im, 
+		VIPS_META_IMAGEDESCRIPTION, &imagedescription ) )
+		return( -1 );
+	TIFFSetField( tif, TIFFTAG_IMAGEDESCRIPTION, imagedescription );
+
+#ifdef DEBUG
+	printf( "vips2tiff: attached imagedescription from meta\n" );
+#endif /*DEBUG*/
+
+	return( 0 );
+}
+
 /* Write a TIFF header. width and height are the size of the VipsImage we are
  * writing (it may have been shrunk).
  */
@@ -543,7 +566,8 @@ write_tiff_header( Write *write, Layer *layer )
 	if( write_embed_profile( write, tif ) ||
 		write_embed_xmp( write, tif ) ||
 		write_embed_ipct( write, tif ) ||
-		write_embed_photoshop( write, tif ) )
+		write_embed_photoshop( write, tif ) ||
+		write_embed_imagedescription( write, tif ) )
 		return( -1 ); 
 
 	/* And colour fields.
@@ -1497,7 +1521,8 @@ write_copy_tiff( Write *write, TIFF *out, TIFF *in )
 	if( write_embed_profile( write, out ) ||
 		write_embed_xmp( write, out ) ||
 		write_embed_ipct( write, out ) ||
-		write_embed_photoshop( write, out ) )
+		write_embed_photoshop( write, out ) ||
+		write_embed_imagedescription( write, out ) )
 		return( -1 );
 
 	buf = vips_malloc( NULL, TIFFTileSize( in ) );

libvips/include/vips/header.h

@@ -85,6 +85,13 @@ extern "C" {
  */
 #define VIPS_META_XML "xml-header"
 
+/**
+ * VIPS_META_IMAGEDESCRIPTION:
+ *
+ * The IMAGEDESCRIPTION tag. Often has useful metadata. 
+ */
+#define VIPS_META_IMAGEDESCRIPTION "image-description"
+
 /**
  * VIPS_META_RESOLUTION_UNIT:
  *

libvips/iofuncs/type.c

@@ -1413,7 +1413,7 @@ vips_value_get_ref_string( const GValue *value, size_t *length )
  * Copies the C string @str into @value. 
  *
  * vips_ref_string are immutable C strings that are copied between images by
- * copying reference-counted pointers, making the much more efficient than
+ * copying reference-counted pointers, making them much more efficient than
  * regular %GValue strings.
  */
 void