add jpeg-chroma-subsample to jpegload

The jpeg loader now sets the field jpeg-chroma-subsample to record
chroma subsample.

See https://github.com/jcupitt/libvips/issues/954

ChangeLog

@@ -16,6 +16,8 @@
 - set "interlaced=1" for interlaced JPG and PNG images
 - add PDFium PDF loader
 - add @format option to magickload
+- jpegload adds a jpeg-chroma-subsample field with eg. 4:4:4 for no
+  chrominance subsampling. 
 
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges [Adrià]

libvips/foreign/jpeg2vips.c

@@ -279,6 +279,25 @@ readjpeg_file( ReadJpeg *jpeg, const char *filename )
 	return( 0 );
 }
 
+static const char *
+find_chroma_subsample( struct jpeg_decompress_struct *cinfo )
+{
+	gboolean has_subsample;
+
+	/* libjpeg only uses 4:4:4 and 4:2:0, confusingly. 
+	 *
+	 * http://poynton.ca/PDFs/Chroma_subsampling_notation.pdf
+	 */
+	has_subsample = cinfo->max_h_samp_factor > 1 ||
+		cinfo->max_v_samp_factor > 1;
+	if( cinfo->num_components > 3 )
+		/* A cmyk image.
+		 */
+		return( has_subsample ? "4:2:0:4" : "4:4:4:4" );
+	else
+		return( has_subsample ? "4:2:0" : "4:4:4" );
+}
+
 static int
 attach_blob( VipsImage *im, const char *field, void *data, int data_length )
 {
@@ -439,6 +458,9 @@ read_jpeg_header( ReadJpeg *jpeg, VipsImage *out )
 	if( jpeg_has_multiple_scans( cinfo ) )
 		vips_image_set_int( out, "interlaced", 1 ); 
 
+	(void) vips_image_set_string( out, "jpeg-chroma-subsample", 
+		find_chroma_subsample( cinfo ) );
+
 	/* Look for EXIF and ICC profile.
 	 */
 	for( p = cinfo->marker_list; p; p = p->next ) {

libvips/foreign/jpegload.c

@@ -374,6 +374,11 @@ vips_foreign_load_jpeg_buffer_init( VipsForeignLoadJpegBuffer *buffer )
  * memory to load, so this field gives callers a chance to handle these
  * images differently.
  *
+ * The string-valued field "jpeg-chroma-subsample" gives the chroma subsample
+ * in standard notation. 4:4:4 means no subsample, 4:2:0 means YCbCr with
+ * Cb and Cr subsampled horizontally and vertically, 4:4:4:4 means a CMYK
+ * image with no subsampling. 
+ *
  * The EXIF thumbnail, if present, is attached to the image as 
  * "jpeg-thumbnail-data". See vips_image_get_blob().
  *