From 4c1e6dd02375a4736166caa8da98f26d07af0780 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Sat, 9 Jul 2016 18:49:22 +0100
Subject: [PATCH] turn off chroma subsample for Q > 90

see https://github.com/jcupitt/libvips/issues/482
---
 libvips/foreign/jpegsave.c  |  3 ++-
 libvips/foreign/vips2jpeg.c | 21 +++++++++++++++------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/libvips/foreign/jpegsave.c b/libvips/foreign/jpegsave.c
index f8b3b454..5289f176 100644
--- a/libvips/foreign/jpegsave.c
+++ b/libvips/foreign/jpegsave.c
@@ -95,7 +95,8 @@ typedef struct _VipsForeignSaveJpeg {
 	 */
 	gboolean trellis_quant;
 
-	/* Apply overshooting to samples with extreme values e.g. 0 & 255 for 8-bit.
+	/* Apply overshooting to samples with extreme values e.g. 0 & 255 
+	 * for 8-bit.
 	 */
 	gboolean overshoot_deringing;
 
diff --git a/libvips/foreign/vips2jpeg.c b/libvips/foreign/vips2jpeg.c
index 91c703ed..dbf25447 100644
--- a/libvips/foreign/vips2jpeg.c
+++ b/libvips/foreign/vips2jpeg.c
@@ -78,6 +78,8 @@
  * 	- add quant_table
  * 26/5/16
  * 	- switch to new orientation tag
+ * 9/7/16
+ * 	- turn off chroma subsample for Q > 90
  */
 
 /*
@@ -1020,7 +1022,9 @@ write_vips( Write *write, int qfac, const char *profile,
 	 */
         g_assert( in->BandFmt == VIPS_FORMAT_UCHAR );
 	g_assert( in->Coding == VIPS_CODING_NONE );
-        g_assert( in->Bands == 1 || in->Bands == 3 || in->Bands == 4 );
+        g_assert( in->Bands == 1 || 
+		in->Bands == 3 || 
+		in->Bands == 4 );
 
         /* Check input image.
          */
@@ -1032,7 +1036,8 @@ write_vips( Write *write, int qfac, const char *profile,
         write->cinfo.image_width = in->Xsize;
         write->cinfo.image_height = in->Ysize;
 	write->cinfo.input_components = in->Bands;
-	if( in->Bands == 4 && in->Type == VIPS_INTERPRETATION_CMYK ) {
+	if( in->Bands == 4 && 
+		in->Type == VIPS_INTERPRETATION_CMYK ) {
 		space = JCS_CMYK;
 		/* IJG always sets an Adobe marker, so we should invert CMYK.
 		 */
@@ -1153,9 +1158,11 @@ write_vips( Write *write, int qfac, const char *profile,
 	if( progressive ) 
 		jpeg_simple_progression( &write->cinfo ); 
 
-	/* Turn off chroma subsampling.
+	/* Turn off chroma subsampling. Follow IM and do it automatically for
+	 * high Q. 
 	 */
-	if( no_subsample ) { 
+	if( no_subsample ||
+		Q > 90 ) { 
 		int i;
 
 		for( i = 0; i < in->Bands; i++ ) { 
@@ -1250,7 +1257,8 @@ vips__jpeg_write_file( VipsImage *in,
 	 */
 	if( write_vips( write, 
 		Q, profile, optimize_coding, progressive, strip, no_subsample,
-		trellis_quant, overshoot_deringing, optimize_scans, quant_table ) ) {
+		trellis_quant, overshoot_deringing, optimize_scans, 
+		quant_table ) ) {
 		write_destroy( write );
 		return( -1 );
 	}
@@ -1535,7 +1543,8 @@ vips__jpeg_write_buffer( VipsImage *in,
 	 */
 	if( write_vips( write, 
 		Q, profile, optimize_coding, progressive, strip, no_subsample,
-		trellis_quant, overshoot_deringing, optimize_scans, quant_table ) ) {
+		trellis_quant, overshoot_deringing, optimize_scans, 
+		quant_table ) ) {
 		write_destroy( write );
 
 		return( -1 );