add force subsample argument that overrides the Q deduced subsampling directive

libvips/foreign/jpegsave.c

@@ -77,6 +77,10 @@ typedef struct _VipsForeignSaveJpeg {
 	 */
 	gboolean no_subsample;
 
+    /* Force chroma subsampling, if Q >= 90 then subsampling is disabled, use this flag to force it
+    */
+    gboolean force_subsample;
+
 	/* Apply trellis quantisation to each 8x8 block.
 	 */
 	gboolean trellis_quant;
@@ -194,6 +198,12 @@ vips_foreign_save_jpeg_class_init( VipsForeignSaveJpegClass *class )
 		G_STRUCT_OFFSET( VipsForeignSaveJpeg, quant_table ),
 		0, 8, 0 );
 
+    VIPS_ARG_BOOL( class, "force_subsample", 19,
+            _( "Force subsample" ),
+            _( "Force chroma subsample" ),
+            VIPS_ARGUMENT_OPTIONAL_INPUT,
+            G_STRUCT_OFFSET( VipsForeignSaveJpeg, force_subsample ),
+            FALSE );
 }
 
 static void
@@ -230,7 +240,7 @@ vips_foreign_save_jpeg_target_build( VipsObject *object )
 		jpeg->Q, jpeg->profile, jpeg->optimize_coding, 
 		jpeg->interlace, save->strip, jpeg->no_subsample,
 		jpeg->trellis_quant, jpeg->overshoot_deringing,
-		jpeg->optimize_scans, jpeg->quant_table ) )
+		jpeg->optimize_scans, jpeg->quant_table, jpeg->force_subsample ) )
 		return( -1 );
 
 	return( 0 );
@@ -297,7 +307,7 @@ vips_foreign_save_jpeg_file_build( VipsObject *object )
 		jpeg->Q, jpeg->profile, jpeg->optimize_coding, 
 		jpeg->interlace, save->strip, jpeg->no_subsample,
 		jpeg->trellis_quant, jpeg->overshoot_deringing,
-		jpeg->optimize_scans, jpeg->quant_table ) ) {
+		jpeg->optimize_scans, jpeg->quant_table, jpeg->force_subsample ) ) {
 		VIPS_UNREF( target );
 		return( -1 );
 	}
@@ -367,7 +377,7 @@ vips_foreign_save_jpeg_buffer_build( VipsObject *object )
 		jpeg->Q, jpeg->profile, jpeg->optimize_coding, 
 		jpeg->interlace, save->strip, jpeg->no_subsample,
 		jpeg->trellis_quant, jpeg->overshoot_deringing,
-		jpeg->optimize_scans, jpeg->quant_table ) ) {
+		jpeg->optimize_scans, jpeg->quant_table, jpeg->force_subsample ) ) {
 		VIPS_UNREF( target );
 		return( -1 );
 	}
@@ -440,7 +450,7 @@ vips_foreign_save_jpeg_mime_build( VipsObject *object )
 		jpeg->Q, jpeg->profile, jpeg->optimize_coding, 
 		jpeg->interlace, save->strip, jpeg->no_subsample,
 		jpeg->trellis_quant, jpeg->overshoot_deringing,
-		jpeg->optimize_scans, jpeg->quant_table ) ) {
+		jpeg->optimize_scans, jpeg->quant_table, jpeg->force_subsample ) ) {
 		VIPS_UNREF( target );
 		return( -1 );
 	}

libvips/foreign/pforeign.h

@@ -166,7 +166,7 @@ int vips__jpeg_write_target( VipsImage *in, VipsTarget *target,
 	gboolean optimize_coding, gboolean progressive, gboolean strip,
 	gboolean no_subsample, gboolean trellis_quant,
 	gboolean overshoot_deringing, gboolean optimize_scans, 
-	int quant_table );
+	int quant_table, gboolean force_subsample );
 
 int vips__jpeg_read_source( VipsSource *source, VipsImage *out,
 	gboolean header_only, int shrink, int fail, gboolean autorotate );

libvips/foreign/vips2jpeg.c

@@ -480,7 +480,8 @@ static int
 write_vips( Write *write, int qfac, const char *profile, 
 	gboolean optimize_coding, gboolean progressive, gboolean strip, 
 	gboolean no_subsample, gboolean trellis_quant,
-	gboolean overshoot_deringing, gboolean optimize_scans, int quant_table )
+	gboolean overshoot_deringing, gboolean optimize_scans, int quant_table,
+	gboolean force_subsample)
 {
 	VipsImage *in;
 	J_COLOR_SPACE space;
@@ -630,8 +631,9 @@ write_vips( Write *write, int qfac, const char *profile,
 	/* Turn off chroma subsampling. Follow IM and do it automatically for
 	 * high Q. 
 	 */
-	if( no_subsample ||
+	if( !force_subsample && (
-		qfac >= 90 ) { 
+	        no_subsample ||
+	        qfac >= 90 )) {
 		int i;
 
 		for( i = 0; i < in->Bands; i++ ) { 
@@ -775,7 +777,8 @@ vips__jpeg_write_target( VipsImage *in, VipsTarget *target,
 	int Q, const char *profile, 
 	gboolean optimize_coding, gboolean progressive,
 	gboolean strip, gboolean no_subsample, gboolean trellis_quant,
-	gboolean overshoot_deringing, gboolean optimize_scans, int quant_table )
+	gboolean overshoot_deringing, gboolean optimize_scans, int quant_table,
+	gboolean force_subsample)
 {
 	Write *write;
 
@@ -802,7 +805,7 @@ vips__jpeg_write_target( VipsImage *in, VipsTarget *target,
 	if( write_vips( write, 
 		Q, profile, optimize_coding, progressive, strip, no_subsample,
 		trellis_quant, overshoot_deringing, optimize_scans, 
-		quant_table ) ) {
+		quant_table, force_subsample ) ) {
 		write_destroy( write );
 		return( -1 );
 	}

test/test-suite/test_foreign.py

@@ -254,6 +254,20 @@ class TestForeign:
             # format area at the end
             assert y.startswith("hello world")
 
+    @skip_if_no("jpegload")
+    def test_jpegsave(self):
+        im = pyvips.Image.new_from_file(JPEG_FILE)
+
+        # higher Q should mean a bigger buffer
+        b1 = im.jpegsave_buffer(Q=10)
+        b2 = im.jpegsave_buffer(Q=90)
+        assert len(b2) > len(b1)
+
+        # force subsampling should result in smaller buffer
+        b1 = im.jpegsave_buffer(Q=90, force_subsample=True)
+        b2 = im.jpegsave_buffer(Q=90)
+        assert len(b2) > len(b1)
+
     @skip_if_no("jpegload")
     def test_truncated(self):
         # This should open (there's enough there for the header)