strict round down on jpeg shrink

libjpeg rounds up on shrink-on-load. In some cases this can leave a dark
line along the right and bottom edge, since it only contains (for
example) 1/4 of a pixel of data.

This change adds a crop after jpeg load so that only complete pixels are
output.

See https://github.com/lovell/sharp/issues/1185

ChangeLog

@@ -1,6 +1,7 @@
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges [Adrià]
 - revise C++ example [fangqiao]
+- strict round down on jpeg shrink on load [davidwood]
 
 12/2/18 started 8.6.3
 - use pkg-config to find libjpeg, if we can

libvips/foreign/jpeg2vips.c

@@ -94,6 +94,8 @@
  * 	- revert previous warning change: libvips reports serious corruption, 
  * 	  like a truncated file, as a warning and we need to be able to catch
  * 	  that
+ * 10/4/18
+ * 	- strict round down on shrink-on-load
  */
 
 /*
@@ -151,8 +153,6 @@
 /* Stuff we track during a read.
  */
 typedef struct _ReadJpeg {
-	VipsImage *out;
-
 	/* Shrink by this much during load. 1, 2, 4, 8.
 	 */
 	int shrink;
@@ -177,6 +177,13 @@ typedef struct _ReadJpeg {
 	 * during load.
 	 */
 	gboolean autorotate;
+
+	/* cinfo->output_width and height can be larger than we want since
+	 * libjpeg rounds up on shrink-on-load. This is the real size we will
+	 * output, as opposed to the size we decompress to.
+	 */
+	int output_width;
+	int output_height;
 } ReadJpeg;
 
 /* This can be called many times.
@@ -228,7 +235,6 @@ readjpeg_new( VipsImage *out, int shrink, gboolean fail, gboolean autorotate )
 	if( !(jpeg = VIPS_NEW( out, ReadJpeg )) )
 		return( NULL );
 
-	jpeg->out = out;
 	jpeg->shrink = shrink;
 	jpeg->fail = fail;
 	jpeg->filename = NULL;
@@ -408,6 +414,18 @@ read_jpeg_header( ReadJpeg *jpeg, VipsImage *out )
 
 	vips_image_pipelinev( out, VIPS_DEMAND_STYLE_FATSTRIP, NULL );
 
+	/* cinfo->output_width and cinfo->output_height round up with
+	 * shrink-on-load. For example, if the image is 1801 pixels across and
+	 * we shrink by 4, the output will be 450.25 pixels across, 
+	 * cinfo->output_width with be 451, and libjpeg will write a black
+	 * column of pixels down the right.
+	 *
+	 * We must strictly round down, since we don't want fractional pixels
+	 * along the bottom and right.
+	 */
+	jpeg->output_width = cinfo->image_width / jpeg->shrink;
+	jpeg->output_height = cinfo->image_height / jpeg->shrink;
+
 	/* Interlaced jpegs need lots of memory to read, so our caller needs
 	 * to know.
 	 */
@@ -691,12 +709,14 @@ read_jpeg_image( ReadJpeg *jpeg, VipsImage *out )
 	if( vips_image_generate( t[0], 
 		NULL, read_jpeg_generate, NULL, 
 		jpeg, NULL ) ||
-		vips_sequential( t[0], &t[1], 
+		vips_extract_area( t[0], &t[1], 
+			0, 0, jpeg->output_width, jpeg->output_height, NULL ) ||
+		vips_sequential( t[1], &t[2], 
 			"tile_height", 8,
 			NULL ) )
 		return( -1 );
 
-	im = t[1];
+	im = t[2];
 	if( jpeg->autorotate )
 		im = read_jpeg_rotate( VIPS_OBJECT( out ), im );
 
@@ -735,6 +755,11 @@ vips__jpeg_read( ReadJpeg *jpeg, VipsImage *out, gboolean header_only )
 		if( read_jpeg_header( jpeg, out ) )
 			return( -1 ); 
 
+		/* Patch in the correct size.
+		 */
+		out->Xsize = jpeg->output_width;
+		out->Ysize = jpeg->output_height;
+
 		/* Swap width and height if we're going to rotate this image.
 		 */
 		if( jpeg->autorotate ) {