test: Ignore single-bit errors for SVG backend.

The interface of the various buffer/image_diff functions is improved to
provide the maximum pixel difference in addition to the number of pixels
that differ. This value can then be used to compare against a per-backend
tolerance.

Currently I've set the SVG backend's tolerance to 1 to handle some issues
we're currently seeing of single-bit differences on different systems, (but
we're not exactly sure why yet).

Also I improved the image_diff routines to properly report a status value
on failure rather than the bogus value of -1 for pixels_changed.

1 files changed, 14 insertions, 5 deletions

diff --git a/test/imagediff.c b/test/imagediff.c
index 903087745..188c1a979 100644
--- a/test/imagediff.c
+++ b/test/imagediff.c
@@ -32,7 +32,8 @@
 int
 main (int argc, char *argv[])
 {
-    int total_pixels_changed;
+    buffer_diff_result_t result;
+    cairo_status_t status;
 
     unsigned int ax, ay, bx, by;
 
@@ -53,10 +54,18 @@ main (int argc, char *argv[])
 	ax = ay = bx = by = 0;
     }
 
-    total_pixels_changed = image_diff (argv[1], argv[2], NULL, ax, ay, bx, by);
+    status = image_diff (argv[1], argv[2], NULL, ax, ay, bx, by, &result);
 
-    if (total_pixels_changed)
-	fprintf (stderr, "Total pixels changed: %d\n", total_pixels_changed);
+    if (status) {
+	fprintf (stderr, "Error comparing images: %s\n",
+		 cairo_status_to_string (status));
+	return 1;
+    }
+
+    if (result.pixels_changed)
+	fprintf (stderr, "Total pixels changed: %d with a maximum channel difference of %d.\n",
+		 result.pixels_changed,
+		 result.max_diff);
 
-    return (total_pixels_changed != 0);
+    return (result.pixels_changed != 0);
 }