diff options
Diffstat (limited to 'test')
| -rw-r--r-- | test/bug-seams.c | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/test/bug-seams.c b/test/bug-seams.c index 3ee82c0a..01e84ff0 100644 --- a/test/bug-seams.c +++ b/test/bug-seams.c @@ -26,6 +26,23 @@ /* Exercises a case of seam appearing between two polygons in the image * backend but not in xlib [using pixman]. + * + * The test case draws two abutting quads both individually on the + * leftm combining them with operator ADD, and in one go on the right. + * Both methods should show no signs of seaming at the common edge + * between the two quads, but the individually drawn ones have a + * slight seam. + * + * The cause of the seam is that there are slight differences in the + * output of the analytical coverage rasterization and the + * supersampling rasterization methods, both employed by + * cairo-tor-scan-converter. When drawn individually, the scan + * converter gets a partial view of the geometry at a time, so it ends + * up making different decisions about which scanlines it rasterizes + * with which method, compared to when the geometry is draw all at + * once. Though both methods produce seamless results individually + * (where applicable), they don't produce bit-exact identical results, + * and hence we get seaming where they meet. */ #include "cairo-test.h" @@ -51,6 +68,9 @@ draw (cairo_t *cr, int width, int height) cairo_scale (cr, 20, 20); cairo_translate (cr, 5, 1); + /* On the left side, we have two quads drawn one at a time and + * combined with OPERATOR_ADD. This should be seamless, but + * isn't. */ cairo_push_group (cr); cairo_set_operator (cr, CAIRO_OPERATOR_ADD); @@ -72,6 +92,8 @@ draw (cairo_t *cr, int width, int height) cairo_set_operator (cr, CAIRO_OPERATOR_OVER); cairo_paint (cr); + /* On the right side, we have the same two quads drawn both at the + * same time. This is seamless. */ cairo_translate (cr, 10, 0); cairo_set_source_rgb (cr, 0, 0.6, 0); |