From: Søren Sandmann Pedersen <s...@redhat.com> The pixel computed by the convolution filter should be rounded off, not truncated. As a simple example consider a convolution matrix consisting of five times 0x3333. If all five all five input pixels are 0xff, then the result of truncating will be
(5 * 0x3333 * 255) >> 16 = 254 But the real value of the computation is (5 * 0x3333 / 65536.0) * 254 = 254.9961, so the error is almost 1. If the user isn't very careful about normalizing the convolution kernel so that it sums to one in fixed point, such error might cause solid images to change color, or opaque images to become translucent. The fix is simply to round instead of truncate. --- pixman/pixman-bits-image.c | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-) diff --git a/pixman/pixman-bits-image.c b/pixman/pixman-bits-image.c index 085dd16..7787ef1 100644 --- a/pixman/pixman-bits-image.c +++ b/pixman/pixman-bits-image.c @@ -413,10 +413,10 @@ bits_image_fetch_pixel_convolution (bits_image_t *image, } } - satot >>= 16; - srtot >>= 16; - sgtot >>= 16; - sbtot >>= 16; + satot = (satot + 0x8000) >> 16; + srtot = (srtot + 0x8000) >> 16; + sgtot = (sgtot + 0x8000) >> 16; + sbtot = (sbtot + 0x8000) >> 16; satot = CLIP (satot, 0, 0xff); srtot = CLIP (srtot, 0, 0xff); -- 1.7.4 _______________________________________________ Pixman mailing list Pixman@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/pixman