Siarhei Siamashka wrote:
On Friday 11 February 2011 12:30:33 Soeren Sandmann wrote:
If I had a time machine, I would go back and make - among others -
these two changes to Render:
[...]
(2) The RGB channels of Alpha-only images would be considered to be
the same as the alpha channel, and not 0 as they are now. For
example, a 0xb9 pixel in an a8 image would be considered
equivalent to 0xb9b9b9b9 and not to 0xb9000000. That is, they
would be considered a translucent white rather than a translucent
black.
These two changes together would have the effect that (a) the equation
would be much easier to understand visually (composite src and dst,
then clip to the mask and write back), and (b) component alpha would
become completely regular with no need for the "component_alpha" bit
in pictures.
Regarding the (b) part, probably as a side effect of current implementation,
right now it is possible to do some operations with images having
non-premultiplied alpha:
src_img = pixman_image_create_bits (
PIXMAN_x8b8g8r8, width, height, src, stride);
msk_img = pixman_image_create_bits (
PIXMAN_a8b8g8r8, width, height, src, stride);
dst_img = pixman_image_create_bits (
PIXMAN_a8r8g8b8, width, height, dst, stride);
pixman_image_composite (PIXMAN_OP_SRC, src_img, msk_img, dst_img,
0, 0, 0, 0, 0, 0, width, height);
That certainly is useful and I thought it was officially supported by
Cairo (using a 4-channel image as a mask is the same as using the alpha
channel). However I don't see how changing the interpretation of the
color of a one-channel image will make any difference to this.
I agree with Soeren that treating one-channel images as though
r==g==b==a (ie it is a white premultiplied image) is much more useful
than as a black premultiplied image and should be done.
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