[cairo] Operator optimization based on source or dest opacity

Soeren Sandmann sandmann at daimi.au.dk
Fri Feb 29 19:24:52 PST 2008

Hi Antoine,

> >- I really don't think it's necessary to have a separate 
> >pixman_transformed_image_is_opaque(). For destination images, the
> >   transform and filter are always going to be NULL, so the 
> > additional check is very quick.
> I'll trust you on this one, but where is this specified?

Not specified as such, but it's just not a useful thing to do, so
people won't. I can't imagine cairo ever does, for example. I don't
think it's worth complicating the code for this case.

> >- The if statement in pixman_transformed_image_is_opaque() is
> >difficult to follow for me. It's also broken in at least one case:
> >convolution filters can introduce translucency even when there
> >weren't any to begin with.
> How can a convolution filter introduce translucency in an opaque 
> image that extends?

If the convolution has negative coefficients, then the alpha channel
of the pixel would get something subtracted from it.

> >   Also, I'm not confident that the pi/2 rotation transformation 
> > check is correct. Doesn't it need the rotation to be around the center of
> >   the image?
> Any affine transform other than non pi/2-multiple rotations (and 
> shears) are allowed, as long as we use a nearest filter. Whether the 
> rotation is around the center of the image or not is irrelevant.

Suppose we have an image with REPEAT_NONE, a NEAREST filter, and this
affine transformation:

    [ 1  0 -10 ]
    [ 0  1 -10 ]
    [ 0  0   1 ]

When we fetch (0, 0, 1), we get the source pixel at (-10, -10, 1),
which will be 0 since we have REPEAT_NONE. But the if statement in
your patch will claim that the image is opaque, as far as I can tell.

I am probably missing something here. What is special about
pi/2-multiple rotations? I could see pi-multiple rotations around the
center of the image being special since that would just turn the image
upside down and therefore not introduce any new blank pixels.


More information about the cairo mailing list