Re: Subtracting two images in a filter

I haven't tested browser support (probably not good), but the CSS filters
spec extends <feBlend> to support all the blend modes from the Blending and
Compositing spec (https://drafts.fxtf.org/compositing-1/#blending),
including "difference".

Would that solve your use case?

I'd prefer to mentally keep <feComposite> for operations that rely on alpha
(like in/out/xor compositing), and `<feBlend>` for color operations in
which reduced alpha means reduced impact.

~ABR

On 16 May 2017 at 10:31, Paul LeBeau <paul.lebeau@gmail.com> wrote:

> Hi all
>
> I've been experimenting with a filter that requires that I subtract two
> inputs. I might be missing something, but there doesn't seem to be a way to
> do that.
>
> The closest you can get, that I can see, is with <feComposite>
>
>     <feComposite operator="arithmetic" in="a" in2="b" k2="1" k3="-1"/>
>
> But because the arithmetic applies to all channels, the alpha channels
> cancel out and you get a blank result (1-1=0).
>
> It seems as if it would be useful if feComposite had a way to preserve
> the alpha, similar to feConvolveMatrix.
>
> Does this idea make sense?  Is it worth creating a feature request to
> discuss this idea?
>
> Paul
>