Re: Subtracting two images in a filter

On Tue, May 16, 2017 at 11:57 AM, Paul LeBeau <paul.lebeau@gmail.com> wrote:

> Ah yes. I forgot about that change.
>
> It was actually "exclusion" that I was trying to emulate.  Looks like it
> works in Chrome and FF at least.
>
>     https://jsfiddle.net/fg2wv22r/1/
>

This should be working on every modern browser.


>
> On 17 May 2017 at 06:20, Amelia Bellamy-Royds <
> amelia.bellamy.royds@gmail.com> wrote:
>
>> 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
>>>
>>
>>
>