Re: First draft of the blending and compositing spec

Hi Nikos

2012/6/1 Nikos Andronikos <Nikos.Andronikos@cisra.canon.com.au>

>
> 5 : Even if it's "standard" to use the word destination when we are
> talking about Porter Duff stuff, it make the spec harder to read and
> understand. IMO it could be easier to chose a word ("backdrop" or
> "destination" whatever you prefer) and remain consistent all along the
> spec. It will avoid confusion.
>
>
>
> I'd be happy to go with backdrop everywhere, except the operators have
> standard names that are well known, and those names include destination
> (e.g. dest-over).
>
> This makes it tricky and is why I decided to use the word destination in
> the chapter and define destination as a synonym of backdrop.
>
> I think that's less confusing than talking about backdrop in the text and
> using operator names with destination in them.
>
> I don't think using destination everywhere is the right way to go.
>

That's find with me, I haven't strong feeling about this. In fact the issue
were that the first time I read the draft I miss the part where you explain
that "backdrop" and "destination" were the same thing. Maybe it just needs
to be a bit more clearly stated. Adding a note at the beginning of section
3 would maybe helpful (and a link to that part on each occurrence of the
"destination" word when it matches "backdrop").


> I'm not an expert in compositing so, I do not understand what the 4 pixel
> subregions mean and the spec does not really help to understand this. As a
> consequence, the example looks wired and it's unclear to me why each pixel
> subregion is 25% in the example. I think it needs some clarification (at
> least for dumb author like me).
>
>
>
>
>
> Hmmm, it's not an easy subject.
>
> I do try to explain why each subregion is 25%. Maybe I haven't quite
> succeeded in making it fully approachable =)
>
>
>
> Basically, in the example, because each object has half coverage
> (equivalent to 50% opacity) and there's always four regions, then in this
> case, the regions work out to be a quarter of the pixel each.
>
> It's a theoretical model of the subpixel. The model has no knowledge of
> the actual subpixel layout so it defines a formula for the size of each
> sub-pixel, based on the coverage.
>
> These formula are given in the table below the image.
>
>
>
> Both = αa x αb
>
> Source only = αa(1 - αb)
>
> Destination only = αb(1 - αa)
>
> None = (1 - αa)(1 - αb)
>
>
>
> The sum of all areas equals 1.
>
> So the more coverage each shape has, the more influence that shapes colour
> has to the final pixel value.
>
> And if both shapes have high coverage then the combined area (both) has
> high influence and the individual areas (source, dest and none) have low
> influence.
>
>
>
> This has some limitations and isn't always accurate, but it works pretty
> well.
>
> To give you an example of an inaccuracy, if the shapes have half coverage
> each and abut without overlapping, the actual subpixel layout would look
> like
>
>
>
> +-------+
>
> |   |   |
>
> | A | B |
>
> |   |   |
>
> +-------+
>
>
>
> A is the area of the source shape
>
> B is the area of the second shape
>
>
>
> So you'd only really have 2 regions with any area and not four, but the
> model doesn't go into this level of detail.
>
> The subpixel layout is only theoretical and not based on the actual
> subpixel layout.
>

Thanks for the explanation, I'm still not a compositing expert but I start
to understand the global idea.


> As long as you understand the way the operators work in 5.1, the theory of
> where the subpixel regions come from isn't so vital.
>

Yes I agree, author does not need to understand the whole theory under the
hood, they will look for concrete result. That say, if there is a big part
about this sub-pixel stuff, there is to way to look at it : 1. it is
informational and in that case it should be stated as is. 2. It is
normative and in that case, it's not really necessary to explain all the
theory about it. Only the math that should be computed are relevant in that
case (and the hell to authors in that case, they will relay on section 5.1
;).


> Having said that, I'll go over it again and see if I can make it easier to
> understand.
>
> Maybe a diagram showing some cases would help.
>

Yes, definitely. IMO schema and diagram are always easier to understand
rather than big long boring tl;dr text.

Thank you again
Cheers
-- 
Jeremie
.............................
Web : http://jeremie.patonnier.net
Twitter : @JeremiePat <http://twitter.com/JeremiePat>