Transparency, opacity, mask editing (Was: Re: A first part of the layers/masks patch)

[Sven Langkamp]

On Sat, Sep 26, 2009 at 11:49 PM, Cyrille Berger <cberger at cberger.net>wrote:

> On Saturday 26 September 2009, Sven Langkamp wrote:
> > At the moment there is a fundamental difference between Gimp/Photoshop
> and
> > Krita in the way we see masks. In both cases we have and (8-bit) one
> >  channel paint device, so not a technical difference.
> > The actual difference is how these channel is interpreted: In
> >  Gimp/Photoshop is a grayscale representation while in Krita it's an
> alpha
> >  representation.
> >
> > The grayscale way is probably what most artists are used too, even if the
> > meaning of black and white is arbitary. The advantage is that it match
> good
> > with gradient and fill.
> >
> > The way Krita uses is closer to the physical representation either there
> is
> > something (color) or you can look through it. The problem that Dmitry
> > descibes is that the only way to "paint" transparency is the eraser tool
> in
> > Krita, which might be unusual for users form other editors. The bigger
> > problem is that you e.g. can't use a transparent color in the fill tool.
> >
> > I hope that's the correctly summarizes it.
> Ok now I get the problem.
>
> First, I will restate that I don't like the use of color for transparancy:
>
> > I can't remember which color is used usually (e.g. in a "well known
> > graphical editor"). I guess, when we paint with white paint the image
> > becomes opaque, when we paint with black paint - becomes transparent,
> with
> > gray color - becomes semi-transparent.
>
> That's exatly my problem with using color. From a technical point of view,
> it
> makes sense to have black==transparent == 0, white == opaque == 255. But I
> don't see how it can have sense for an user (and from my point of view,
> because photoshop does it isn't a valid answer, we are not aiming at
> cloning
> all its broken behaviour, if it's good lets use it, otherwise lets
> innovate).
>
> Maybe we should reinvestigate the whole transparancy with the color
> selector
> thing. We had that debate see [1] and [2] (around 18:38 in the log).
>
> In our previous discutions on the subject we considered that the choice
> between transparancy and opacity was wether to put the choice in the tool
> or
> in the color selector. But what if, actually, we need both. Because
> currently,
> the opacity is mainly used to control how much of the color is mixed. While
> the transparancy in the color selector could be used to paint with
> transparent
> color, either on a layer (either for filling or not), or on a selection.
>
> I am trying to think about the math for that, and also about wether it
> would
> make sense for an user, and we would probably have to rethink about the
> "opacity" label, to make it have more sense.
>
> [1]
>
> http://wiki.koffice.org/index.php?title=Colordialogs#Opacity_as_part_of_color
> [2] http://lists.kde.org/?l=koffice-devel&m=123060912408849&w=2
>

There is another potential flaw I see when you paint with transparency vs
grayscale (from user point of view):

If you have a completey white/opaque mask in the Gimp system and paint with
black color over it you will get black.

In the Krita system that would mean that you have an opaque mask and paint
with a transparent color over it you would get transparency.
Does that make sense? With the opacity label it would get even more weird:
What is e.g. a transparent color painted with 50% opacity?
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