[Gimp-user] Enhancement request: Transparency as a "paintable" color

[Brendan Scott <disposableemail apps opensourcelaw biz>]

Hi All,

I have lodged a request for GIMP to support use of transparency as a color.  By this I mean mainly that when 
using a paint tool, painting with that tool paints only the chosen level of transparency.  The enhancement 
request is here:
https://bugzilla.gnome.org/show_bug.cgi?id=704775

This email is to invite comments on the proposed enhancement.

What?
Ability to tap a palette region with the paintbrush and then "paint" with full transparency/erase.

Why?
This request occurred to me after watching online digital painting tutorials from a couple of different 
sources.  In them, the artist chooses a particular color as their background (typically white) and, rather 
than erasing their mistakes, simply switches to the background color and paints over with it.  I have seen 
several comments on these videos (replicated across the sources) to the effect that this swapping to 
background color rather than using the eraser is a neat trick - mainly because it simplifies the workflow.

How would I use it?
I recently worked on an ecorche
http://brendanlearnstosee.deviantart.com/art/An-onion-skinnable-ecorche-dorsal-view-387530797

To do this I basically had 9 tool presets
- a solid brush, no dynamics in each of 3 sizes (small, medium, large)
- the same brushes but with pressure->opacity dynamics; and
- erasers in each of the three sizes (with no dynamics)

I also had a small palette of colors.

The tool preset tab separates the brushes from the erasers.  So each time I swap from paint to erase and back 
again I spend a little time hunting, and breaking my concentration.

I have a Wacom tablet and the pen had an eraser on the other end from the stylus.  If I spin the pen around, 
it gives me an eraser with the right size and dynamics. However:
- the palm rejection on the tablet often skips while the pen is being spun in my hand, so I can end up in a 
changed window, screen or activity (I am on Linux); and
- the GIMP's interface doesn't seem as responsive to the eraser as to the stylus.

I was specifically creating an onion-skinnable picture, so there had to be multiple layers and each layer had 
to have transparency so I couldn't simply use a background color. Any time that there are multiple 
overlapping layers a transparency brush will be useful.


Implementation
I don't know how it would be implemented or whether transparent should be treated as a color internally.  The main 
point is that the workflow would be simplified if fully transparent were accessible through the color selection 
interface.  If I could add fully transparent as a color to a custom palette, then I could have my "eraser" 
and color selection in the same place. I know that conceptually it may be useful to draw a distinction between an 
eraser and a paintbrush but, in practise erasure is equivalent to painting with transparency.

Also, having transparent as a color is more consistent as the background "color" for layers.

I thinking only of a fully transparent brush, simply because that would be my use case. There would probably 
also be a use case for partial levels of transparency (this is already sort of implemented through eraser 
opacity).

Thanks


Brendan