I cheat.

I used MS Paint and made a bunch of quick smooth gradients and saved them to use in Tux. http://www.youtube.com/watch?v=Wc0JAYoU6aM

For Tux, maybe you could Imagine, not a fill, but a super size gradient paintbrush, bigger than the page diagonal; left click to pick one end's current color; right click to pick the other end color thru transparent value then swipe a split font (gradient) across the picture, any angle.  Any easier?

Tux and Paint make good partners

From: Bill Kendrick <nbs@sonic.net>
To: Discussion list for parents and teachers using Tux Paint <tuxpaint-users@lists.sourceforge.net>
Cc: LGallaz <LGallaz@gmail.com>
Sent: Fri, March 11, 2011 5:12:23 PM
Subject: Re: [Tuxpaint-users] A simple questiion

On Mon, Mar 07, 2011 at 03:27:46AM -0500, LGallaz wrote:
>    Hello,
>    My question is rather simple, I just try to remember since a week how I
>    can get a vertical gradient on an entire drawing, the best example for
>    that is a dark sky on top with a lighter colour underneath. I obtained
>    this effect a few weeks ago, but I can absolutely not remember how I
>    proceeded successfully!
>    I tried many tools which have an effect on the whole canvas, without
>    obtaining this gradient effect until today.

Hi there (your posts didn't go to the list, since you were not subscribed,
but I've approved them; Cc'ing you, in case you're still not subscribed).

Here's what I do:

(1) Paint a line across the middle of the canvas (your 'horizon')
(2) Use 'Fill' to fill the top half with the general sky color (e.g., blue),
    and to fill the bottom with the general ground color (e.g., purple).
(3) Use the 'Lighten' and 'Darken' Magic tools to 'paint' stripes of the scene
    darker or lighter;  Or you could use the 'Paint' tool with a fuzzy,
    transparent brush (or one of the textured ones).
(4) Use tools like 'Chalk' to 'mix up' the stripes of color
(5) Use 'Smudge' and/or 'Blur' to blend them together

A gradient-based fill tool would be very nice, but we don't really have
a concept of "the other color" at the moment, so I can't really imagine
what the interface would be like to do this... unless it simply did
"current color thru transparent"... in other words, a fill that
'faded out' in some direction.

I put together an example  w/ screenshots, which you can see here:



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