While there are great improvements in the brush engine in Gimp 2.4, such as scaling and jitter, it still lack the native ability to rotate a brush while you paint. You can, however, create an animated brush (.GIH) that track the angle of your motion and uses a different brush image depending on the angle. This can be done by duplicating a brush, along with a bunch of rotated copies of itself, and saving it out as a .GIH animated brush. Unfortunately, this is a pain in the butt to manually set up an rotate. I was having troubles with GAP (which I was told would do what I wanted). I also couldn't get python-fu working on my windows machine so this plugin[?] wouldn't work for me.
There is a way to script for Gimp using tiny-fu, which is a small scheme interpreter, but I had never used scheme before. (Very RPN'esque, by the way.) After banging my head on it for a while, figuring out how the Gimp procedure s can be accessed, and generally copying allot from other plugins, I finished my plugin. The download link is at the bottom of the page.
Just start with an image oriented in the "drawing upwards" direction:
and run the script. With the defaults it will create a new image having all the rotated copies:
Now save it as a .gih in your brush folder, and refresh the brush list.
Save Brush Dialog
Here is a sample drawing with this brush. The right squiggle uses tablet pressure sensitivity to change the size.
You have to start with a square for each brush image, but the script only rotates a circle, so try to keep the image inside a circle that would fill the square canvas to avoid clipping. To make a colour (RGB) brush, start with a RGBA image with transparency, and check off the option in the dialog. For a normal brush, start with a greyscale image with no alpha. White will end up transparent, black will be opaque and greys will end up partially transparent.
It will also rotate multiple images if you start with them stacked vertically, and tell the script how many images you have:
Start with this
End up with this!
Here is how to set up the save dialog to follow the angle and alternate between the rows when drawing:
and a sample. (I toggled on "Use Color from Gradient" in the paint tool dialog in the lower half)...
The save dialog figures it out, as long as you specify the cell size correctly. So if you want to save a 20x20 "cell" (or brush size), you could make the file as as a 1 layer, 20x100 with 5 copies arranged in a column; a 1 layer 100x20 with 5 copies arranged in a row; or a 5 layer 20x20 image.