How do I get a colored texture brush to show up in Open GL ES on white background?

Answer 1

Ok, so you're trying to "paint" a given colour (in this case "red") on to a background, using a mask for the brush shape.

You need to do the following before you start rendering the "paint":

1. First make sure your brush has an alpha channel that corresponds with its shape - that is the alpha channel should look similar to the brush image you posed.

2. Render with these states set (note space to get around wiki markup):

// Make the current material colour track the current color
glEnable( GL_COLOR_MATERIAL );

// Multiply the texture colour by the material colour.
glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE );

// Alpha blend each "dab" of paint onto background
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );

See also:
http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/gl/colormaterial.html

http://www.khronos.org/opengles/documentation/opengles1_0/html/glTexEnv.html

Answer 2

Hi Ben,

I'm looking also for this solution, but I can't get the right result using the sample application "GLPaint". Can you share the part of your code where you setup the OpenGL environment ?

Answer 3

I'm in the same boat as strange99. I'm completely new to OpenGL, and I'm trying draw on a white background by reverse engineering GLPaint. I've done what you've suggested above, plus I've gone through every combination of glBlendFunc, AND just about every combination I can think of for brush texture (black on white, white on black, white on trans, alias/no alias, etc).

The best I've been able to achieve is with a white-on-trans circle, with glBlendFunc as above (GL_ SRC_ ALPHA, GL_ ONE_ MINUS_ SRC_ ALPHA), but this still gives me a dull colour, and the semi-trans outer bits are interpreted as black. It's as though it still assumes I'm on a black background.

Any advice?