OpenGL - Textures - Replacing black (white) with color

Question

I am generating a bitmap for a text display with OpenGL using Cairo/Pango. I generate the bitmap as RGBA with a transparent background and with the text in either black or white. (Let's assume black.)

If I load this bitmap as an OpenGL texture and display it, it appears as black text or white text, as expected.

I'd like to be able to color the text using only the original texture, but with OpenGL taking care of the coloring.

Preferably, I'd like to use glColor to set the color, but I'm willing to use glBlendColor or GL_TEXTURE_ENV_COLOR.

However, I can't get any of those options to work.

I've try what seem like innumerable combinations of - white text or black text - enabling blending or using GL_TEXTURE_ENV_MODE's GL_BLEND - trying GL_MODULATE, GL_REPLACE, and GL_COMBINE - trying various differnt glBlendFunc combinations

I've been searching online and reading the spec for a few hours and I'm really at the end of my rope.

Can anyone point me to the right place to get the answer to this?

score 3 · Accepted Answer · answered Apr 23 '11 at 07:14

3

The simplest way to go is to have your text bitmap with white text and transparent background.

Then, to color it, you have to make sure that TEXTURE_ENV_MODE is set to MODULATE :

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

With this, the draw color will be glColor * textureColor == glColor (as textureColor is white)

Then you have to enable blending to handle your transparent background :

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

or, if your bitmaps are in premultiplied alpha form :

glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

answered Apr 23 '11 at 07:14

rotoglup

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This answer helped a lot. It helped me find the biggest problem, which is pretty much that I'm an idiot... normally when I use glColor I'll do something like glColor(1.,0.,0.,0.) for opaque red. That's what I was doing before... but that final 0. was setting the alpha on the entire texture to 0. (because it was being multiplied.) I really need to use glColor(1.,0.,0.,1.) so that whatever the alpha from the texture was would take effect. Wow I feel dumb. – Jay McCarthy Apr 23 '11 at 15:02

score 1 · Answer 2 · answered Apr 23 '11 at 09:44

1

Instead of supplying a full RGBA set, let the text just determinine the non-/transparency, i.e. just supply an alpha channel. Then use the normal glColor to set the text color.

answered Apr 23 '11 at 09:44

datenwolf

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and how to do this? This is exactly what I'm searching for – Prokop Hapala Oct 09 '16 at 10:06
@ProkopHapala: Rasterize the glyphs into a grayscale format and upload it to a GL_ALPHA format texture (fixed function OpenGL) or GL_RED format texture and in the fragment shader assign the R component of the texture to the A component of the output fragment. – datenwolf Oct 09 '16 at 12:19
aha, OK, using fragmet shader It may be easy (although I did not tested), but I'm trying to do it now old-fashined GL<2.0 way. If you have some idea see [this question](http://stackoverflow.com/questions/39943456/using-single-channel-texture-opengl-2) – Prokop Hapala Oct 09 '16 at 12:26

score 0 · Answer 3 · answered Apr 23 '11 at 06:45

It sounds like you can create the bitmap as you see fit. If that's the case, instead of black on white, I'd draw the text into the alpha channel. Then to color the text, you draw a rectangle (or whatever) of the color you want. Then draw your texture over it. Where the texture is opaque, you'll get the texture color. Where the texture is transparent, the background color will show through.

To do that, you will have to set up glBlendFunc. The usual should be fine:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

You also have to turn on blending for texturing:

glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_BLEND);

OpenGL - Textures - Replacing black (white) with color

3 Answers3