OpenGL changing color of generated texture - c++

Question

I'm creating a sheet of characters and symbols from a font file, which works fine, except on the generated sheet all the pixels are black (with varying alpha). I would prefer them to be white so I can apply color multiplication and have different colored text. I realize that I can simply invert the color in the fragment shader, but I want to reuse the same shader for all my GUI elements.
I'm following this tutorial: http://en.wikibooks.org/wiki/OpenGL_Programming/Modern_OpenGL_Tutorial_Text_Rendering_02
Here's a snippet:
// Create map texture
glActiveTexture(GL_TEXTURE0);
glGenTextures(1, &map);
glBindTexture(GL_TEXTURE_2D, map);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, mapWidth, mapHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
// Draw bitmaps onto map
for (uint i = start; i < end; i++) {
charInfo curChar = character[i];
if (FT_Load_Char(face, i, FT_LOAD_RENDER)) {
cout << "Loading character " << (char)i << " failed!" << endl;
continue;
}
glTexSubImage2D(GL_TEXTURE_2D, 0, curChar.mapX, 0, curChar.width, curChar.height, GL_ALPHA, GL_UNSIGNED_BYTE, glyph->bitmap.buffer);
}
The buffer of each glyph contains values of 0-255 for the alpha of the pixels. My question is, how do I generate white colored pixels instead of black? Is there a setting for this? (I've tried some blend modes but without success)

Answer 1

Since you create the texture with
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, mapWidth, mapHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
you can either change the GL_RGBA to GL_RED (or GL_LUMINANCE for pre-3.0 OpenGL) or you can create the RGBA buffer and copy the glyph data there.
I.e., you have
glyph->bitmap.buffer
then you do
unsigned char* glyphRGBA = new unsigned char[ curChar.width * curChar.height * 4];
for(int j = 0 ; j < curChar.height ; j++)
for(int i = 0 ; i < curChar.width ; i++)
{
int ofs = j * curChar.width + i;
for(int k = 0; k < 3 ; k++)
glyphRGBA[ofs + k] = YourTextColor[k];
// set alpha
glyphRGBA[ofs + 3] = glyph->bitmap.buffer[ofs];
}
In the code above YourTextColor is unsigned char[3] array with RGB components of the text color. The glyphRGBA array can be fed to glTexSubImage2D.