How do I get my textures to bind properly for multitexturing? - opengl

Question

I'm trying to render colored text to the screen. I've got a texture containing a black (RGBA 0, 0, 0, 255) representation of the text to display, and I've got another texture containing the color pattern I want to render the text in. This should be a fairly simple multitexturing exercise, but I can't seem to get the second texture to work. Both textures are Rectangle textures, because the integer coordinate values are easier to work with.
Rendering code:
glActiveTextureARB(GL_TEXTURE0_ARB);
glEnable(GL_TEXTURE_RECTANGLE_ARB);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, TextHandle);
glActiveTextureARB(GL_TEXTURE1_ARB);
glEnable(GL_TEXTURE_RECTANGLE_ARB);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, ColorsHandle);
glBegin(GL_QUADS);
glMultiTexCoord2iARB(GL_TEXTURE0_ARB, 0, 0);
glMultiTexCoord2iARB(GL_TEXTURE1_ARB, colorRect.Left, colorRect.Top);
glVertex2f(x, y);
glMultiTexCoord2iARB(GL_TEXTURE0_ARB, 0, textRect.Height);
glMultiTexCoord2iARB(GL_TEXTURE1_ARB, colorRect.Left, colorRect.Top + colorRect.Height);
glVertex2f(x, y + textRect.Height);
glMultiTexCoord2iARB(GL_TEXTURE0_ARB, textRect.Width, textRect.Height);
glMultiTexCoord2iARB(GL_TEXTURE1_ARB, colorRect.Left + colorRect.Width, colorRect.Top + colorRect.Height);
glVertex2f(x + textRect.Width, y + textRect.Height);
glMultiTexCoord2iARB(GL_TEXTURE0_ARB, textRect.Width, 0);
glMultiTexCoord2iARB(GL_TEXTURE1_ARB, colorRect.Left + colorRect.Width, colorRect.Top);
glVertex2f(x + textRect.Width, y);
glEnd;
Vertex shader:
void main()
{
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_TexCoord[1] = gl_MultiTexCoord1;
}
Fragment shader:
uniform sampler2DRect texAlpha;
uniform sampler2DRect texRGB;
void main()
{
float alpha = texture2DRect(texAlpha, gl_TexCoord[0].st).a;
vec3 rgb = texture2DRect(texRGB, gl_TexCoord[1].st).rgb;
gl_FragColor = vec4(rgb, alpha);
}
This seems really straightforward, but it ends up rendering solid black text instead of colored text. I get the exact same result if the last line of the fragment shader reads gl_FragColor = texture2DRect(texAlpha, gl_TexCoord[0].st);. Changing the last line to gl_FragColor = texture2DRect(texRGB, gl_TexCoord[1].st); causes it to render nothing at all.
Based on this, it appears that calling texture2DRect on texRGB always returns (0, 0, 0, 0). I've made sure that GL_MULTISAMPLE is enabled, and bound the texture on unit 1, but for whatever reason I don't seem to actually get access to it inside my fragment shader. What am I doing wrong?

Answer 1

The overalls look fine. It is possible that your texcoords for unit 1 are messed up, causing sampling outside the colored portion of your texture.
Is your color texture fully filled with color ?
What do you mean by "causes it to render nothing at all." ? This should not happen except if your alpha channel in color texture is set to 0.
Did you try with the following code, to override the alpha channel ?
gl_FragColor = vec4( texture2DRect(texRGB, gl_TexCoord[1].st).rgb, 1.0 );

Answer 2

Are you sure the the font outline texture contains a valid alpha values? You said that the texture is black and white, but you are using the alpha value! Instead of using the a component, try to use the r one.
Blending affects fragment shader output: it blends ths fragment color with the corresponding one.