Opengl: Use single channel texture as alpha channel to display text

Question

What I\'m trying to do is load a texture into hardware from a single channel data array and use it\'s alpha channel to draw text onto an object. I am using opengl 4.

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Answer 1

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texture->x, texture->y, 0, GL_RED, GL_UNSIGNED_BYTE, reinterpret_cast(full_texture));

This is technically legal but never a good idea.

First, you need to understand what the third parameter to glTexImage2D is. That's the actual image format of the texture. You are not creating a texture with one channel; you're creating a texture with four channels.

Next, you need to understand what the last three parameters do. These are the pixel transfer parameters; they describe what the pixel data you're giving to OpenGL looks like.

This command is saying, "create a 4 channel texture, then upload some data to just the red channel. This data is stored as an array of unsigned bytes." Uploading data to only some of the channels of a texture is technically legal, but almost never a good idea. If you're creating a single-channel texture, you should use a single-channel texture. And that means a proper image format.

Next, things get more confusing:

new_texture->datasize = texture_width * texture_height*4;

Your use of "*4" strongly suggests that you're creating four-channel pixel data. But you're only uploading one-channel data. The rest of your computations agree with this; you don't seem to ever fill in any data pass full_texture[texture_width * texture_height]. So you're probably allocating more memory than you need.

One last thing: always use sized internal formats. Never just use GL_RGBA; use GL_RGBA8 or GL_RGBA4 or whatever. Don't let the driver pick and hope it gives you a good one.

So, the correct upload would be this:

glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, texture->x, texture->y, 0, GL_RED, GL_UNSIGNED_BYTE, full_texture);

FYI: the reinterpret_cast is unnecessary; even in C++, pointers can implicitly be converted into void*.