Question

How to render a texture with alpha?

I have a texture, and need to render it with different alpha values at different locations. Any way to do so? (My texture is GL_RGBA)

If not possible to change alpha value on the fly, I have to create different textures for different alpha levels?

Answer 1

Your texture is GL_RGBA so it has a different alpha value for each texel.

If you want to change the alpha value used for render, I can think of the following methods:

• Change the texture alpha values (not sure if you say that you don’t want to do that).
• Use glColor4f to change the alpha value of the vertices. It will multiply the texture values. You may need to use glEnable(GL_COLOR_MATERIAL) and/or glColorMaterial().
• Use a vertex shader to change the vertex alpha values. It will multiply the texture values.
• Use a fragment shader to change the sampled texture values on the fly.
• Use two texture stages and multiply them. The second one will have the modified alpha values (see glActiveTexture() and friends).
• Use a fragment shader and two (or more) texture stages. This is the coolest!