这应该有效:

glEnable(GL_BLEND);
// Use a simple blendfunc for drawing the background
glBlendFunc(GL_ONE, GL_ZERO);
// Draw entire background without masking
drawQuad(backgroundTexture);
// Next, we want a blendfunc that doesn't change the color of any pixels,
// but rather replaces the framebuffer alpha values with values based
// on the whiteness of the mask. In other words, if a pixel is white in the mask,
// then the corresponding framebuffer pixel's alpha will be set to 1.
glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO);
// Now "draw" the mask (again, this doesn't produce a visible result, it just
// changes the alpha values in the framebuffer)
drawQuad(maskTexture);
// Finally, we want a blendfunc that makes the foreground visible only in
// areas with high alpha.
glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
drawQuad(foregroundTexture);

这相当棘手，所以如果有任何不清楚的地方，请告诉我.

This is fairly tricky, so tell me if anything is unclear.

不要忘记请求 alpha 缓冲区创建 GL 上下文.否则有可能在没有 alpha 缓冲区的情况下获得上下文.

Don't forget to request an alpha buffer when creating the GL context. Otherwise it's possible to get a context without an alpha buffer.

在这里，我做了一个插图.

Here, I made an illustration.

自从写下这个答案，我了解到有更好的方法来做到这一点:

Since writing this answer, I've learned that there are better ways to do this:

• 如果您仅限于 OpenGL 的固定功能管道，请使用纹理环境
• 如果您可以使用着色器，请使用片段着色器.

此答案中描述的方法有效，并且在性能上并不比这 2 个更好的选项差，但不够优雅且不那么灵活.

The way described in this answer works and is not particularly worse in performance than these 2 better options, but is less elegant and less flexible.