Can I have a default Framebuffer without alpha and depth? - opengl

Question

I am looking to save some video card memory by not allocating what I do not use. I am far from running out of memory, but it would feel 'cleaner' to me.
I can't really think of a reason to have an alpha value in the default framebuffer since my window is not going to alpha-blend with my desktop anyway. I was wondering if I could save a few bytes or have more color depth by removing its alpha.
Likewise, I am doing some deferred lighting and all my depth calculations occur in a framebuffer that is not the default one. Then I simply render a quad (two tris) to the default frame buffer with the texture in which I rendered my scene as well as a few GUI elements, none of which requires depth-testing. I call glDisable(GL_DEPTH_TEST) when rendering the default framebuffer, but I wouldn't mind not having a depth buffer at all instead of a depth buffer that I don't use.
Can I do that within OpenGl ? Or within SDL with which I create my OpenGl context ?
I try to create my OpenGl context with the following SDL attributes
sdl.GL_SetAttribute(sdl.GL_DOUBLEBUFFER, 1)
sdl.GL_SetAttribute(sdl.GL_DEPTH_SIZE, 0)
sdl.GL_SetAttribute(sdl.GL_ALPHA_SIZE, 0)
info := sdl.GetVideoInfo()
bpp := int(info.Vfmt.BitsPerPixel)
if screen := sdl.SetVideoMode(640, 480, bpp, sdl.OPENGL); screen == nil {
panic("Could not open SDL window: " + sdl.GetError())
}
if err := gl.Init(); err != nil {
panic(err)
}
Unfortunately, SDL's binding for GoLang lack the sdl_gl_GetAttribute function that would allow me to check whether my wishes are granted.
As I said, there is no emergency. I am mostly curious.

Answer 1

I can't really think of a reason to have an alpha value in the default framebuffer since my window is not going to alpha-blend with my desktop anyway.
That's good, because the default framebuffer having an alpha channel wouldn't actually do that (on Windows anyway).
The framebuffer alpha is there for use in blending operations. It is sometimes useful to do blending that is in some way based on a destination alpha color. For example, I once used the destination alpha as a "reflectivity" value for a reflective surface, when drawing the reflected objects after having drawn that reflective surface. It was necessary to do it in that order, because the reflective surface had to be drawn in multiple passes.
In any case, the low level WGL/GLX/etc APIs for creating OpenGL contexts do allow you to ask to not have alpha or depth. Note that if you ask for 0 alpha bits, that will almost certainly save you 0 memory, since it's more efficient to render to a 32-bit framebuffer than a 24-bit one. So you may as well keep it.
However, since you're using SDL, and the Go binding of SDL, that's up to SDL and it's Go binding. The sdl_gl_SetAttribute function should work, assuming SDL implements it correctly. If you want to verify this, you can just ask the framebuffer through OpenGL:
glBindFramebuffer(GL_FRAMEBUFFER, 0); //Use the default framebuffer.
GLint depthBits, alphaBits;
glGetFramebufferAttachmentParameteriv(GL_DRAW_FRAMEBUFFER, GL_DEPTH, GL_FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE, &depthBits);
glGetFramebufferAttachmentParameteriv(GL_DRAW_FRAMEBUFFER, GL_BACK_LEFT, GL_FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE, &alphaBits);