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OpenGL Beginning OpenGL question

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I need someone to explain exactly what this code means. I read the documentation and I am really confused. I am trying to understand some code and here is what is really throwing me for a loop. All of this Binding, and Generating is really confusing the hell out of me. I dont know whats coming or going. I am completely fluent in DirectX programming, but this OpenGL is messing with my head big time only on this. I also noticed that in OpenGL, a backbuffer is never created. Is there a default one created when this is called?
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowSize(width, height);

Does the above code create a Render target with Red, Green, Blue and also a Depth buffer? Does it also automatically bind these for use upon glewInit(); ?


glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glGenFramebuffersEXT(1, &a);// what does a reference now?
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, a);// does this mean a now points to the backbuffer, or what ever is currently bound as a render target?
glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);// does this mean that a is allowed to read from the backbuffer?
glDrawBuffer(GL_COLOR_ATTACHMENT0_EXT);// does this mean a is allowed to draw to the backbuffer?
glFramebufferTextureEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, b, 0);// what exactly does this mean?
glFramebufferTextureEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT1_EXT, c, 0);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);// is this a way to unbind a render target, or is this considered the normal back buffer?

glGenFramebuffersEXT(1, &d);// here we go again with this generating stuff
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, d);
glDrawBuffer(GL_COLOR_ATTACHMENT0_EXT);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);


The Genframebuffer function just dishes out unique id's? Then, you bind those ID's to a render target? Am i close here? I guess it is confusing because I never see any of these FrameBuffers defined. Their size, format, nothing. Then they are used later on in the code.

I realize this is alot of questions. I understand most of it, but a few of these I could not get a nice answer to yet.

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I'll take a stab at what I know:

Quote:
I also noticed that in OpenGL, a backbuffer is never created. Is there a default one created when this is called?
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowSize(width, height);


Yes, this sets up a basic double buffering setup (because of the GLUT_DOUBLE). At this point all draw calls will go into the back buffer, and then the swap command at the end of each frame will switch the buffers. At this point you're pretty much ready to go with rendering. Or at least you'd be ready except for ultra-modern opengl which doesn't allow you to use the default framebuffer anymore. But for most people they'll just start drawing their triangles here.

Quote:

Does the above code create a Render target with Red, Green, Blue and also a Depth buffer?


Yes

Quote:

Does it also automatically bind these for use upon glewInit(); ?

No I don't think so. glewInit only initializes the function pointers for the opengl extensions. These buffers should be the default binding. glew isn't a requirement for opengl, but it makes managing the extension pointers a lot easier (that's a whole 'nother topic there). But for now just call glewInit at the start and don't worry about it.


Quote:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glGenFramebuffersEXT(1, &a);// what does a reference now?

'a' is just an id for an unused framebuffer.

Quote:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, a);// does this mean a now points to the backbuffer, or what ever is currently bound as a render target?

No, the backbuffer still exists as it's own entity seperate from 'a'. A is an 'off-screen' render target. When you're binding 'a' to the GL_FRAMEBUFFER target, you're saying that you want any further operations on the GL_FRAMEBUFFER target to be applied to 'a', as opposed to say another framebuffer that you don't currently have bound.

Quote:

glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);// does this mean that a is allowed to read from the backbuffer?
glDrawBuffer(GL_COLOR_ATTACHMENT0_EXT);// does this mean a is allowed to draw to the backbuffer?

No. The default to glRead/DrawBuffer is the backbuffer in a double buffering setup. This is saying 'anytime I do a read or write command, do the operations on the GL_COLOR_ATTACHMENT0 renderbuffer, not the backbuffer'. I think these might not be necessary in a modern shader pipeline, I think there's another method to map outputs of your fragment shader (glBindFragDataLocation).

Quote:

glFramebufferTextureEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, b, 0);// what exactly does this mean?
glFramebufferTextureEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT1_EXT, c, 0);

This is attaching two color textures to the currently bound framebuffer ('a'). Then when you render to 'a' the results will be burned into these textures ('b' and 'c' are texture id's) instead of a renderbuffer. The shader picks which data goes into which attachment, so you could be burning two separate images in a single pass.

Quote:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);// is this a way to unbind a render target, or is this considered the normal back buffer?

This just means 'I'm done doing things with framebuffer 'a', go back to the default framebuffer (the one provided by the windowing system that you're used to, with the frontbuffer and backbuffer attachments).

Quote:

The Genframebuffer function just dishes out unique id's? Then, you bind those ID's to a render target? Am i close here?

Yes I think that is correct. The term 'render target' is pretty loose here, but you seem to have the right idea.

Quote:

I guess it is confusing because I never see any of these FrameBuffers defined. Their size, format, nothing. Then they are used later on in the code.

This is probably because it is rendering into textures 'b' and 'c', which have probably had this data defined before. A framebuffer is really just a container for attachments (GL_COLOR_ATTACHMENTi, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT), and then each of these attachments either renders into a renderbuffer (for blitting onto the screen), or a texture (for use like any other texture). If you wanted to draw into a renderbuffer you'd define the specifics with glRenderbufferStorage, which has the width/height/pixelformat arguments that you're probably looking for.

It seems overly convoluted but it's pretty flexible, for example this would allow you to, in a single pass of the pixel shader, generate 5 distinct color textures, a stencil texture, while at the same time drawing color and depth onto the screen. Don't know why you would ever need to do that, but if you wanted to you could :)

I hope I've got this mostly correct, but I think the gist of it is there. If you really want to get into the tiny details I'd pick up a 'Red Book', and it has detail about all of these functions and what they do.

Addendum:

One way to think about all these terms that might clarify things is to look at the typical default double buffering setup. In the default, you have:

1 Framebuffer
3 Attachments (GL_FRONT, GL_BACK, and GL_DEPTH_ATTACHMENT)
and each attachment has an associated RenderBuffer.

When you're creating a new framebuffer, it doesn't have any attachments, so that's where you add them (GL_COLOR_ATTACHMENT0 instead of GL_BACK, for example). Then for each attachment you either define a texture target or a render target (glFramebufferTexture or glFramebufferRenderbuffer).

[Edited by - karwosts on November 12, 2010 3:27:02 AM]

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