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OpenGL Question about OpenGL initialization for offscreen rendering

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Hi,

Back from a very long break from GameDev, and the forum looks more shiny now :)

I'm having some confusion about OpenGL initialization if I want to render to memory only. I know windowing is handled differently across platforms and so must be the creation of OpenGL context if it is to be used with a window on the screen, but if I only want offscreen rendering, this shouldn't depend on the specific platform I'm on. I did some digging on the web and found that I can create OpenGL context with PBuffer or "X Pixmap" (no idea what that is) on Linux, so that I can indeed avoid dealing with specific windowing APIs, but still this is Linux only and not cross-platform. I'm wondering why this has to be platform dependent (or is there a platform-independent way to do this)?

Thanks!

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It doesn't have to be platform dependent. It's just that pretty much any OpenGL interface you deal with is platform dependent. It has nothing to do with off-screen rendering itself, but the interface you use, and what it offers in terms of off-screen rendering. There are exceptions, like the off-screen renderer from Mesa, which is as platform independent as you can get, but other solutions brings other issues, like lack of hardware rendering for example.

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the way to do this now is by using framebuffer objects. There are some articles on gamedev under the OpenGL section or you could try this one: http://www.songho.ca...ngl/gl_fbo.html


I've read this article before. But it doesn't mention OpenGL initialization, it's only about how to use FBO after OpenGL is ready. I still have to create a valid OpenGL context in order to use FBO, right?

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That's right, as far as i know you always need a context in order to be able to use opengl which is platform dependent. There are wrappers for that though, i think glut does such a thing

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It doesn't have to be platform dependent. It's just that pretty much any OpenGL interface you deal with is platform dependent. It has nothing to do with off-screen rendering itself, but the interface you use, and what it offers in terms of off-screen rendering. There are exceptions, like the off-screen renderer from Mesa, which is as platform independent as you can get, but other solutions brings other issues, like lack of hardware rendering for example.


Thanks Bob, but then I guess the question is, why are those interfaces platform dependent, if they don't have to be? Isn't cross-platform a big advantage of having OpenGL? I just don't quite get the implications, like why bad things like losing hardware rendering can happen when you insist on platform independency.

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That's right, as far as i know you always need a context in order to be able to use opengl which is platform dependent. There are wrappers for that though, i think glut does such a thing


I see. Thanks.

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[quote name='Murdocki' timestamp='1319991447' post='4878596']
That's right, as far as i know you always need a context in order to be able to use opengl which is platform dependent. There are wrappers for that though, i think glut does such a thing


I see. Thanks.
[/quote]

Could you explain how exactly to use glut for this? I tried creating a window and then returning to my own code without going into glutMainLoop(), but there is an icon appearing on my dock that doesn't respond and can't be hidden with glutHideWindow() (supposedly it will only respond after glutMainLoop() gets control). So do I have to give control to glutMainLoop()? I'm using OpenGL for offscreen rendering only, and I'd love to retain my own control flow.

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[quote name='Brother Bob' timestamp='1319990945' post='4878592']
It doesn't have to be platform dependent. It's just that pretty much any OpenGL interface you deal with is platform dependent. It has nothing to do with off-screen rendering itself, but the interface you use, and what it offers in terms of off-screen rendering. There are exceptions, like the off-screen renderer from Mesa, which is as platform independent as you can get, but other solutions brings other issues, like lack of hardware rendering for example.


Thanks Bob, but then I guess the question is, why are those interfaces platform dependent, if they don't have to be? Isn't cross-platform a big advantage of having OpenGL? I just don't quite get the implications, like why bad things like losing hardware rendering can happen when you insist on platform independency.
[/quote]
OpenGL is cross platform, and that is its advantage, yes. But there is much more around OpenGL, because OpenGL by itself, without any side-support, is pretty much useless. OpenGL has to have a buffer to render to, and that buffer has to be provided by something, but it's not provided by OpenGL. That is why you have platform dependent layers, like wgl and glX, that provides the layer between the application and OpenGL and provides the necessary side-support for making OpenGL useful. The OpenGL code is platform independent, but the layer between your application and OpenGL may not be.

Hardware support is typically provided by vendor-specific drivers. As long as you only find hardware drivers for Microsoft's own wgl-layer, you will likely not have hardware acceleration on other layers like Mesa's wgl-layer or off-screen renderer. Mesa does have some hardware rendering capabilities, but not for the off-screen renderer which is software only as far as I remember.

Whether these layers are platform dependent or not is entirely up to vendor of the layer. There is nothing that prevents someone from making an layer that is supported by many platforms, and indeed Mesa has such a layer, so it is possible. But why would Microsoft provide platform support for its wgl-layer on all platforms, when they are only concerned with its own platform? They have no reason to do so, and if hardware manufacturers make drivers for Microsoft's layer but not Mesa's layer, the one will have hardware rendering and one will not.

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Thank you very much for the explanation Bob. That clears it up a lot for me.

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