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Do I have to generate and bind a framebuffer for every renderbuffer I create? Or is there a chance to create renderbuffer only (and map it to a texture or submit somehow to the sahders)?

I just want to render to a one channel buffer to create some mask for later use. I think setting up a complete framebuffer would be overhead for this task.

Thanks.

Nicol Bolas
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Geri Borbás
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2 Answers2

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A renderbuffer is just an image. You cannot bind one as a texture; if you want to create an image to use as a texture, then you need to create a texture. That's why we have renderbuffers and textures: one of them is for things that you don't intend to read from.

Framebuffers are collections of images. You can't render to a rendebuffer or texture; you render to the framebuffer, which itself must have renderbuffers and/or textures attached to them.

You can either render to the default framebuffer or to a framebuffer object. The images in the default framebuffer can't be used as textures. So if you want to render to a texture, you have to use a framebuffer object. That's how OpenGL works.

"setting up a complete framebuffer" may involve overhead, but you're going to have to do it if you want to render to a texture.

Nicol Bolas
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You could use a stencil buffer instead, and just disable the stencil test until you are ready to mask your output.

edit: have a look at the following calls in the opengl docs:

glClearStencil
glClear(GL_STENCIL_BUFFER_BIT)
glEnable(GL_STENCIL_TEST)
glDisable(GL_STENCIL_TEST)
glStencilFunc
glStencilOp

http://www.opengl.org/sdk/docs/man/xhtml/glStencilFunc.xml
http://www.opengl.org/sdk/docs/man/xhtml/glStencilOp.xml
http://developer.nvidia.com/system/files/akamai/gamedev/docs/stencil.pdf?download=1

CuriousGeorge
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  • Whoa. Sound exactly what I need. Can I render to the stencil buffer directly? Simply bind it as a renderbuffer before render (and disable framebuffer)? – Geri Borbás Apr 14 '12 at 16:21
  • basically how you would use this, is disable the color buffer, enable the stencil buffer, then draw some shapes. Those shapes would be drawn to the stencil buffer. After you are done drawing your mask, you would re-enable the color buffer and draw your scene. Your drawing would be masked out depending on the content of the stencil buffer, and which settings you used in glStencilFunc(). glStencilFunc() and glStencilOp() are the two most important functions above, as they determine how the stencil buffer will be updated, and how it will affect your rendering. – CuriousGeorge Apr 14 '12 at 16:31
  • Ah see. Draw mask with stencil enabled, draw content with stencil disabled. stencilFunc is something similar to blendFunc? To set the source/destination multipliers? – Geri Borbás Apr 14 '12 at 16:39