OpenGL ES examples that don't use EAGLContext - ios

Question

I'd like to better understand the creation, allocation, and binding of OpenGL ES framebuffers, renderbuffers, etc under iOS. I understand that the EAGLContext and EAGLSharegroup classes normally manage the allocation and binding of such objects. However, the apple docs suggest that it is possible to do GL offscreen rendering without using the EAGLContext class and I'm interested in how. Does anyone have any pointers to code examples?
I would also be interested in examples showing how to accomplish offscreen rendering with EAGLContext.

Answer 1

The only way to render content using OpenGL ES on iOS, offscreen or onscreen, is to do so through an EAGLContext. From the OpenGL ES Programming Guide:
Before your application can call any OpenGL ES functions, it must
initialize an EAGLContext object and set it as the current context.
I think the following lines might be what are causing some confusion:
The EAGLContext class also provides methods your application uses to
integrate OpenGL ES content with Core Animation. Without these
methods, your application would be limited to working with offscreen
images.
What that means is that if you want to render content to the screen, you use some extra methods only provided by the EAGLContext class, such as -renderbufferStorage:fromDrawable:. You still need an EAGLContext to manage OpenGL ES commands even if you're going to draw offscreen, but these particular methods which are specific to EAGLContext are needed to draw onscreen.
To your second question, how you setup your offscreen rendering will depend on the configuration of this offscreen render (texture-backed FBO, depth buffer, etc.). For example, the following code will set up a simple FBO that has no depth buffer and renders to the already set up outputTexture texture:
glActiveTexture(GL_TEXTURE1);
glGenFramebuffers(1, &filterFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, filterFramebuffer);
glBindTexture(GL_TEXTURE_2D, outputTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, (int)currentFBOSize.width, (int)currentFBOSize.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, outputTexture, 0);
For code examples, you could look at how I do this within the open source GPUImage framework (which just does simple image rendering) or my open source Molecules application (which does more complex offscreen rendering using depth buffers).