Openg GL ES draw offscreen to provide the contents for a CALayer

Is it is possible use Open GL ES to draw offscreen to create a CGImageRef to use as content for a CALayer.

I intend to alter the image only once. In detail I'm looking for an efficient way to change onl开发者_Python百科y the hue of an image without changing the brightness as well. The other solution might be to create a pixel buffer and to change the data directly but it seems computationally expensive.

Although it's not something I've done, it should be possible.

If you check out the current OpenGL ES template in Xcode, especially EAGLView.m, you'll see that the parts that bind the OpenGL context in there to the screen are:

• line 77, [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer *)self.layer];, which tells the CAEAGLLayer to provide sufficient details to the framebuffer object there being created so that it can be displayed on screen.
• line 128, success = [context presentRenderbuffer:GL_RENDERBUFFER];, which gives the CAEAGLLayer the nod that you've drawn a whole new frame and it should present that when possible.

What you should be able to do is dump the CAEAGLLayer connection entirely (and, therefore, you don't need to create a UIView subclass), use glRenderbufferStorage or glRenderbufferStorageMultisampleAPPLE to allocate a colour buffer for your framebuffer instead (so that it has storage, but wherever OpenGL feels like putting it), do all your drawing, then use glReadPixels to get the pixel contents back.

From there you can use CGDataProviderCreateWithData and CGImageCreate to convert the raw pixel data to a suitable CGImageRef.

The GPU stuff should be a lot faster than you can manage on the CPU normally, but your main costs are likely to be the upload and the download. If you don't actually need it as a CGImageRef other than to show it on screen, you'll be better just using a CAEAGLLayer toting UIView subclass. They act exactly like any other view — updating if and when you push new data, compositing in exactly the same way — so there's no additional complexity. The only disadvantage, if you're new, is that most tutorials and sample code on OpenGL tend to focus on setting things up to be full screen, updating 60 times a second, etc, that being what games want.