CODEWITHSUNDEEP
iosipadopengl-es
James Edge
James Edge

Reputation: 245

picking code for opengl-es on ios

At the moment I am using snapshot to do my picking. I change the render code to render out object ids, grab the snapshot, then take the value for the pixel under the user tap. I think this is quite inefficient though - and I'm getting reports of slowness on some ipads (my mini is fine).

Is it possible to render to the backbuffer, and use a call of glreadpixels to retrieve only the pixel under the user tap without the object-ids being rendered to the screen? I am using GLKView for my rendering. I've tried glreadpixels with my current code - and it always seems to return black. I know that the documentation for GLKView recommends only to use snapshot, but surely it is more efficient for picking to only retrieve a single pixel.

Upvotes: 0

Views: 173

Answers (2)

James Edge
James Edge

Reputation: 245

Actually it is quite possible to read from the backbuffer, even using GLKView. The documentation states that it is not advised - but after a bit of fiddling I got it to work. The only thing which was a problem is that glreadpixels can only take GL_RGBA as argument (not GL_RGB). So long as you ensure that glClear is called after the picking you will not get object ids rendered to the screen.

Using snapshot to do the picking on an ipad mini slowed down the app 50%. Using glReadPixels leads to no noticable slowdown at all. You could do this by allocating an extra framebuffer - but I don't think it is necessary.

Upvotes: 0

Hugh Fisher
Hugh Fisher

Reputation: 2516

You are correct, a much better way is to render the object ids to the back buffer and read back a particular pixel (or block of pixels).

(If you're doing a lot of selection, you could even use a second offscreen renderbuffer and generate the object ids every frame in a single render pass.)

But you will have to write your own view code to allocate offscreen render buffers, depth buffers, and whatnot. GLKView is a convenience class, a high level wrapper, and the Apple doco specifically says not to mess with the underlying implementation.

Setting up your own GL render buffers isn't too difficult, and there's example code all over the place. I've used the example code on the Apple dev site and from the OpenGL SuperBible.

Upvotes: 1

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