iOS Convert TouchBegan coordinates to OpenGL ES Coordinates - ios

Question

New to OpenGL ES here.
I'm using the following code to detect where I tapped in a GLKView (OpenGL ES 2.0). I would like to know if I touched my OpenGL drawn objects. It's all 2D.
How do I convert the coordinates I am getting to OpenGL ES 2.0 coordinates, which are seemingly -1.0 to 1.0 based? Are there already built in functions to do so?
Thanks.
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
CGRect bounds = [self.view bounds];
UITouch* touch = [[event touchesForView:self.view] anyObject];
CGPoint location = [touch locationInView:self.view];
NSLog(#"x: %f y: %f", location.x, location.y);
}

Answer 1

-1 to 1 is clipping space. If your coordinate space is in clipping space when it displays on the screen, I'd say you forgot to convert the spaces using a projection matrix. If you're using GLKBaseEffect (which I don't recommend later down the road since it tends to memory leak everywhere) then you need to set <baseEffect>.transform.projectionMatrix to a matrix that will convert the space correctly. For example,
GLKBaseEffect* effect = [[GLKBaseEffect alloc] init];
GLKMatrix4 projectionMatrix = GLKMatrix4MakeOrtho(0, <width>, 0, <height>, 0.0f, 1.0f);
self.effect.transform.projectionMatrix = projectionMatrix;
width and height would be the width and height of the device's screen/your GLKView/etc. This is automatically applied to the coordinates you pass in so that you can use normal coordinates ranging from 0 to <width> on the x axis and 0 to <height> on the y axis, with the origin in the lower left corner of the screen.
If you are using custom shaders like I am then you can pass in the projection matrix as a uniform using:
glUniformMatrix4fv(shaderLocations.projectionMatrix,1,0,projection.m)
where projection is the matrix and and shaderLocations.projectionMatrix is the identifier for the uniform-its name, as they say. You then need to multiply your position by the projection matrix.
Once you've converted away from clipping space, either by passing in the matrix manually or setting the correct property on GLKBaseEffect, the only difference between OpenGL space an UIKit space is that the y axis is flipped. I convert touches I receive through the touches methods and gesture recognizers like this.
CGPoint openGLTouch = CGPointMake(touch.x, self.view.bounds.size.height - touch.y);
I'll try my best to clarify if you have any questions but keep in mind I'm relatively new to OpenGL myself. :)