GLKIT Object Picking

For anyone who has played with OpenGL ES, its really hard work (iOS or Android). I have spent many hours on OpenGL ES 1.0 and 2.0, making my own frameworks etc.

Apple introduced GLKit in iOS 5 if I remembered correctly and it was designed to make the maths side easier. One of the things I struggle with is object picking, this is touching the screen and turning that in to a 3D position in the OpenGL world. Generally there is 2 ways to do this, Colour picking and Ray Picking.
Colour picking is when the user touches the screen, every object is rendered as a different colour and then you detect what colour that person touched to work out where they touched.

Ray Picking, is where you fire off a ray and work out what the first object is that it hits. This requires quite a bit of maths which people have already done for you but in different programming languages.

However, the point of this article is that I discovered that Apple have added functions to help with Ray Picking namely GLKMathUnproject

Code based from this StackOverFlow post

This works out when you touch the floor (self.plane) and then puts self.cube at that position

- (BOOL)didTouchObject: (CGPoint) tapLoc
tapLoc.x *= [UIScreen mainScreen].scale;
tapLoc.y *= [UIScreen mainScreen].scale;

bool testResult;

GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

GLKVector3 nearPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-viewport[3])*-1, 0.0), _baseModelViewMatrix, _projectionMatrix, &viewport[0] , &testResult);
GLKVector3 farPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-viewport[3])*-1, 1.0), _baseModelViewMatrix, _projectionMatrix, &viewport[0] , &testResult);

//farPt = GLKVector3Subtract(farPt, nearPt);

float xDif = (farPt.x – nearPt.x) / 1000;
float yDif = (farPt.y – nearPt.y) / 1000;
float zDif = (farPt.z – nearPt.z) / 1000;

for (int i = 0; i < 100; i ++)
if ((nearPt.x + (xDif * i)) > self.plane.position.x – self.plane.scale.x && (nearPt.x + (xDif * i)) < self.plane.position.x + self.plane.scale.x &&
(nearPt.y + (yDif * i)) > self.plane.position.y – self.plane.scale.y && (nearPt.y + (yDif * i)) < self.plane.position.y + self.plane.scale.y &&
(nearPt.z + (zDif * i)) > self.plane.position.z – self.plane.scale.z && (nearPt.z + (zDif * i)) < self.plane.position.z + self.plane.scale.z)

NSLog(@”Hit cube”);
self.cube.position = GLKVector3Make(nearPt.x + (xDif * i), nearPt.y + (yDif * i), nearPt.z + (zDif * i));
return YES;


return NO;


3D Stereoscopic demo for iOS

So the main reason I created a GitHub account was to upload my work on BurfWorld3D (Sorry for the rubbish name).

I have a keen interest in virtual reality and I decided to buy a (they also sent me one as I signed up as a developer.) This allows you to turn your Android/iPhone in to a head mounted display giving the user a virtual reality experience for a small cost. The issue I had was that the Durovis OpenDive SDK is for Unity only and so I started the hunt to find something I could use or convert so that native OpenGL apps could be made.

I also got for Christmas a DuoGamer iPad controller which was made for Gameloft games only, however luckily someone had made their own SDK for it which I found on GitHub. I thought this would be a great way to move around in the 3D space.

So, I created a simple 3D demo where you can walk around buildings, up stairs, etc. Has collision control and gravity, still needs a lot of work but may help others.

2014-03-20 17.52.00

BurfWorld 3D on GitHub

Thanks to mringwal for the Duo Gamer SDK.

Thanks to sphereinabox For the initial OpenGl iOS split screen view for the iPad.