Question

I am using OpenGL ES 2.0 and the latest iOS.

I have a 3D model that I wish the user to be able to select different parts of the model by tapping on the screen. I have found this tutorial on converting a pixel-space screen coordinate to a world-space ray, and have implemented a ray-AABB intersection test to determine the intersection portion of the model.

I get some hits on the model at seemingly random sections of the model. So I need to debug this feature, but I don’t really know where to start.

I can’t exactly draw a line representing the ray (since it is coming out of the camera it will appear as a point), so I can see a couple of ways of debugging this:

1. Check the bounding boxes of the model sections. So is there an easy way with OGL ES to draw a bounding box given a min and max point?

2. draw some 3D object along the path of the ray. This seems more complicated.

3. Actually debug the raycast and intersection code. This seems like the hardest to accomplish since the algorithms are fairly well known (I took the intersection test straight ouf of my Real-Time Collision Detection book).

If anyone can help, or wants me to post some code, I could really use it.

Here is my code for converting to world space:

- (IBAction)tappedBody:(UITapGestureRecognizer *)sender
{
    if ( !editMode )
    {
        return;
    }
    CGPoint tapPoint = [sender locationOfTouch:0 inView:self.view];
    const float tanFOV = tanf(GLKMathDegreesToRadians(65.0f*0.5f));
    const float width = self.view.frame.size.width,
                height = self.view.frame.size.height,
                aspect = width/height,
                w_2 = width * 0.5,
                h_2 = height * 0.5;

    CGPoint screenPoint;
    screenPoint.x = tanFOV * ( tapPoint.x / w_2 - 1 ) / aspect;
    screenPoint.y = tanFOV * ( 1.0 - tapPoint.y / h_2 );

    GLKVector3 nearPoint = GLKVector3Make(screenPoint.x * NEAR_PLANE, screenPoint.y * NEAR_PLANE, NEAR_PLANE );
    GLKVector3 farPoint = GLKVector3Make(screenPoint.x * FAR_PLANE, screenPoint.y * FAR_PLANE, FAR_PLANE );

    GLKVector3 nearWorldPoint = GLKMatrix4MultiplyVector3( _invViewMatrix, nearPoint );
    GLKVector3 farWorldPoint = GLKMatrix4MultiplyVector3( _invViewMatrix, farPoint );

    GLKVector3 worldRay = GLKVector3Subtract(farWorldPoint, nearWorldPoint);
    NSLog(@"Model matrix: %@", NSStringFromGLKMatrix4(_modelMatrix));
    worldRay = GLKVector4Normalize(worldRay);

    [male intersectWithRay:worldRay fromStartPoint:nearWorldPoint];

    for ( int i =0; i < 3; ++i )
    {
        touchPoint[i] = nearWorldPoint.v[i];
    }
}

And here’s how I get the matrices:

- (void)update
{
//  _rotation = 0;

    float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
    GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, NEAR_PLANE, FAR_PLANE);

    self.effect.transform.projectionMatrix = projectionMatrix;
    GLKMatrix4 baseModelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -5.0f);

    // Compute the model view matrix for the object rendered with ES2
    _viewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, 0.0f);
    _modelMatrix = GLKMatrix4Rotate(baseModelViewMatrix, _rotation, 0.0f, 1.0f, 0.0f);
    _modelViewMatrix = GLKMatrix4Rotate(_viewMatrix, _rotation, 0.0f, 1.0f, 0.0f);
    _modelViewMatrix = GLKMatrix4Multiply(baseModelViewMatrix, _modelViewMatrix);

    _invViewMatrix = GLKMatrix4Invert(_viewMatrix, NULL);
    _invMVMatrix = GLKMatrix4Invert(_modelViewMatrix, NULL);

    _normalMatrix = GLKMatrix3InvertAndTranspose(GLKMatrix4GetMatrix3(_modelViewMatrix), NULL);

    _modelViewProjectionMatrix = GLKMatrix4Multiply(projectionMatrix, _modelViewMatrix);

    male.modelTransform = _modelMatrix;

    if ( !editMode )
    {
        _rotation += self.timeSinceLastUpdate * 0.5f;
    }
}

Answer 1

I’m not sure why this fixed the problem I was having but changing

screenPoint.x = tanFOV * ( tapPoint.x / w_2 - 1 ) / aspect;

to

screenPoint.x = tanFOV * ( tapPoint.x / w_2 - 1 ) * aspect;

and

GLKVector4 nearPoint = GLKVector4Make(screenPoint.x * NEAR_PLANE, screenPoint.y * NEAR_PLANE, NEAR_PLANE, 1 );
GLKVector4 farPoint = GLKVector4Make(screenPoint.x * FAR_PLANE, screenPoint.y * FAR_PLANE, FAR_PLANE, 1 );

to

GLKVector4 nearPoint = GLKVector4Make(screenPoint.x * NEAR_PLANE, screenPoint.y * NEAR_PLANE, -NEAR_PLANE, 1 );
GLKVector4 farPoint = GLKVector4Make(screenPoint.x * FAR_PLANE, screenPoint.y * FAR_PLANE, -FAR_PLANE, 1 );

seems to have fixed the raycasting issue. Still not sure why my view matrix seems to be indicating that the camera is looking down the positive Z-axis, but my objects are translated negative along the z axis