My GLKMathUnproject always returns the geometry that's center-screen

Question

I'm trying to use GLKMathUnproject() to translate screen coordinates to geometry, but my routine always returns whatever geometry is in the center of the screen.

My unproject code looks like this:

+ (GLKVector3) gluUnproject: (GLKVector3) screenPoint
                     inView: (UIView*) view
            withModelMatrix: (GLKMatrix4) model
                 projection: (GLKMatrix4) projection
{
    CGSize viewSize = view.bounds.size;
    int viewport[4];
    viewport[0] = 0.0f;
    viewport[1] = 0.0f;
    viewport[2] =  viewSize.width;
    viewport[3] =  viewSize.height;

    bool success;
    GLKVector3 result = GLKMathUnproject(screenPoint, model, projection, &viewport[0], &success);

    return result;
}

And my call to it looks like this:

- (void) handleSingleTap: (UIGestureRecognizer*) sender
{
    UITapGestureRecognizer *tapper = (UITapGestureRecognizer*) sender;
    CGPoint tapPoint = [tapper locationInView: self.view];

    // Find tapped point on geometry
    MyObject *bd = [worldObjects objectAtIndex: 0];  // the object on which I'm trying to sense the tap.
    // NOTE: bd is planar, along X/Z, with Y=0.  Visually, it's several little
    // ..squares (like a big checkerboard), and I'm trying to determine
    // ..in which square the user tapped.
    // At this point. eyesAt is { someX, y, someZ } and lookAt is { someX, 0, someZ } and
    // ..upVector is { 0, 0, -1 }.  We Zoom our view of the board in/out by
    // ..changing eyesAt.y
    float tapZ = (self.eyesAt.y - self.zNear) / (self.zFar - self.zNear);  // % of the Z-depth.  eyesAt.y = distance to geometry.
    GLKVector3 tapPoint3 = GLKVector3Make(tapPoint.x, tapPoint.y, tapZ);
    GLKVector3 tapAt = [MFglkUtils gluUnproject: tapPoint3 inView: self.view withModelMatrix: bd.modelviewMatrix projection: [self projectionMatrix]];
    // etc., snip -- do stuff with the tapAt point.

The problem is: my gluUnproject always returns the geometric point of whatever is in the middle of the screen, regardless of where I tap.

EDIT: The "always near the center" problem is that the returned X/Z values are always very-near zero, and my geometry is centered on the origin.

It looks as if the problem lies with my tapZ value. My understanding is that this should be a value from 0-1, where 0 means "at the nearZ plane" and 1 means "at the farZ plane" and in-between values are a percentage in between. Experimenting with various tapZ values, I've seen:

    // gives very-very small results
    tapZ = (self.eyesAt.y - self.zNear) / (self.zFar - self.zNear); // % deep through the view frustum 

    // gives Z=0, and very small results
    glReadPixels(viewPoint.x, viewPoint.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &tapZ);

    // gives approximately-correct results
    tapZ = 0.99943;  // found through trial & error

    // gives Z=1, and too-big results
    glReadPixels(viewPoint.x, viewPoint.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &tapZ);
    tapZ = 1. - tapZ;

Clearly, I need to pass a correct tapZ value, but I don't understand how to arrive at it!

score 0 · Answer 1 · answered Jun 07 '13 at 02:31

Ok, whew! It turns out that the problem was my tapZ value, and the problem is related to the fact that depth-buffer Z values are not linear. (More on that whackiness here.)

At any rate, using glProject() to project a piece of the geometry in question up to the screen (remember, I'm dealing with an overhead view of a flat checkerboard!), gives me the z-value I wanted. The end-result code turned out like this:

- (GLKVector3) vectorFromTapPoint: (CGPoint) tapPoint
{
    tapPoint.y = (self.view.bounds.size.height - tapPoint.y);

    BlockDiagram *bd = [worldObjects objectAtIndex: 0];

    // find Z-depth of center of board
    GLKVector3 v3Zero = { 0, 0, 0 };
    int viewport[4];
    glGetIntegerv(GL_VIEWPORT, viewport);
    GLKVector3 worldZero = GLKMathProject(v3Zero, bd.modelviewMatrix, [self projectionMatrix], viewport);

    // Find tapped point on geometry
    float tapZ = worldZero.z;
    GLKVector3 tapPoint3 = GLKVector3Make(tapPoint.x, tapPoint.y, tapZ);
    GLKVector3 tapAt = [MFglkUtils gluUnproject: tapPoint3 inView: self.view withModelMatrix: bd.modelviewMatrix projection: [self projectionMatrix]];

    return tapAt;
}

and gluUnProject was modified to:

+ (GLKVector3) gluUnproject: (GLKVector3) screenPoint
                     inView: (UIView*) view
            withModelMatrix: (GLKMatrix4) model
                 projection: (GLKMatrix4) projection
{
    int viewport[4];
    glGetIntegerv(GL_VIEWPORT, viewport);

    bool success;
    GLKVector3 result = GLKMathUnproject(screenPoint, model, projection, &viewport[0], &success);

    return result;
}

NOTE: asking glGet*() for the viewport gave the correctly oriented (my app is landscape) values, which also matters!

My GLKMathUnproject always returns the geometry that's center-screen

1 Answers1