projectScreenPointToPlane Issue

Hi,

I recently have been learning the iOS side of the Vuforia SDK (I started with Android). I am trying to replicate an App I made on Android, and in this app I use the projectScreenPointToPlane function to translate a touch in 2D screen space to the 3D AR Target space. I then check the translation and determine if it is within a certain area on the target.

I have this functionality working perfectly on Android. I mirrored my implementation in my iOS project. However, when I print the translated values of my touch in iOS ( intersection.data[0] and intersection.data[1] ), the values are inconsistent.

For instance, if I positon my device to view the target straight on, I get a translated value A [ ie: (0.0, 0.0) ] when I touch the center of the target through my screen. But as I move the device, the translated coordinates are skewed. If I view the target with my device from a greater distance/different angle and touch the same point on the target through the screen, I receive a different translated value B [ ie: (50.0, -20.0) ]. The translation value becomes so skewed from different view points that it is impossible to run it through any sort of conditionals.

I didn't have this problem with Android, is there something I am missing here? I would really like to resolve this issue, as I am frustrated that what I thought I understood now suddenly doesn't work for iOS. 

- Alex

PS: Here is the function from the code provided in the Video Playback sample. In my app, I simply changed the conditional statement to check for a smaller range than the entire target:

 

- (int)tapInsideTargetWithID

{

    QCAR::Vec3F intersection, lineStart, lineEnd;

    // Get the current projection matrix

    QCAR::Matrix44F projectionMatrix = [vapp projectionMatrix];

    QCAR::Matrix44F inverseProjMatrix = SampleMath::Matrix44FInverse(projectionMatrix);

    CGRect rect = [self bounds];

    int touchInTarget = -1;

   

    // ----- Synchronise data access -----

    [dataLock lock];    

    // The target returns as pose the centre of the trackable.  Thus its

    // dimensions go from -width / 2 to width / 2 and from -height / 2 to

    // height / 2.  The following if statement simply checks that the tap is

    // within this range

    for (int i = 0; i < NUM_TARGETS; ++i) {

        SampleMath::projectScreenPointToPlane(inverseProjMatrix, videoData[i].modelViewMatrix, rect.size.width, rect.size.height,

                                              QCAR::Vec2F(touchLocation_X, touchLocation_Y), QCAR::Vec3F(0, 0, 0), QCAR::Vec3F(0, 0, 1), intersection, lineStart, lineEnd);        

        // print out the translated values

        NSLog(@"---->intersection[%i]: x: %f  y: %f  z:%f", i, intersection.data[0], intersection.data[1], intersection.data[2]);  

        if ((intersection.data[0] >= -videoData[i].targetPositiveDimensions.data[0]) && (intersection.data[0] <= videoData[i].targetPositiveDimensions.data[0]) &&

            (intersection.data[1] >= -videoData[i].targetPositiveDimensions.data[1]) && (intersection.data[1] <= videoData[i].targetPositiveDimensions.data[1])) {

            // The tap is only valid if it is inside an active target

            if (YES == videoData[i].isActive) {

                touchInTarget = i;

                break;

            }

       }

    }