If your 3D image “floats”: Test your coordinate system
Floating in LED and projection VR: why screen size and tracking origin must be accurate to the millimetre — and how to verify them with a glasses quick test.
Why objects “float” in space
You see it far too often — and it ruins the 3D effect: poorly calibrated coordinate systems. In projection-based or LED-based VR systems, the relative position of the screens in the tracking coordinate system must be accurate to the millimetre.
If it is, objects appear to stay fixed in space — you can move around them or interact with them using tracking. If it isn’t, objects float and seem to move as soon as the viewer moves.
Why tracking is so critical for screen-based VR is also covered in Why good tracking is essential for stereo LEDs and projections.
What has to be right — and why it is not trivial
Two things must be exact; neither is as straightforward as it first appears.
1. Screen size and right angles
LED screens: A tape measure or the datasheet is enough. LED walls are rectangular with right angles and exact dimensions.
Projections: Rectangularity must come from proper setup or keystone correction. Size is where the first pitfall appears: To get a sharp edge, rear-projection images are often aligned slightly too large on the screen so the frame crops a clean border. Software still assumes the full image fills the measured screen size — while only part of it is actually visible.
Better method:
- Create a test image with a frame at 80 % of the image size.
- Calculate the true image size from that frame.
- Also measure the image centre in the test image — it is not necessarily exactly at the centre of the frame.
2. Tracking origin and parallelism to the screen
The only reliable approach: measure carefully and, if needed, fine-tune with Room Adjustment in DTRACK until the coordinates are exact. If tracking can “see” the screen, a tracked target helps: use Body Adjustment to move the measurement position onto a marker.
Background on room vs. body calibration: Tracking quality dropping? → Room calibration first. Practical target tips: Target calibration in DTRACK.
Quick test: distortion cones at the stereo glasses
A geometric effect of the 3D rendering tells you whether the alignment is correct.
When the eye point approaches the display, the image becomes severely distorted. Usually unwanted — here useful:
- Run a 3D scene with tracking active.
- Move the stereo glasses slowly toward the screen.
- The image distorts into cones pointing toward the eyes.
- When the glasses pass through the image plane, the image jumps to the opposite side.
The closer to the screen, the stronger the distortion must become. The test covers all three dimensions: the cones must point exactly at both eyes — at the centres of the lenses. If they miss left/right or high/low, that origin axis is wrong.
Fine check: Deliberately shift depth by 5 cm and verify that the jump occurs 5 cm in front of the screen.
Takeaway
This quick test shows immediately whether the coordinate system is correctly placed. Precise alignment increases realism and reduces fatigue in longer sessions — especially in CAVEs, powerwalls, and mixed-reality setups.