Introduction

During calculation of geometry, nControl uses the measurements to calculate a model of the projector position, orientation, and lens parameters to best match the measured image positions in space.

To create an unambiguous set of projector parameters, the algorithm needs a set of measurements, ideally

• A large number of measurements
• Accurate positions of sensors
• Measurements that cover most of the image area
• A set of measurements that are not in the same 3D plane

All these ideal conditions cannot always be met. For example, if the screen is flat or only slightly curved, all measurements will be in nearly the same plane in space, since all sensors are in the screen surface, which is nearly flat.

Depending on projection geometry, it may not be possible to get a large number of measurements, or coverage of the entire image area. In these cases, it may be necessary to give some hints to the algorithm to improve the calibration. The "Calibration Options" are essentially hints to the calibration algorithm. These can be found in the "Auto calibration" dialog window.

Description of Calibration Options

• Lock projector position
• When set, the algorithm will assume the projector is already positioned correctly, and adjust other parameters to create the proper calibration. Note: If the position is not correct, there may be an offset in the final calculation.

• Lock projector orientation
• When set, the algorithm will assume the orientation (yaw, pitch, and roll) is already set, but it may adjust other parameters, including moving the position of the projector

• Lock focal length/zoom
• When set, the algorithm assumes that the zoom setting, or throw ratio, is correct as set. Note that even for fixed lenses, there may be tolerances in the actual focal length, that can be compensated by CompactClick. However, when projecting on a flat or near-flat screen, it is recommended to lock this setting. When the option is set, the algorithm will generally try to move the projector to match the measured image size.

• Lock horizontal shift (horizontal shift tolerance) / Lock vertical shift (vertical shift tolerance)
• When set, the lens shift (horizontal/vertical) is locked. Even for fixed shift and on-axis projectors, there may be a slight offset due to tolerances in the optics. If good measurements are made, it is worth trying to uncheck these options to let the algorithm calculate such tolerances.

• Lock lens distortion
• When set, the algorithm will not try to calculate lens distortion. In general, lens distortion calculation requires a large number of measurements, to be able to accurately model this effect. The algorithm will also require a minimum of 10 valid measurements to attempt calibration when this option is set. Note: If a lens distortion adjustment has been defined, the parameters of this lens distortion will not be reset by setting the Lock Lens distortion option, but the values will not be changed.

• Fine tuning only
• This option will use the initial position of the projector (before calibration) as a reference. Any measurements that deviate from the expected position by more than a given tolerance, will be discarded. It is normally recommended to set this option after initial calibration, since future re-calibration will normally be very close to the initial calibration (it is normally safe to assume that the projector does not move much between successive calibrations).

General recommendations

• When calibrating on flat screens, or only slightly curved screens, lock either the projector position, or the focal length/zoom. On a flat screen, it is impossible to distinguish between a wide angle lens at short distance, and a telephoto lens at long distance.
• When only a few measurements are available, lock the lens distortion setting.
• Remember also to set the projector orientation correcly in CompactControl (normal/rotate/mirror etc).
• If calibration fails, or the projector is positioned in what appears to be a "wrong" positionan, try manually moving the projector to a near correct position, then recalibrate.
• If calibration still fails, try to lock parameters such as lens shift and zoom (it may be necessary to reset shift and zoom to nominal values) and calibrate again.