>Psychtoolbox>PsychGLImageProcessing

s = VignetCalibration([filename] [, screenId])

Vignetted Luminance calibration procedure for undistortion of distorted
display luminance.

This code was created by Jorrit S. Montijn of the Laboratoire Psychologie
de la Perception at Universite Paris Descartes.

This function allows the user to create a calibration for a vignetted
luminance distortion. It enables the user to choose from either a
Gaussian or Exponential approximation of the luminance vignetting. Users
can change the position and steepness of the luminance fall-off in
horizontal and vertical direction independently. It is also possible to
adjust the minimal luminance.

Once the calibration procedure is finished, the gain matrix (and other
possibly useful parameters and variables) are saved to disk for use by a
vignetting compensation program (see ‘VignettingCorrectionDemo’ for an
example of how to do this).

How to use:

Start the script, providing a filename and screennumber:

‘filename’ is the name of the file to which calibration results will be
saved. If none is provided, a default name will be used. The default file
will be stored in the directory returned by PsychtoolboxConfigDir(‘ShadingCalibration’).

‘screenId’ is the optional index of the screen that you want to
calibrate. If only one screen is present, it is not necessary to include
this parameter.

When started, the program will show either a gaussian or exponential
approximation of the required gain function. You can adjust the
parameters so that the screen will look approximately isoluminant. It is
recommended to use a photometer, since human eyes are far from perfect when
doing this.

You can use the following keys to issue commands:

Escape - use this key to save and exit
Tab - show/hide help text
Space - change the parameter you wish to adjust
‘<’ and ‘>’ - use these keys to change the increment size of the
parameter adjustments
Arrow Keys - adjust parameters (depending on selected parameter)
1. Luminance/Function type
2. Width/Steepness of function
3. Position of centre