

Explanation of how to measure scene complexity of a virtual environment (adopted from So, Ho and Lo, 2001)
The use of Spatial Frequency in quantifying scene complexity
The changes of gray scale values along each row and column are then extracted. Figure1 illustrates the extraction of the gray scale values along one row of a snap shot. As shown in the figure, the plot of gray scales across the horizontal pixel space clearly illustrates the changes of scales due to objects in the VE. Fast Fourier Transforms (FFTs) are then applied to this extracted gray scale history and the result is the power spectral density (PSD) function of this gray scale series. Using a method reported in So, Ho and Lo (2001) (called 'Combined method'), the Spatial Frequency (SF) of this row is calculated. This process is then repeated for all the rows and the average SF is the SF_horizontal of this snap shot. When the whole process is repeated for the columns of this snap shot, the average SF_vertical is calculated. Finally, the SF_radial is calculated as the geometric mean of the SF_horizontal and SF_vertical. A program to perform this routine automatically has been developed under the signal processing software MATLABR. A Webenabled demonstration has been set up. Using this demo, users can upload a .bmp or .jpg picture and our server will calculate and email the SF_horizontal, SF_vertical, and SF_radial to you. [NB: The FTP facilitate has to be turned off to avoid frequent hacking problem. Please send your .jpg file with a description of the fieldofview in degree to rhyso@ust.hk and we send you the calculated spatial frequency information.
Step 1 is repeated for many snap shots of the same VE. So far, we have been sampling two snap shots per second through out the entire duration of a VR simulation. This, of course, is oversampling and our MPhil student (YSL) is studying the optimal number of samples needed.
Significant relationships between SFs and levels of cybersickness If you are interested in tring this algorithm, please email your picture to Dr. Richard SO with a description of the fieldofview of the picture. 

