usage: dots width speed direction correlation density_spec density frames
dots produces all the separate frames of a moving-dot stimulus. Dots are computed and tracked within a square frame, but only those that fall within a circular window inscribed in this frame are actually displayed. Frames are output as four-digit filenames with the extension ".bw". An explanation of parameters follows:
| PARAMETER | TYPE | UNITS | |
| width | integer | pixels | |
| speed | float | pixels per frame | |
| direction | float | degrees from right horizontal | |
| correlation | float | percent | |
| density_spec | char | Dots-per-pixel | Number-of-dots | |
| density | float | see density_spec above | |
| frames | float | number of frames |
A final, optional parameter, "snow", indicates that the position of each non-coherent dot is to be reassigned randomly on each frame. If the "snow" option is not present, non-coherent dots move the same distance as coherent dots but in a randomly chosen direction.
speed and direction will suffer from round-off errors due to the discrete nature of pixels.
The density_spec parameter selects one of two methods of interpreting the dot density parameter that immediately follows it on the command line. In the dots-per-pixel mode, the dot density parameter specifies how many dots should be set in the figure per pixel of area. 0.02 is a reasonable value; it gives one dot per fifty pixels. In the number-of-dots mode, the dot density parameter specifies an absolute number of dots in the display. This absolute number is multiplied by the ratio of non-displayed area to displayed area in order to compute the total number of dots in the frame.
To produce output in a format other than SGI's, set the environment variable VISTIM_PLUGIN to the name of a converter that takes the filename of an SGI image as its first parameter and creates an image whose name is its second parameter. An extension for this second image can be specified in the VISTIM_EXT environment variable. For example, to produce PICT images use setenv VISTIM_PLUGIN topict; setenv VISTIM_EXT pct.
Author: Matthew Belmonte
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