| T O P I C R E V I E W |
| weissman |
Posted - Jan 16 2004 : 2:21:06 PM
Dear Forumers,
I was wondering whether anyone knows the current conversion from stimulus size in pixels to degrees of visual angle when viewing stimuli through the mirror at the 1.5T.
That is, how many screen pixels per degree are there?
The answer will likely vary with the screen's resolution, which is 640 X 480 in my particular case.
Thanks in advance,
Daniel
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| 1 L A T E S T R E P L I E S (Newest First) |
| jim.voyvodic |
Posted - Apr 06 2004 : 2:54:58 PM
The conversion depends on where the screen is placed
as well as the pixel resolution. In CIGAL I convert stimuli
from visual angle to pixels using screen width and height (cms),
screen width and height (pixels), an distance to screen (cm).
alpha = 2 * atan(scrwidcm/scrdist); factor = scrwidpixels/alpha
factor = scrwidpixels/(2 * atan(scrwidcm/scrdist))
The trouble is that the distance to the screen is not fixed
and the active screen area changes with the screen position
and zoom.
But, if you assume the screen is 67 cm from the eyes and the
active viewing area is 33 cm across, then the conversion factors
will be:
640x480 23 pixels/degree
800x600 29 pixels/degree
That's close; if you want to be precise you should put up one
of your stimuli with the screen in the bore and measure for
yourself.
Jim |
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