[visionegg] questions on eye - screen distance adjustment
- From: "Alexandre Santos" <alexandre.santos@xxxxxxxxxxxx>
- To: visionegg@xxxxxxxxxxxxx
- Date: Fri, 29 Aug 2008 14:32:50 +0200
Hello,
I'm planning to use VisionEgg for my experiments, and the script
gratingTCP.py fits nearly all my needs except for one thing: The
distance between the mouse and the screen is not always the same, and
I need to create a new parameter (distance) so that the spatial_freq
parameter remains consistent for different mouse distances from the
screen. This brings me two questions:
1) Which parameter should I adjust?
Initially I wanted to manipulate spatial_freq or size directly with
distance, but according to the documentation (p.9 of
http://visionegg.org/visionegg.pdf) size is expressed in arbitrary eye
coordinate units and converted to pixel coordinates through a
Projection class. If so, it makes more sense to manipulate this
projection class with the distance parameter than size or spatial_freq
which refer to arbitrary eye coordinates. The problem is that while
reading the SinGrating2D source code I saw no reference to this
Projection class, it seems that the documentation is either outdated
or not yet implemented. Unless I completely misunderstood the code,
the projection is directly calculated in SinGrating2D itself through
the floating_point_sin variable. Should I adjust for distance at the
following line of code?
floating_point_sin =
numpy.sin(2.0*math.pi*p.spatial_freq*numpy.arange(0.0,w,inc,dtype=numpy.float)+(phase/180.0*math.pi))*0.5*p.contrast+p.pedestal
2) How to implement the change?
Unfortunately I'm learning python as I learn to use VisionEgg, so this
question is more python than VisionEgg related - sorry for the bother:
once I figure out how to correct spatial frequency for distance I
could simply copy SinGrating2D to gratingTCP.py and modify it
accordingly. But this solution is cumbersome and not very sustainable.
Since SinGrating2D is an object, I guess the best would be to use a
new object which would inherit from SinGrating2D, but with the added
distance parameter. Would the following syntax work?
gratingTCP.py:
[snip]
# Create a new class modified from SinGrating2D
class ModSinGrating2D(SinGrating2D):
"""
Modified SinGrating2D allowing to enter mouse distance from screen in cm
"""
def __init__(self, distance = None):
"""
if distance not specified don't bother adjusting spatial frequency
"""
self.distance = distance
# the following adjustment is totally wrong, I'm just using it as an example
if self.distance: p.spatial_freq = 1.0 / ( 2.0 * self.distance *
numpy.tan( ( ( 1.0 / p.spacial_freq ) * numpy.pi / 180.0 ) / 2.0 ) *
1024.0 / screen.size )
# Create the instance SinGrating with appropriate parameters
stimulus = ModSinGrating2D(anchor='center')
[snip]
Regards,
Alexandre Santos
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