Minoru camera parameters

[motters]

> I have some questions about the Minoru camera you are using. What are the parameters of
> the Minoru cam for doing stereo? Especially the parameters as specified in your page:
> focal_length_pixels = distance_mm * disparity_pixels / baseline_mm;

If you're trying different lenses the focal length of the lens will be
given in the specification. I'm not sure what the focal length for
the default lenses is, but it's likely to be in the range 2-3mm. The
baseline distance is 60mm.


> sensor_pixels_per_mm = focal_length_pixels / focal_length_mm;

The CMOS sensor itself measures 3mm across. Assuming that the image
is focussed over the entire sensor, and we're using the default
320x240 resolution, each image pixel corresponds to 3/320 =
0.009375mm. If the focal length is 3mm this of course would mean that
the focal length in pixels was 320.

There are some assumptions here - especially the area of the sensor
upon which the image is focussed. However, we can find this out at
least approximately by experiment if the cameras observe an object at
a known distance, and we manually measure the disparity in pixels, and
we know that the baseline is 60mm.


> And for the lens, which one did you get? I have looked at Mars, and the most
> wide lens I can find is about 120 degrees.

The lenses which I found to work the best were:

Angle of view: 180 degrees
Focal length: 1.8mm
Back focal length: 5.44mm
Format: 1/3", 1/4"
Aperture: F2.0

However, when using these lenses the disparities become smaller as
measured in pixels, due to the wider field of view. This limited the
effective stereo range to maybe 1.5 metres or less. This might be ok
if I could use a higher image resolution, but on my system I'm limited
to 320x240 due to USB bandwidth constraints so I have ended up going
back to using the original lenses.

The ideal solution is to have a longer baseline, which allows you to
get bigger disparities but still use a low image resolution which
enables fast processing. Unfortunately on the Minoru the baseline is
not adjustable, so for my main robot project still uses two separate
USB webcams (Quickcam Pro 9000) spaced 120mm apart. The Minoru is
however being used on my Rodney robot, which only needs short range
vision.

A video of Rodney with the Minoru fitted:

http://www.youtube.com/watch?v=gNRdcwrTOM0