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### Author Topic: stereo vision questions  (Read 1439 times)

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#### sinista

• Jr. Member
• Posts: 14
##### stereo vision questions
« on: November 22, 2010, 07:34:59 PM »
hey,
Im setting up something like the stereo vision tutorial (http://www.societyofrobots.com/programming_computer_vision_tutorial_pt3.shtml bottom of the page) and have a few questions

1. I know the focal length of the cams, but the image on the image plane, how does this relate to the web cams, its not like I can touch or see it so how do I know where it is?

2. also is there some conversion from he distance that I will receive? I'm expecting a the distance to be in pixels, and if so do I do a DPI conversion to get the distance, if this is the case all i need to do is measure the position of the abject on the screen, but i doubt that this will give a true representation of the distance between the object and the cameras?

#### dunk

• Expert Roboticist
• Supreme Robot
• Posts: 1,086
##### Re: stereo vision questions
« Reply #1 on: November 23, 2010, 06:58:53 AM »
hi Sinista,
you are on the right track assuming the distance will be in pixels early in your calculations.

you don't ever really need to know the exact size of the image plane providing you can calculate the correct multiplier to convert pixels to distance. (inches, mm, meters, whatever you use...)

you are going to have to work this multiplier out experimentally.
just compare the output from your program to the actual distance to work out the multiplier.

dunk.

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: stereo vision questions
« Reply #2 on: November 23, 2010, 08:06:36 AM »
The angular distance between pixels is easily calculated. This is used is astro-imaging:

angular distance (arc-sec) = 206.265 * pixel pitch (um)/lens focal length (mm)

The pixel pitch comes from the data sheet of the imaging sensor.

#### sinista

• Jr. Member
• Posts: 14
##### Re: stereo vision questions
« Reply #3 on: November 23, 2010, 03:14:51 PM »

that what i was thinking dunk, get the figures and convert them according to the actual dist i.e if i get 5 pixels and the dist is 20 cm than i have a scale of 5/20 ( of what ever ) found a few similar posts to this and they all pretty much say the same thing,

waltr, im sorry i dont even get what your saying im not so hot when it comes to maths, can you explain a little further

cheers guys.

#### azy

• Full Member
• Posts: 107
##### Re: stereo vision questions
« Reply #4 on: November 23, 2010, 06:27:00 PM »
cut out a bunch of large square targets with gridlines,place at different distances, find a ruler and hold it up to your screen

the maths may be hard with this kind of thing, but u can beat using stoneage approach when things get brain heavy.

seeing is believing

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: stereo vision questions
« Reply #5 on: November 23, 2010, 07:45:48 PM »
The lens projects the real image (object at some distance) onto the surface of the image sensor. The width (say) of object at some distance can be measure as an angle. It is this angle that can then calculated on the image sensor. That is what the this equation expresses.
angular distance (arc-sec) = 206.265 * pixel pitch (um)/lens focal length (mm)
The units seem to need explanations:
an arc-second is 1/3600 degrees
um is micro-meter
mm is millimeter
206.265 is a conversion factor of the units in the equation.

Does that help?

• Jr. Member
• Posts: 14