Distance Traveled

[jramborger78]

Hey guys!

First off, excited to meet most of you in person at the Jackson Lab course this month, super excited.

I had a question. How do you guys go about calculating distance traveled from pixels to cms? I am losing my mind trying to find something that helps take my camera specifications and distance of the rat to get anything concrete. I am kind of surprised how difficult it is to find the little information on this. ChatGPT provided something that seems reasonable but i cant find any sources to verify it.

I know how far my camera is from the grid floor, the resolution, the focal length, etc, and just trying to find anything i can plug some numbers into to know that "2.5px = X cms". Thanks!

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[eberrigan]

Hello @jramborger78,

To convert the distance you've measured in pixels to real-world units, you'll need a scale conversion factor. This factor should be determined using an object in your image whose actual size you know and that is at the same distance from the camera (unless you have a telecentric lens) as the object you're measuring.

If you have such an object, you can use FIJI, an image processing software, to find the conversion factor. Here's how:

Download and Install FIJI:
Get FIJI from here and install it on your computer.
Open Your Image:
Launch FIJI and open your image file.
Measure the Known Object:
Select the Straight Line tool from the toolbar.
Draw a line over the object whose real-world length you know.
Determine Pixel Length:
Go to Analyze -> Measure or press CTRL + M.
The length in pixels will appear in the "Results" window.
Calculate the Conversion Factor:
Since you know the actual length of the object (e.g., in centimeters), divide that by the pixel length you just measured.
This gives you the conversion factor in units per pixel (e.g., cm/pixel).
With this conversion factor, you can convert any pixel measurements in your image to real-world units.

Best,

Elizabeth