How do I measure the conversion between pixels to cm from an image?

I have a picture that includes part of a ruler. I want to measure the length of the ruler by tracing a line on the image and see how many pixels correspond to the length of it. How do I trace a line on the picture manually after it is already displayed and save the length in pixels? I can easily do the conversion once I have the length in pixels, but I don't know the function to trace the line.

Thank you.

[Rik]

You mean like the imdistline function?

Answer 1

[Image Analyst]

Run my attached spatial calibration demo. It will show you exactly what to do.

[kamran zahid]

Excellen.

Do I need to know the area of my camera covering rectangular frame for this or not?

[Image Analyst]

No, but if you do that makes it easy. Just divide the real world distance of your field of view by the number of pixels across. The demo asks you to draw a line across some known distance, like a ruler or whatever, and then it computes the ratio of real world distance over pixels, which you can then multiply by any pixel-based distance to get the real distance, or multiply its square by areas in pixels to get areas in real world units.