Mathematica Calculating length from pixel count

wil3

Hello. Does anyone know of a fast way of converting pixel count into a length metric (preferably using Mathematica)? I have an imaging program that segments and measures the lengths of objects in an image, but the only parameter that I can extract is pixel count, which then can get converted to length if I measure a ruler using the program.

But if objects are diagonal in the image, then the program will obviously see them take up fewer pixels because they have to cross an extra √(2) in order to get detected. So I can't just multiply the pixel count by a constant ratio, since orientation matters.

A lot of my objects are curved, so this isn't a matter of just measuring orientation and then multiplying by an appropriate factor. I'm sure this problem has been dealt with before, and I could probably flesh out a brute force way of solving this pixel-by-pixel, but there's gotta be an easier way to do this.

Thanks much,

wil3