What program can I use to determine unknown lengths in a photo?

I am trying to find the exact dimensions of an object in a photo, I know its length, but I was wondering if there is maybe a program where I could input that one value, then like drag a line or something to determine the other lengths based off that one, I am looking to avoid any tedious math, thanks!

Comments

[jahg]

The newer versions of Photoshop have this ability. I believe the feature was added in CS3, this page http://arstechnica.com/old/content/2007/04/photoshop-cs3.ars/5 mentions is near the bottom.

[rlarge]

Thats about right lemonie and i just looked for a hour to find a program that would do it but to no success, this kind of software would be good for cnc machining to make parts that fit on other parts that you didn't make and don't know the exact dimensions. Maybe thats a place to look....

[paganwonder]

Sketchup on Google. Fun program to play with.

[NobodyInParticular]

As lemonie said, if perspective isn't involved, you can just use proportions.

actual length ======= actual width

---

photo length ======= photo width

Or to put that another way,
actual length * photo width / photo length = actual width

But if perspective is involved, you best bet may be to build a 3D model that matches the photo. All you need is a rough idea.

Can you describe the situation more fully? Maybe someone here already knows the dimensions of the object in your photo.

[lemonie]

This sounds tricky, but maybe not. People like archaeologists photograph things next to a ruler or similar. If you know one dimension, and the other lengths are in the same plane - i.e. there's no depth / perspective distorting things you can do it by simple proportion: Actual length ( which you say you know) divided by the measured length on the photo gives you a conversion factor. Multiply the measured length on the photograph by the conversion factor to get the actual length (that you don't know) L