Suppose I want to take the dimensions of an object of negligible width on a flat surface from a photograph and am able to calibrate in the sense that I know the distance per pixel of some line drawn on the object via doing real world measurements.

How could I go about getting the other dimensions without manually finding a pixel to distance ratio or measuring each individual desired dimension irl?

Is there some program that can use this photograph to craft a 2D outline of it that I might then use with CAD?

  • This isn't really a graphic design question. I'd suggest looking at 3 modeling software. There might be something out there that can do this, but note that various camera lenses will distort axis in different ways so I can't imagine there's a truly accurate way to handle this. – DA01 Dec 23 '14 at 18:50
  • Yes this can be done with multiple pictures or uncompressed video. – joojaa Dec 23 '14 at 20:05
  • If you know the "distance per pixel of some line" than you should be able to use a ratio to figure out other dimensions. As far as automating that in some program I'm not familiar with one. In school we would often use doors as base measurements because they're a consistent width. – Ryan Dec 23 '14 at 20:14

(Assumes Photoshop)

Measure your reference object.

(Use physical measurements—inch, mm, etc.—not pixels. The ruler tool is hidden under the eyedropper. You can also use the line tool, which is actually a little better in that it reads out next to your mouse, not on some distant info palette.)

Divide the real-world size of the reference object by the image size of the reference object. Multiply by 100 to get a percentage.

Go to Image->Image Size and scale the image by that amount. (You want resample off. The only catch is that Photoshop won't let you take the ppi below 1.0, so you might have to uprez before scaling.)

Go back and measure stuff. Things are actual size. (At least things co-planar with the reference object.)

If you're dealing with really huge physical objects (say, a measuring buildings on a satellite photo.) convert to something that's off by a power of ten to make the manual conversion easier.

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