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Suppose you have an image of a real object which contains also a measuring stick (or another objekt of exactly known length).

Now I am looking for a software where I can measure the real distances betweeen any two points of the picture after setting a calibration stick to the measuring stick (like in tracker, but just for pictures, not for videos) on the picture.

Clearly I can measure the pixels in gimp and make the calculation manually. But is there any tool which does it automatically?

  • According to Tracker's documentation, you can also open still images (jpg, gif, png). – ComFreek Jul 25 '15 at 11:03
  • I don't understand how this would be possible (assuming that by “image” you mean a 2D picture taken by an ordinary camera). The distance between objects represented in the picture would depend on their depths. – Gilles 'SO- stop being evil' Jul 25 '15 at 11:30
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    @Gilles Sure, however, if you have two objects in the same 2D plane (orthogonal to the lens direction), then the scale is retained. – ComFreek Jul 25 '15 at 11:50
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Tracker

Tracker is an open-source video analysis software aimed at tracking coordinates of moving objects in videos. As part of this feature, it includes measuring tools, which allow to measure a distance between two points in a frame (or over the course of multiple frames) after having set a calibration stick.

Matched requirements:

  • Apart from video files, Tracker handles still images (JPEG, GIF, PNG) as well (cf. its documentation).

  • It runs on Windows, Mac OS X, Linux 32-bit and Linux 64-bit.

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GIMP

You can measure real distances (in mm, cm, m and so on) in GIMP as follows:

  1. Select the Measure Tool.

  2. Measure the reference element of known size in pixels, and calibrate the image resolution under "Image → Print size …" accordingly.

    For example, when your reference element measures 100 pixels and is 35 mm long, you enter into the boxes "X resolution" and "Y resolution: 100/35 pixels/mm. Just enter it as "100/35", GIMP will do the calculation for you.

  3. In the status bar on the bottom left of the image, change the unit from pixels to something else.

    enter image description here

  4. In the tool options, there is also the checkbox "Use info window". This will show a small overlay window (or real window if you like). Unlike the status bar, you can select the measurement results here and copy them to the clipboard.

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