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In a digital image, size (or distance) is measured in pixels.

Is there any method to find the actual size of an object in an image without knowing the calibration factor?

Dev-iL
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ali samah
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    There's no "actual" size. Depending on the device you print the printed size may change : ex an image will be bigger on A3 paper than on A4 paper. Basically when dealing with image the unit of measurement is the pixel. – PTRK Mar 22 '18 at 08:05
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    Is this really a programming question? – sco1 Mar 22 '18 at 13:53

1 Answers1

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To my knowledge, there is no general solution to this.

  • For specific objects (mostly those that are mass produced, or have specs/drawings available) you might be able to find the "calibration factor" after the fact. Alternatively, if there's anything else in the photo that has known dimensions, you might be able to use that to compute the size of the object you care about (for example - consider images that come from fancy microscopes, that have a "scale" indicator somewhere on the image).
  • If you have the details of the photodetector (e.g. pixel size; assuming it's an electronic device), the parameters of the lens being used when taking the photo, the distance to the object etc., you might be able to estimate the object size using straightforward optical computations (like Geometrical optics).
Dev-iL
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    That is the entire point yes. The only way to know the real dimensions is knowing the dimensions of each pixel, or of something in the image. In other words, to have a calibrated camera – Ander Biguri Mar 22 '18 at 11:34