How to calculate the real size of an object from image

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Discussion Overview

The discussion revolves around methods to calculate the real size of an object from images or videos, particularly in the context of CCTV footage. Participants explore various mathematical and geometrical approaches to relate 2D image data to 3D real-world dimensions, considering factors such as object movement and camera perspective.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that if the object is moving, the rate at which its size appears to change on screen is related to its distance from the camera, proposing that this relationship could be used to deduce actual size and position.
  • Another participant introduces the concept of using two viewpoints for distance measurement, referencing stereo vision and triangulation as a method to determine object size if the distance between viewpoints is known.
  • A clarification is sought regarding whether the camera lens has a fixed focal length or is a zoom lens, as this could affect the calculations.
  • It is noted that the method of deducing size from changing image size may only be valid under the assumption of constant velocity of the object, as variations in speed and distance could complicate the relationship.
  • One participant mentions converting perspective projection to orthographic projection from a still image and emphasizes the necessity of having at least one object of known length in the image for accurate measurements.

Areas of Agreement / Disagreement

Participants express various viewpoints and methods without reaching a consensus. There are differing opinions on the feasibility of certain approaches, particularly regarding the assumptions about object movement and camera characteristics.

Contextual Notes

Limitations include the dependence on the type of camera lens, the assumption of constant object velocity, and the need for reference objects in the scene for accurate calculations. The discussion does not resolve these complexities.

Who May Find This Useful

Individuals interested in image processing, computer vision, or applications involving surveillance technology may find this discussion relevant.

DetectiveT
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Hello everyone, I have been doing researches on this, but those topics I found are a bit different from what I am doing,

I am trying to determine the actual size of an object in image/video (CCTV video tracks), my thinking is, if this object is moving, the size appears on the screen changes, and depending on the position of this object (its distance from the camera), the size changing rate should be different, vice versa, if the object image size changing rate is known, it should be possible to deduce its actual position and actual size? If this is not right, "put a reference object in the scene" method is also acceptable.

However, the current problem is, I do not really understand the 3D real world projecting to 2D image geometrical math, perhaps with a reference object, some mapping equation f(x,y)→g(x_real, y_real, z_real) can be found? Or with the changing rate way, f(dr/dt, x, y)→g(x_real, y_real, z_real) is possible?

Please, any fundamental ideas, corrections or suggestions will be appreciated, thanks!
 
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If you have two view points then you can determine the distance between the view point and the object, such as in stereo vision.

https://en.wikipedia.org/wiki/Triangulation

You require however two view points and a known distance between them. For example our eyes are two view points with a distance of 2 inches in between them. From there you can determine the other lengths of the triangle and size of the object.
 
DetectiveT said:
I am trying to determine the actual size of an object in image/video (CCTV video tracks)

For clarification, is the lens a fixed focal length or is it a zoom?

DetectiveT said:
my thinking is, if this object is moving, the size appears on the screen changes, and depending on the position of this object (its distance from the camera), the size changing rate should be different, vice versa, if the object image size changing rate is known, it should be possible to deduce its actual position and actual size

That sounds like it might only work if the velocity of the object is constant. If it's close by and moving slowly, it's image size will change slowly, similar to it being far away and moving quickly.
 
From a still image, you convert perspective projection to orthographic projection, and you'd also need at least one object in the image of known length.
 

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