Is there a way to measure size of the object without having the reference object (Aruco marker/coin) always in the frame? As in after calculating the pixel per metric ratio, can we use this value only to compute the size of the object? The camera will also always be from a fixed height. Thanks so much!
1 Answers
The short answer is no. This problem occurs in 3D->2D projection (also called homography or perspective projection). Here is an example:
Here is a simplified version of this projection:
As you can see- if you decrease z but increase x&y you can get the same result, even though you know the pixel-distance ration (that is f in the equation).
check out this two links for a more mathematical proof.
For measuring in 2D images you first need the f (intrinsic calibration) and also one of two: By knowing a size of an object in the screen (like the aruco) you can infer z, and if you know z of the object (=distance from the camera) you can infer size.
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