When viewing a circular target through a camera at an angle I see an elliptical shape as such:
If I select two arbitrary points on the image is it possible to calculate the distance between the two points in millimetres?
What we know:
- The $X$ and $Y$ coordinates (in pixels) of both points on the image.
- The lengths of the major and minor axis of the ellipse in pixels and millimetres (also just the diameter of the circle)
Is it necessary to calculate the angle of the camera? I found this: Calculating the angle of a disc to provide given ellipse but I can only measure the major and minor axis in pixels. I tried the equation using pixels:
$$\arcsin\left(\frac{1429}{925}\right)$$
This produced no result.
I have tried calculating the average size of a pixel. I did this by by moving the centre of the target from the top left of the screen to the bottom right, recording the distance in millimetres and dividing the distance by the difference in pixels between the two points on the image.
However, this proves only to be an estimation of the distance. This is because pixel size increases farther up the image as each pixel represents more distance.