How to Determine Distance from an Object in a Video

How can I determine distance from an object in a video?

When you have moving video, you can use temporal parallax to determine the relative distance of objects. Parallax: (definition).

The effect would be the same we get with our eyes which which can gain depth perception by looking at the same object from slightly different angles. Since you are moving, you can use two successive video frames to get your slightly different angle.

Using parallax calculations, you can determine the relative size and distance of objects (relative to one another). But, if you want the absolute size and distance, you will need a known point of reference.

You will also need to know the speed and direction being traveled (as well as the video frame rate) in order to do the calculations. You might be able to derive the speed of the vehicle using the visual data but that adds another dimension of complexity.

The technology already exists. Satellites determine topographic prominence (height) by comparing multiple images taken over a short period of time. We use parallax to determine the distance of stars by taking photos of night sky at different points in earth's orbit around the sun. I was able to create 3-D images out of an airplane window by taking two photographs within short succession.

The exact technology and calculations (even if I knew them off the top of my head) are way outside the scope of discussing here. If I can find a decent reference, I will post it here.

Finding distance from camera to object of known size

You can use triangle similarity to calibrate the camera angle and find the distance.

You know your ball's size: D units (e.g. cm). Place it at a known distance Z, say 1 meter = 100cm, in front of the camera and measure its apparent width in pixels. Call this width d.

The focal length of the camera f (which is slightly different from camera to camera) is then f=d*Z/D.

When you see this ball again with this camera, and its apparent width is d' pixels, then by triangle similarity, you know that f/d'=Z'/D and thus: Z'=D*f/d' where Z' is the ball's current distance from the camera.

Is it possible to measure distance to object with camera?

Well you should read how ithinkdiff.com "measures" the distance:

Uses the angle of the iPhone to estimate the distance to a point on the ground.
Hold the iPhone in front of you, align the point in the camera and get a direct
reading of the distance. The distance can then be used in the speed tool.

So basically it takes the height of where you hold the phone (eye-level), then you must point the camera to the point where object touches the ground. Then the phone measures the inclination and with simple trigonometry it calculates distance.

This is of course not very accurate. It gets less accurate the further the object is. Also it assumes that the ground is level.

How do I find an object in image/video knowing its real physical dimension?

I wouldn't worry too much about the exact geometry and calibration and rather find the window by its own characteristics.

Binarization works relatively well, be it on the whole image or in a large region of interest.

Then you can select the most likely blob based on it approximate area and/or circularity.

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