How can I estimate an object's real size from photographs if one object in the image has a known size?
Asked 2/9/2018
3 views
2 answers
0
I want to estimate the real-world size of an object from photos. In the scene, there is another object with known dimensions that can be used as a reference. Is there a standard way to calculate the unknown object's size from one or two photos taken from different perspectives? What information is needed, such as focal length, subject distance, or image coordinates?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
1
There are quite a few methods to determine this. The most common that you probably noticed would be in crime scene photos when an object of known size such as a pencil or even better a ruler is placed next to the object of interests. From this it is very easy to estimage the size of the object of interest.
As long as you know the focal length and the object distance, both of which some lenses return to the camera you can calculate the real size of an item in the image. The focal length will give you the lens field of view in terms of degrees.
Measuring size of objects in an image with OpenCV
I'd try looking at these to start off
You also might find these of use:
Originally by user70677. Source · Licensed CC BY-SA 4.0
user70677
8y ago
0
Generated from our catalog & community — verify before relying on it.
Yes, but the method depends on the photo setup.
If the known-size object is in the same plane and at the same distance from the camera as the unknown object, the simplest approach is just scaling: compare their sizes in the image and apply the known real dimension.
If they are at different distances or the perspective changes, simple scaling is no longer accurate. Then you need camera geometry: at minimum, focal length and subject distance, since focal length determines field of view. With those, you can relate image size to real size.
With two photos from different viewpoints, parallax/stereo methods can also be used, but that requires matching image points between the two photos and knowing the camera geometry well enough to reconstruct distances.
In practice, the easiest and most reliable method is to place a ruler or other object of known size right next to the subject and photograph them together. If that is not possible, look into photogrammetry or computer-vision tools such as OpenCV for measuring from images.
Recommended products
UniqueBot
AI8y ago
Your Answer
Related Questions
How can I estimate an object's size in pixels from its real size, distance, and camera settings?
How can I estimate an object's real size from one image if I know the camera-to-subject distance?
Can focal length be calculated from vanishing points in a single photo?
Can moving the camera and comparing object size in the frame be used to estimate distance?
Can I calculate an object's size from a single photo if it sits closer to the camera than a known reference?