How to measure the real size of an object from an unknown picture?
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Guys how can I measure the height and width of an object when I don't know the distance from the object nor any other objects' dimensions in the image.
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Walter Roberson
2018-3-12
... except in the case of the Moon, you have the size of your own body to compare against, such as by holding out your arm and measuring how much of your hand the moon spans, and you can go around and do that same test with other objects and from other angles. By moving measured distances, you can do trigonometry to figure out how big the Moon is.
When all you have is a single picture from an unknown distance, or even multiple pictures from locations whose physical relationship is unknown, then you cannot figure out how large the object being pictured is.
Adam
2018-3-12
That's true, but measured distances appear to be not on the agenda for this question!
回答(2 个)
Image Analyst
2018-3-13
If you ever do or can get the size of known object in the image, you can adapt my attached example. If you don't have any objects of known size (like you said), then you're out of luck (like the others said).
5 个评论
Walter Roberson
2018-3-11
Q: In the movie Star Wars, how large is the Imperial Battle Cruiser?
A: About 3 feet long. It just looks huge because of the camera angles and very very careful use of perspective.
Which distance would you want returned: the actual distance (3 feet or so), or the distance intended to be conveyed ? https://en.wikipedia.org/wiki/Death_Star
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Walter Roberson
2018-3-12
编辑:Walter Roberson
2018-3-12
Paint the box to look like the original Death Star. Now when you take the pictures from various distances will it tell you the answer as the (say) 40 cm that is the physical box size or should it tell you the the 100 km to 150 km that the story says that the distance should be?
If I use CGI software to render a picture of a box, a box that never physically existed, then what size should the program say?
"When one with the Force you are, matters not size does."
Steven Lord
2018-3-12
On my desk I have some six-sided dice that are 16 mm to a side, others that are 20 mm to a side, and two dice-shaped tins that are 7 cm (70 mm) to a side. At home I have two dice that are 15 cm (150 mm) to a side.
I'm sure I could take pictures against a plain white background that make each of these dice or dice-like objects appear to be the same size, depending on how far away from the dice the camera is located. Barring any other information, how could you distinguish between those four cases? The answer is, you can't.
And even if you knew the sizes of all the objects in your scene, the picture can trick your eyes. In the Lord of the Rings movies Sir Ian McKellen (Gandalf) is 5 feet 11 inches tall and Elijah Wood (Frodo) is 5 feet 6 inches tall, a difference of only 5 inches. Through forced perspective and other techniques Frodo looks much shorter than Gandalf, a difference of much more than 5 inches.
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