How to measure the real size of an object from an unknown picture?

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.

[Adam]

You can't unless you have something else in the image right next to it too whose dimensions you do know. Even then it would still depend on the angles at which the image was taken.

It's no different to trying to decide how big the moon is just by looking at it with no additional information about how far away it is.

[Walter Roberson]

... 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]

That's true, but measured distances appear to be not on the agenda for this question!

Answer 1

[Image Analyst]

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).

[Christakis Loizou]

I guess im out of lack....

[Image Analyst]

Is this really allowed at your university??? To submit others code as your own to satisfy your graduation project? Anyway, I don't have that code. The Mathworks does and it's posted in their web pages for the Computer Vision System Toolbox.

The most I can give you is the frame differencing demo I've posted many times before.

[Jaswanth kumar Jalagam]

Thank you sir and I tried was unable to get result so kinda desperate and asked ,thank you sir now I feel to try harder

[Nafiseh Yazdi]

Thank you so much. It was very useful for me

[Abdul Qadir]

thanks

Answer 2

[Walter Roberson]

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

[Chris Loizou]

for example if i take a picture of a box, and i would like to measure its height. But if i take the picture from different distances, (assuming the camera is perpendicular to the box) the pixel count will be different.

[Stephen23]

"...But if i take the picture from different distances..."

Do you know these distances?

[Chris Loizou]

no because i want to take phitos of the box from random distances which i will not know the distance each time.

[Walter Roberson]

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]

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.