How to measure area of a rectangle from an image?

Suppose, I have this image and I want to measure the area of the table surrounded by a red line. This table is rectangular but not looking as a rectangle. Here we can't take pixel values uniform how can we measure the area of this table?

[Walter Roberson]

You cannot measure the real-world area of anything from an image unless you have an item of known length in the image and also information about the lens settings (preferably information about the lens distortion characteristics as well.)

To use a different key phrase : you need to calibrate.

Answer 1

[KSSV]

see the links:

http://in.mathworks.com/matlabcentral/answers/154484-how-can-i-calculate-area-of-an-object-in-the-image-with-png-extension

https://in.mathworks.com/matlabcentral/answers/12032-how-to-get-areas-of-specified-objects-in-an-image