converting 16 bit image to 12 bit

i have read a .pgm image in imread. it is in 16 bit

can i know how to recover to 12 bit .

[Steve Eddins]

There is no 12-bit integer type in MATLAB, so it's not obvious to me what you mean by "recover to 12 bit." Can you clarify?

[Varsha Nataraj]

But pgm will be 12 bit but matlab stores it in 16 bit. How to get 12 bit value of pgm from 16 bit which is stored

Answer 1

[Walter Roberson]

Different programs deal with storing lower number of bits (such as 12) in higher number of bits (such as 16) in different ways.

Sometimes they just store the data as the "bottom" bits, padding with 0 for the top bits. In such a case, the "12 bit" version of the data has the same numeric value as the 16 bit version does, except in the case where the "12 bit" data is intended to be signed numbers.

Sometimes instead the data is stored as the "top" bits, padding with 0 for the bottom bits. This has the advantage of preserving relative intensity -- full intensity of 12 bit translates to (nearly) full intensity of 16 bit. In this situation, to convert the 16 bit values down to 12, just divide the 16 bit values by 2^(16-12) = 2^4 = 16.

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/93578-why-are-12-bit-pgm-images-scaled-up-to-16-bit-value-representation-in-image-processing-toolbox-7-10

has the authorative answer as to how MATLAB reads 12 bit PGM, and has functions to scale the value.

... They come down to dividing by 16, just like I suggested.