MATLAB: Are 12-bit PGM images scaled up to 16-bit value representation in Image Processing Toolbox 7.10 (R2010a)

Image Processing Toolbox

I am loading 12-bit PGM images into MATLAB. However, the image values when loaded are between 0 and 65535 suggesting that this is actually a 16-bit PGM image. I would expect to see values between 0 and 4095. Additionally, the values do not appear to be an exact multiply of original values by 16.

Best Answer

This is not a bug in MATLAB 7.10 (R2010a). MATLAB does load PGM 12-bit images correctly. However, after MATLAB loads the images, the image values are rescaled from 12-bit to 16-bit.
MATLAB uses the following algorithm to scale the values from 12-bit to 16-bit:
% W contains the the 12-bit data loaded from file. Data is stored in 16-bit unsigned integer
% First 4 bits are 0. Consider 12-bit pixel color value of ABC
% Then W = 0ABC
X = bitshift(W,4); % X = ABC0
Y = bitshift(W,-8); %Y = 000A
Z = bitor(X,Y); %Z = ABCA 
% Z is the variable that is returned by IMREAD.
To work around this issue you need to manually rescale the result of IMREAD from 16-bit to 12-bit. To do that divide color values for the image by 16. Below is a sample function that acomplishes that.
function out_image = imreadPGM12(filename)
out_image = imread(filename);
out_image = floor(out_image./16);
return
Alternatively perform a 4 bit shift to the right:
function out_image = imreadPGM12(filename)
out_image = imread(filename);
out_image = bitshift(out_image,-4);
return
Related Question