How to convert uint16 grayscale (obtained from .tiff) to uint8 grayscale?

1. I used this:

X=imread(a .tiff file);
[Y,newmap] = imapprox(X,map,64);
imshow(Y, newmap);

Problem 1: I am getting an empty matrix for map

Problem 2: I used map=0:1:2^16; but getting all 255 in Y.

2. I want to use the converted grayscale image for GLCM. Should the grayscale image be indexed for measuring GLCM?

Answer 1

[Image Analyst]

Not all images are stored with a colormap, like color images for example don't have one. You don't need an indexed image or a quantized image for graycomatrix (GLCM). You can use a uint8 or uint16 image directly without alteration.

[Nasser]

But it is taking a long time to calculate GLCM for uint16 images. What to do?

[Image Analyst]

How big are your images? Quantizing them won't speed it up. How long does it take?