Extending palette of indexed images in MATLAB

Question

I extracted the color palette of an indexed image - a 256x3 matrix, duplicated the palette to 512x3 matrix with duplicate values in each half. What I want to do is steganography. When the secret message bit is 0,I want to refer to one half of palette, else to the other half. In this way, we can get lossless steganography in indexed images!

But when I try to save the image as bitmap with the new color map, it says bmp/gif files cannot have more than 256 entries in the color palette!

[im,map]=imread('mandril_color.gif');
nmap=zeros(512,3);
nmap(1:256,1:3)=map(1:256,1:3);
nmap(257:512,1:3)=map(1:256,1:3);
imwrite(im,nmap,'palette1.gif');

The above was my code to just test whether saving an image with an extended palette works or not.. unfortunately it did not. How can I avoid this problem and have a custom palette with more than 256 values?

Answer 1

The standard for .bmp and .gif only supports color palettes of length 256. There is no way around that for you.

To use color palettes with more than 256 entries, you can use .jpg, for example. Make sure you choose lossless compression, since otherwise, your message will be scrambled.