Why an error occurs in cmunique with images with colour palettes larger than 256*256 colours?

Found an error in cmunique when working with images with large colourmaps.

>> im=rand(256*257,1,3);
>> [Y,newmap] = cmunique(im);
Error using  - 
Matrix dimensions must agree.

Error in cmunique (line 107)
loc = (1:nmap) - cumsum(d);

The error appears to be caused by line 105 in cmunique

n = histcounts(c,'BinMethod','integers','BinLimits',[1 nmap]);

If nmap<=256*256, all works well, but if nmap>=256*257, the length of n suddenly drops by an order of magnitude (and the number of counts for each color changes from 1 to 10!)

Any reason why this should be so?

[Walter Roberson]

The code should not be relying on those parameters for histcounts :(

[Maria Jose Luque]

For some reason, histcounts limits the maximum number of bins Line 387 function mb = getmaxnumbins mb = 65536; %2^16 end

Answer 1

[Walter Roberson]

I suggest

[newmap, ~, uidx] = unique( reshape(im, [], 3), 'rows');
Y = reshape(uidx, size(im, 1), size(im, 2) );
mlen = length(newmap);
if mlen <= 256
  Y = uint8(Y - 1);
elseif mlen <= 65536
  Y = uint16(Y - 1);
%else leave it as-is
end

[Maria Jose Luque]

Yes, this works very nicely. Many thanks!

Answer 2

[Darshan Ramakant Bhat]

It seems MATLAB has an issue with this function. I recommend you to use ' rgb2ind ' function instead. Document link for the function is given below:

https://www.mathworks.com/help/matlab/ref/rgb2ind.html

I hope this will serve your purpose.

Regards

Darshan Bhat

[Walter Roberson]

rgb2ind is not suitable for this situation. Notice that

"n must be less than or equal to 65,536"

which is 256*256.

[Maria Jose Luque]

In fact, rgb2ind uses cmunique in line 86