two dimensional histogram of an image in matlab

Question

I have an image. i am calculating I,u,v components for it.

I = (R+G+B)/3

u = RG

v= GB;

Now , I want to find two-dimensional histograms over the chromatic information (u; v).

Thanks in advance.

Answer 1

You can use sparse to creatre a sparse 2D matrix that counts the u - v entries.
Note that you'll have to adjust the indices in the u - v dimension to be in range 1...|u| and 1...|v| (and not negative or fractional).

[uu ui] = unique( round(u(:)) ); % adjust u index using unique command
[vv vi] = unique( round(v(:)) );
twoDhist = sparse( ui, vi, 1, numel(uu), numel(vv) );
twoDhist = spfun( @(x) x/numel(ui), twoDhist ); % normalize hist to sum to 1

figure;
imagesc( vv, uu, twoDhist ); colormap jet; colorbar; axis image