Can't detect a board with light gray squares

[zoharl3]

[zoharl3]

Quantizing to three colors and changing the middle one resolves some of these boards. Otherwise, some of the white queens are detected as black.

Image source: "Chess Tactics Book for Beginners 200 Puzzles Learn How to Play and Checkmate in One and Two Moves"

[zoharl3]

A solution that seems to work so far:

1. Quantize 3 colors and detect black square color as the one that isn't white or black (max and min color norm). Find a pixel with such color in the image.
2. Quantize 8 colors. Get the color of the pixel above and change it to red.

Matlab:

% process image

function chess_util( fin, fout )
    % read
    a = imread( fin );
    assert( size( a, 3 ) == 3 );

    % quantize colors
    [ia, cm] = rgb2ind( a, 3, 'nodither' ); % 3, 4

    %imshow( ia, cm )

    % impixelinfo()
    % colormapeditor

    % change color
    if 1
        % visualize color map
        if 0
            rgbplot(cm)
            hold on
            colormap(cm)
            colorbar('Ticks',[])
            hold off
        end

        % find dark square color in color map
        % eliminate black and white
        ii = 1:size( cm, 1 ); % candidates

        dd = sum( cm.^2, 2 );
        [~, imin] = min( dd(ii) ); % black
        ii(imin) = [];

        [~, imin] = min( -dd(ii) ); % white
        ii(imin) = [];

        [~, imin] = min( dd(ii) ); % darkest
        icl = ii(imin);
        %cl = cm( icl, :);
        [row, col] = find( ia == icl - 1 ); % find a pixel with that value

        % load more colors
        [ia, cm] = rgb2ind( a, 8, 'nodither' );
        icl = ia( row(1), col(1) ); % get color from pixel location
        cm(icl + 1, :) = [1, 0, 0]; % change
    end

    % write
    imwrite( ia, cm, fout );
end