Find the bug and fix it~

README.md

A friend of mine has trouble with his program. He was going to implement lanczosFilter. He compared his implementation of lanczosFilter with the built-in imresize function and found his results is buggy (See result.jpg).

Try to find the reason causing the result so ugly.

Have fun! Please visit my blog to find the answer.

Itest.png

lanczosFilter.m

function [ret_val] = lanczosFilter(lobe, src, texCoord, scale, W, H)
% texCoord: [w, h]    
    % Parameters
    coef = -inf;
    coef_sum = 0;
    res = 0;
    idx_x =0;
    idx_y = 0;
    ret_val = 0;
    
    for n = -lobe : 1 : lobe-1
       for m = -lobe : 1 : lobe-1
          idx_x = floor(texCoord(1) / scale) + m + 1;
          idx_y = floor(texCoord(2) / scale) + n + 1;
          
          coef = lanczos3Coef(texCoord(1) / scale - idx_x, lobe) * lanczos3Coef(texCoord(2) / scale - idx_y, lobe);
          
          % when the neib. pixel is outside the boundary, using the boundary pixels
            if (idx_x < 1)
                idx_x = 1;
            else
                idx_x = idx_x;
            end
            
            if (idx_y < 1)
                idx_y = 1;
            else
                idx_y = idx_y;
            end
            
            if (idx_x > W)
                idx_x = W;
            else
                idx_x = idx_x;
            end
            
            if (idx_y > H)
                idx_y = H;
            else
                idx_y = idx_y;
            end
            
            res =  res + src(idx_y, idx_x) * coef;
            coef_sum = coef_sum + coef;
       end
    end
    
    if (coef_sum ~= 0)
       ret_val = res / coef_sum;
       ret_val = min(max(ret_val, 0), 255);
    end
    
end

function [coef] = lanczos3Coef(x, lobe)
    if (x == 0.0)
        coef = 1.0;
        return;
    end
     s = sin(pi * x) * sin(pi * x / lobe);
     t = pi^2.0 * x^2.0 / lobe;
    if(abs(x) < 3.0)
       coef = s / t;
    else
       coef = 0.0;
    end
end

main.m

Itest = imread('Itest.png'); 
Iones = ones(50,50,3);

for dim3 = 1 : 3 
    for row = 1 : 2*50
        for col = 1 : 2*50
           resize_I(row, col, dim3) = lanczosFilter(3, Itest(:,:,dim3), [col, row], 2, 50, 50);
        end
    end
end

resize_I2 = imresize(Itest, [100,100], 'Lanczos3', 'Antialiasing', false);

figure(1); imshow(Itest); title('source image')

h = figure(2)
subplot(1,2,1); imshow(resize_I); title('lanczosFilter');
subplot(1,2,2); imshow(resize_I2); title('imresize');

ezDump(h);

result.jpg