Implementing a Harris corner detector

Question

I am implementing a Harris corner detector for educational purposes but I'm stuck at the harris response part. Basically, what I am doing, is:

1. Compute image intensity gradients in x- and y-direction
2. Blur output of (1)
3. Compute Harris response over output of (2)
4. Suppress non-maximas in output of (3) in a 3x3-neighborhood and threshold output

1 and 2 seem to work fine; however, I get very small values as the Harris response, and no point does reach the threshold. Input is a standard outdoor photography.

[...]
[Ix, Iy] = intensityGradients(img);
g = fspecial('gaussian');
Ix = imfilter(Ix, g);
Iy = imfilter(Iy, g);
H = harrisResponse(Ix, Iy);
[...]

function K = harrisResponse(Ix, Iy)
    max = 0;
    [sy, sx] = size(Ix);
    K = zeros(sy, sx);
    for i = 1:sx,
        for j = 1:sy,
            H = [Ix(j,i) * Ix(j,i), Ix(j,i) * Iy(j,i)
                Ix(j,i) * Iy(j,i), Iy(j,i) * Iy(j,i)];
            K(j,i) = det(H) / trace(H);
            if K(j,i) > max,
                max = K(j,i);
            end
        end
    end
    max
end

For the sample picture, max ends up being 6.4163e-018 which seems far too low.

Answer 1

A corner in Harris corner detection is defined as "the highest value pixel in a region" (usually 3X3 or 5x5) so your comment about no point reaching a "threshold" seems strange to me. Just collect all pixels that have a higher value than all other pixels in the 5x5 neighborhood around them.

Apart from that: I'm not 100% sure, but I think you should have:

K(j,i) = det(H) - lambda*(trace(H)^2) Where lambda is a positive constant that works in your case (and Harris suggested value is 0.04).

In general the only sensible moment to filter your input is before this point:

[Ix, Iy] = intensityGradients(img);

Filtering Ix2, Iy2 and Ixy doesn't make much sense to me.

Further, I think your sample code is wrong here (does function harrisResponse have two or three input variables?):

H = harrisResponse(Ix2, Ixy, Iy2);
[...]

function K = harrisResponse(Ix, Iy)