+% Hough Transform for circles with gradient.
+% img : gradient image
+% radius_range is a vector : [min, max]
+function [acc_votes, acc_radius] = HTCircleGradient(img, radius_range)
+
+ [rows, columns] = size(img);
+
+ acc_votes = zeros(rows, columns);
+ acc_radius = zeros(rows, columns);
+
+ phi_range = linspace(0, 2 * pi, 100);
+ phi_range(end) = [];
+
+ indexes_non_zero = find(img)';
+ s = size(img);
+
+ for r = radius_range(1):radius_range(2)
+ acc = zeros(rows, columns);
+ for i = indexes_non_zero
+ [y, x] = ind2sub(s, i);
+ for phi = phi_range
+ x0 = round(x - r * cos(phi));
+ y0 = round(y - r * sin(phi));
+ if x0 < columns && x0 > 0 && y0 < rows && y0 > 0
+ acc(y0, x0) = acc(y0, x0) + img(y, x);
+ end
+ end
+ end
+
+ for x = 1:columns
+ for y = 1:rows
+ if acc(y, x) > acc_votes(y, x)
+ acc_votes(y, x) = acc(y, x);
+ acc_radius(y, x) = r;
+ end
+ end
+ end
+ end
+
+ acc_votes = imgaussfilt(acc_votes, 1.0);
+end
+