Undefined function or variable in tracking green ball example

I'm attempting to follow the tracking a green ball example found here:

Tracking A Green Ball

using my built in webcam. When I attempt to run the code I am getting an error undefined function or variable bwbw. I am using the following code for the trackball function:

function [img, bw] = trackball(img, thresh)

r = img(:, :, 1);
g = img(:, :, 2);
b = img(:, :, 3);

justGreen = g - r/2 - b/2;
bw = justGreen > thresh;

[x, y] = find(bw);
if ~isempty(x) && ~isempty(y)
    xm = round(mean(x));
    ym = round(mean(y));
    xx = max(1, xm-5):min(xm+5, size(bw, 1));
    yy = max(1, ym-5):min(ym+5, size(bw, 2));
    bwbw = zeros(size(bw), 'uint8');
    bwbw(xx, yy) = 255;
end
imagesc(justGreen + bwbw);

and am calling it using this code (I've tried running it both from the command window and by creating a script and running it):

function [img, bw] = trackball(img, thresh)

r = img(:, :, 1);
g = img(:, :, 2);
b = img(:, :, 3);

justGreen = g - r/2 - b/2;
bw = justGreen > thresh;

[x, y] = find(bw);
if ~isempty(x) && ~isempty(y)
    xm = round(mean(x));
    ym = round(mean(y));
    xx = max(1, xm-5):min(xm+5, size(bw, 1));
    yy = max(1, ym-5):min(ym+5, size(bw, 2));
    bwbw = zeros(size(bw), 'uint8');
    bwbw(xx, yy) = 255;
end
imagesc(justGreen + bwbw);

I was able to run it successfully once last week without obtaining this error, but now I cannot fix it. I do make sure to create a camera object (using cam = webcam) before I run the loop that captures the snapshots. Any help would be greatly appreciated. Thanks!

Answer 1

[Image Analyst]

This poorly written code never assigns bwbw in the case where no green is found. To fix, add this line after you threshold it to find bw:

bwbw = zeros(size(bw), 'uint8');

Even better, use my code instead. It's attached.

[LKR]

Thank you!

Answer 2

[John D'Errico]

Um, think about how and when the variable bwbw is defined.

[x, y] = find(bw);
if ~isempty(x) && ~isempty(y)
    ...
    bwbw = zeros(size(bw), 'uint8');
    bwbw(xx, yy) = 255;
end

Is it possible that the if statement never triggers? Better, is it a fact?

Under what circumstances will that the clause in the if statement not be true?

When you have a problem like this in a function, LEARN TO USE THE DEBUGGER. The debugger is your friend.