Finding 3 consecutive zero values row by row in an image.

Hi,

I would like to be able to go through every row in an image looking for when there are 3 consecutive pixels with a zero value. What I need is it to print the row(X) and column(Y) for the pixel before those 3 pixels. And then continue to move through the rest of the rows. So in the end I would have a list for the Xs and Ys. Can anyone help me make this loop?

Thanks

[Sean de Wolski]

What if the three consecutive pixels occur int he first three columns? If there are four consecutive zeros, does this count? If so, which column(s) should be returned? A small example would be best.

[Image Analyst]

And is the image grayscale or color?

[Ross]

color

[Ross]

No it wouldn't count and I don't think it will happen. For an example think of a jagged line down the middle of the image. One side has color the other side is dark(zeros). Does that help?

[Image Analyst]

Care to post the image (so we can see it), and let us know why you want/need to find locations of exactly 3 (no more and no less) black pixels? What are you going to do once you know the location of the pixels before the black line starts?

[Ross]

So what I am trying to do is get the coordinates of that line. So it doesn't have to be exactly 3. It could be more but going less I thought might be an issue. Once I have a list of the Xs and Ys then I might doing some curve fitting. That's why I need the Xs and Ys loction.

Answer 1

[Andrei Bobrov]

x1 = all(img==0,3);
rst = conv2(x1+0,[1 1 1],'same');
[ii,jj] = find(rst == 3);
jj = bsxfun(@plus,jj,-1:1);

[Sean de Wolski]

conv2 and bsxfun! +1

Answer 2

[Geoff]

Simplest (and rather inefficient) approach:

for y = 1:size(img,1)
  blacks = 0;
  for x = 1:size(img,2)
    if all(squeeze(img(y,x,:)) == 0)
      blacks = blacks + 1;
      if blacks == 3
        % BTW, what do you do if first three pixels in the row are black??
        disp( [x-blacks, y] );
        break;  % Stop processing row
      end
    else
      blacks = 0;
    end
  end
end