Outline polygon with square region

I have a polygon image(mask, 0's and 1's).

Let us pretend it is a octagon of width 200 pixels in a 500x500 pixel image. I want to draw a bounding box around the octagon, however, I want the bounding box to only be made up of 90 degree angles.

Another way to imagine the problem is, take a polygon and put it on a checkerboard. The sub-squares of the checkerboard that has the polygon in it gets a value of 1, and the squares that does not gets a value of 0.

Any ideas on how to easily do this?

Answer 1

[Walter Roberson]

left = find(any(TheMask,1), 1, 'first');
right = find(any(TheMask,1), 1, 'last');
top = find(any(TheMask,2), 1, 'first');
bottom = find(any(TheMask,2), 1, 'last');

These are going to be in pixels.

[Shane]

Hi, thanks for the response. To clarify a little more. I do not just want to find the square region it is in, but actually trace out the checkerboard pattern of the polygon.

Imagine if it is a U shaped mask. Then I went to have the U equivalent in the checkerboard pattern, not just a block covering the entire area of the U.

[Walter Roberson]

The only way that I can see a difference between what you ask and poly2mask() is if you want the checkerboard to be lower resolution (larger grained) than that.

If you do have a bitmask generated by a polygon and you want to lower its resolution, then blockproc:

   set_if_any = @(blockstruct) ones(size(blockstruct.data),class(blockstruct.data)) * any(blockstruct.data(:));

   grainsize = 6;   %if you want the new boxes to be 6 x 6
   grainedMask = blockproc(ExistingMask, grainsize, set_if_any);

[Shane]

Thanks! That is exactly the type of function I was looking for!