Bounding Box largest rectangle?

When I use regionprops BoundingBox, it seems to always capture smaller blobs on the left most side of my image and not my largest blob. How do I bound the box around the largest blob?

[David Young]

regionprops finds all blobs - there must be something else happening. Please could you post the image and code that demonstrates the problem.

[Ivan Q]

how are all blobs stored in regionprops? How do i choose the largest blob? Here's my code.

diff_im=imsubtract(image(:,:,1),rgb2gray(image));
diff_im-medfilt2(diff_im,[3,3]);
diff_im=im2bw(diff_im, threshold);
diff_im=imfill(diff_im,'holes');

%%find card
[L,num]=bwlabel(diff_im,8);
stats=regionprops(diff_im,'Centroid','Area','BoundingBox');
center=cat(1,stats.Centroid);
area=[stats.Area];
[pixelArea, index]=max(area);
box=stats.BoundingBox;
area=box(3)*box(4);
rectangle('Position',box,'EdgeColor','r');

[Walter Roberson]

box=stats(index).BoundingBox;

[Ivan Q]

Thanks everyone!

Answer 1

[Image Analyst]

stats is a structure array. Each element is a structure and has it's own bounding box. So stats(1).BoundingBox is the bounding box (x,y,width,height) of the first blob. And stats(2).BoundingBox is the bounding box (x,y,width,height) of the second blob, and so on. To display them all:

hold on;
for k = 1 : length(stats) % Loop through all blobs.
  % Find the bounding box of each blob.
  thisBlobsBoundingBox = stats(k).BoundingBox;  
  rectangle('Position', thisBlobsBoundingBox);
end

Answer 2

[nilton brites]

explain what is definition of bounding box on image?

[Image Analyst]

It's the box, with edges parallel to the edges of the image, that contains object(s) of interest, such as blobs you got by thresholding. You can have a box around each blob, or around the whole group of them.

[Walter Roberson]

In particular a bounding box is most often computed as the smallest box with horizontal and vertical sides that encloses the object(s), but bounding boxes can, in general, be larger than that: they are basically a box large enough to be sure you are enclosing the portion of interest.