How to crop away convexity defects?

Question

I\'m trying to detect and fine-locate some objects in images from contours. The contours that I get often include some noise (maybe form the background, I don\'t know). The

Answer 1

I came up with the following approach for detecting the bounds of the rectangle/square. It works based on few assumptions: shape is rectangular or square, it is centered in the image, it is not tilted.

• divide the masked(filled) image in half along the x-axis so that you get two regions (a top half and a bottom half)
• take the projection of each region on to the x-axis
• take all the non-zero entries of these projections and take their medians. These medians give you the y bounds
• similarly, divide the image in half along y-axis, take the projections on to y-axis, then calculate the medians to get the x bounds
• use the bounds to crop the region

Median line and the projection for a top half of a sample image is shown below.

Resulting bounds and cropped regions for two samples:

The code is in Octave/Matlab, and I tested this on Octave (you need the image package to run this).

clear all
close all

im = double(imread('kTouF.png'));
[r, c] = size(im);
% top half
p = sum(im(1:int32(end/2), :), 1);
y1 = -median(p(find(p > 0))) + int32(r/2);
% bottom half
p = sum(im(int32(end/2):end, :), 1);
y2 = median(p(find(p > 0))) + int32(r/2);
% left half
p = sum(im(:, 1:int32(end/2)), 2);
x1 = -median(p(find(p > 0))) + int32(c/2);
% right half
p = sum(im(:, int32(end/2):end), 2);
x2 = median(p(find(p > 0))) + int32(c/2);

% crop the image using the bounds
rect = [x1 y1 x2-x1 y2-y1];
cr = imcrop(im, rect);
im2 = zeros(size(im));
im2(y1:y2, x1:x2) = cr;

figure,
axis equal
subplot(1, 2, 1)
imagesc(im)
hold on
plot([x1 x2 x2 x1 x1], [y1 y1 y2 y2 y1], 'g-')
hold off
subplot(1, 2, 2)
imagesc(im2)