How to smooth the perimeter output of bwperim? - matlab

Question

I have a segmented image
When I apply bwperim function on this I get the output as below
I want to have a thin line of perimeter - just one pixel-thick. This is essential for further processing work. What is the best approach?
Please suggest.
======
BoundingBox
%%% ComputeBoundingBox
%%%
function [stats, statsAlreadyComputed] = ...
ComputeBoundingBox(imageSize,stats,statsAlreadyComputed)
% [minC minR width height]; minC and minR end in .5.
if ~statsAlreadyComputed.BoundingBox
statsAlreadyComputed.BoundingBox = 1;
[stats, statsAlreadyComputed] = ...
ComputePixelList(imageSize,stats,statsAlreadyComputed);
num_dims = numel(imageSize);
for k = 1:length(stats)
list = stats(k).PixelList;
if (isempty(list))
stats(k).BoundingBox = [0.5*ones(1,num_dims) zeros(1,num_dims)];
else
min_corner = min(list,[],1) - 0.5;
max_corner = max(list,[],1) + 0.5;
stats(k).BoundingBox = [min_corner (max_corner - min_corner)];
end
end
end

Answer 1

That is happening because your image had quantization error when you were saving the image. Did you save your image using a lossy compression algorithm, like JPEG? If you want to preserve the intensities so that they don't change when you save the image, use a lossless compression algorithm, like PNG.
To eliminate these "noisy" effects, threshold your image first to eliminate any quantization errors so that you can set these pixels to completely white, then try using bwperim again. In other words, do something like this:
im = im2bw(imread('http://i.stack.imgur.com/dagEc.png'));
im_noborder = imclearborder(im);
out = bwperim(im_noborder);
imshow(out);
The first line of code reads in your image directly from StackOverflow and we use im2bw to threshold your image. This image was originally grayscale, and so we want to convert this into black and white only. This will also remove any quantization artifacts as it thresholds anything higher than 128. The next line of code removes the white border with imclearborder that surrounds your shape because the image you uploaded has a white border surrounding it for some reason. Once we remove this border, we then apply bwperim and we show the image.
This is the image I get: