How to save the image WITHOUT the white background using imwrite (advanced problem)

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Salad Box
Salad Box on 3 Sep 2019
Edited: Salad Box on 3 Sep 2019
Hi
RGB = imread('bird1.png');
figure
imshow(RGB)
Then I get:
Then:
s = size(RGB);
rectangle('position',[1 1 s(2) s(1)], 'edgecolor', [1 0 0])
rectangle('position',[0 0 s(2) s(1)], 'edgecolor', [1 0 0])
I get:
Now I would like to save this image WITHOUT the white background, only the content within the red box (including the red box).
Imwrite needs to use 'RGB', however after the red box been added onto the image, RGB is no longer the original RGB. It WON'T be correct just to use
imwrite(RGB, 'bird1.png');
Could anyone provide correct solution to this question please?
  3 Comments
Adam
Adam on 3 Sep 2019
Ah well, that is different! None of the images you previously showed had any white background outside the red rectangle so I assumed you meant the white within it.

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Accepted Answer

Image Analyst
Image Analyst on 3 Sep 2019
Try this:
% To burn the color into the image itself.
[rows, columns, numberOfCOlorChannels] = size(RGB)
RGB(:, 1, 1:3) = repmat([255, 0, 0], rows, 1);
RGB(:, end, 1:3) = repmat([255, 0, 0], rows, 1);
RGB(1, :, 1:3) = repmat([255, 0, 0], columns, 1);
RGB(end, :, 1:3) = repmat([255, 0, 0], columns, 1);
imshow(RGB);
imwrite(RGB, filename);
It's much more robust than saving the bitmap in the overlay, which is subject to change whenever you resize the figure window.

More Answers (1)

Johannes Fischer
Johannes Fischer on 3 Sep 2019
% read image as NxMx3 rgb matrix
RGB = imread('bird1.png');
imshow(RGB)
s = size(RGB);
rectangle('position',[1 1 s(2) s(1)], 'edgecolor', [1 0 0])
rectangle('position',[0 0 s(2) s(1)], 'edgecolor', [1 0 0])
% get a handle of the axis
F = getframe(gca);
% and save the color information of the axis, which is stored as RGB in
% the field 'cdata'
imwrite(F.cdata, 'bird2.png')
  5 Comments
Johannes Fischer
Johannes Fischer on 3 Sep 2019
Interesting... it seems to be larger by one pixel in row or column dimension, depending on the input size. I thought it would keep the size as long as no resizing is performed.
But you're right, you're solution is more robust, and in terms of batch processing probably much faster.

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