Cropping an image with coordinate values

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Sterne_17 2023-1-2

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https://ww2.mathworks.cn/matlabcentral/answers/1887222-cropping-an-image-with-coordinate-values

评论： Image Analyst 2023-1-4

001.jpg

I have an image with 848*1102 pixels after edge detection, and I want to crop out a part of the image accurately, so I need to use the coordinate values of the top left and bottom right points. How do I get the coordinates of these two points and crop the image?

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Answer 1

Image Analyst 2023-1-2

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1887222-cropping-an-image-with-coordinate-values#answer_1139877

编辑：Image Analyst 2023-1-2

在 MATLAB Online 中打开

001.jpg

Try this:

grayImage = imread('001.jpg');

[rows, columns, numberOfColorChannels] = size(grayImage)

rows = 962

columns = 1286

numberOfColorChannels = 3

if numberOfColorChannels == 3

grayImage = grayImage(:, :, 2);

end

subplot(2, 1, 1);

imshow(grayImage);

axis('on', 'image')

% Binarize

mask = grayImage > 128;

% Get rid of things touching the border.

mask = imclearborder(mask);

% Label the image

[LabeledImage, numBlobs] = bwlabel(mask);

% Eliminate the outermost blob, which will have a label of 1. Just keep blobs labeled 2 and up.

mask = ismember(LabeledImage, 2:numBlobs);

% Find the corners of the remaining blobs -- the bounding box.

[r, c] = find(mask);

row1 = min(r)

row1 = 424

row2 = max(r)

row2 = 790

col1 = min(c)

col1 = 437

col2 = max(c)

col2 = 542

% Do the cropping

mask = mask(row1:row2, col1:col2, :);

subplot(2, 1, 2);

imshow(mask);

axis('on', 'image');

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DGM 2023-1-3

编辑：DGM 2023-1-3

在 MATLAB Online 中打开

These images all look a lot like they were saved from a figure. If that's the case, then that's why there's a bunch of white padding on the images. You'll probably find that the sizes don't match the original image either.

Don't save screenshots of images. Write clean, correctly-sized images with imwrite() instead of trying to fix screenshots.

% create a test image

inpict = imread('cameraman.tif');

edgemap = edge(inpict);

% display the image and save a screenshot

imshow(edgemap)

saveas(gcf,'savedfig.png')

% write the image directly

imwrite(edgemap,'edgemap.png')

% to test, read the images back again

badpict = imread('savedfig.png');

goodpict = imread('edgemap.png');

% original size

size(inpict)

ans = 1×2

256 256

% a screenshot is resized, padded, and has changed depth and class

size(badpict)

ans = 1×3

656 874 3

% the properly saved image is fine

size(goodpict)

ans = 1×2

256 256

Also, don't use JPG for any files that need to be processed further, especially not images with hard edges. PNG takes up less space (for this type of content) and doesn't damage the images. This is a section from one of your images (levels adjusted for emphasis).

Sterne_17 2023-1-4

Dear Sir,

The program you have written has helped me a lot and I thank you very much. Because I'm still a beginner, I don't have in-depth knowledge about it.

I would like to describe how my images were generated: a line laser was shone on a metal sphere, which was rotating, and I captured a series of images under that condition with an industrial camera. First I grayed out the images with MATLAB, then I used the canny algorithm for edge detection, after that the resulting images need to be processed with the program you wrote for them. I'm confused because the resolution and file size of the images have become smaller after processing with the program I wrote (grayscale and edge detection), because there is something wrong with the way I saved the images?

I have converted the initial image from jpg to png and then repeated the steps I described above: grayscale and edge detection. I have a question: as I tried to ask you before, I want to crop a series of images to generate a 3D image, and I don't know if the size of the cropped image will affect this step, this is what I want to ask you. If this will not affect my subsequent steps, then just removing the white border as the crop line will be fine.

Attached are a few samples of the image I converted and reprocessed from the initial image format and the image I marked with a color pen, first I want to remove the white border on the image, I draw the approximate location of the crop line with a red dotted line (just remove the white border), after cropping the image I don't need the axes and titles. After cropping the image, I have to remove some invalid white parts due to the reflection of the laser on the surface of the sphere, so that only a part of the surface of the sphere remains. I have circled in red the invalid white parts to be eliminated. The final part to be retained is circled in blue. I need to process a series of images, each with different details, so I need a generalized method.

Best wishes.

Image Analyst 2023-1-4

在 MATLAB Online 中打开

mask.png

Since it's a sphere, and the curved part won't move very much from image to image, I suggestion you just make up a template mask that you can multiply by your image to erase everything outside it. Just take the attached mask and set your gray scale image, or segmented binary image, outside it to zero.

grayImage(~mask) = 0;

That way nothing on the border or any of those spurious reflections will appear after masking.

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Answer 2

KALYAN ACHARJYA 2023-1-2

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1887222-cropping-an-image-with-coordinate-values#answer_1139577

在 MATLAB Online 中打开

[rows,colm]=find(image==1);

Get the indices as rows & colm

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Sterne_17 2023-1-2

编辑：Sterne_17 2023-1-2

Hello, my aim is to remove the white border and keep the rest of the pattern. Please help me. Thanks!

Sterne_17 2023-1-2

Hi, can you explain the code of the first step (Get the indices as rows & colm) in more detail? Because I can't get it to run. Thanks!

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Cropping an image with coordinate values

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Cropping an image with coordinate values

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