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convert from binary to grayscale without affecting the binary image

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I am using the zhang-suen skeletonization on the following image
The result of skeletonization is as follows
I would like to perform subpixel edge detection on this,and for that to work the image needs to be changed back to grayscale or double.How do I go about that.In the end I am interested in finding the points corresponding to the skeleton.
Basically i want something like this
I tried running skeletonization on this but i was unable to produce the result similar to the one shown in the second image.

Accepted Answer

Image Analyst
Image Analyst on 7 Sep 2021
Why do you think you need edge detection? Why do you want/need to "perform subpixel edge detection"?
The vast majority of time people here do edge detection they don't need it and it's the wrong thing to do.
If you really want the edges, you can use bwperim() or bwboundaries() but they are on pixels, not subpixels.
To cast a uint8 image to double, use
dblImage = mat2gray(grayImage) ; % Puts into 0-1 range.
dblImage = double(grayImage); % Leaves as original range
sparsh garg
sparsh garg on 7 Sep 2021
ok i will try that and get back to you.Also wouldn't that give me the foreground values also,the ones in the white(Just asking)?
Image Analyst
Image Analyst on 7 Sep 2021
Yes. If you want the black pixels for some reason, do
[rows, columns] = find(~yourSkeletonImage);
Also, they may not be in the order you want. They will be in column major order. Basically all rows in a column, then all rows in the next column and so on across the image. So, for example it would not be adjacent coordinates like would follow a sprial.

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