Line detection against darker shades

Question

I wants to detect lines from an image with black line drawing over white papers. It could be easy if its ideal 'black and white', using histogram threshold would do.

But, as the image attached shows, some lines (e.g. in the light red circle) are in gray lighter than the shades (e.g. in the dark red circle). So some shades are obtained before light lines using histogram threshold.

Is there any ideas to divide lines from shades with some 'knowledge'? Thanks!

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Edit: Here are the raw images, a bit small because they are of original resolution.

Thanks :-)

Answer 1

I would add another method using Gaussian blur other than erosion+dilation, for your reference:

file='https://i.sstatic.net/oEjtT.png';
I=imread(file);
h = fspecial('gaussian',5,4);
I1=imfilter(I,h,'replicate');
h = fspecial('gaussian',5);
I2=imfilter(I,h,'replicate');
I3=I1-I2;
I3=double(I3(:,:,1));
I3=I3.*(I3>5);
imshow(I3)