MATLAB: How to make image background white

Question

I have a binary image. I want to make the background white keeping the black straight lines intact.I mean those lines which intersecting image objects will remain intact. Picture has attached.

Answer 1

I would start by closing in small gaps to get a solid hand.

imgin_close = imclose(imgin, strel('square', 3));

Then I would find the overlap between that solid hand, and the inverse of the original image, leaving me with just white lines on a black background. You may have some additional noise around the edges of the hand. The could be filtered out with something like bwareaopen, depending on your application.

imgin_lines = imgin_close & ~imgin;
imgin_lines = bwareaopen(imgin_lines, 3);

I would then take the inverse of that image to get just the black lines.

imgout = ~imgin_lines;