在不触摸物体的情况下增加位掩码边框

Question

I've written a bitmask filter in C#, to grow a border, where found pixels should grow border if there is enough space, but the areas may not touch each other

input mask   output mask
..........    .....xxx...
.....x....    .....xxxx...
.....x....    xxx..xxxxxx.
..x..xxxx.    xxx..xxxxxx.
..........    xxx..xxxxxx.

Using a 5x5 binary convolution that checks the border.
The border consists of 8 pixels (x-2,y-2) , (x,y-2) , (x+2,y-2) ... ie corners and centers of borders.
If a pixel is found, I set its corresponding array P[i] to true .
So these 8 pixels are represented in a boolean array starting from P[0] trough p[7] .

Next I count boolean true s.

If the count of P[.] > 3 then there is another object nearby and I don't grow the border there.

If the count is 1, so if for example (x-2,y)==true then write mask (x-1,y) .

If the count is 2, so if for example (x+2,y)==true && (x+2,y+2)==true then write mask (x+1,y+1) .

I've got quite a lot of boolean comparisons to eventually make this work. I won't bore you width the dull code, but I'm wondering if the principle I use is ideal for this, or if there exists other methods to do this.

As I got a side effect (though not really a problem) that makes the shapes more cubic over every iteration. Which seems to happen less in programs such as PhotoShop.

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some additional info, the code had to run on a video stream, so i prefer fast ways. I wrote "dull" code, if i would print this its 80+ lines most is binary compare code and switch cases too make it fast, 80+ is a to long post for here, much easier to understand when explained in normal language.

Answer 1

Have you heard of mathematical morphology? You more or less implemented a thickening operator (though thickening is defined differently, I think the result is more or less similar), but there are more efficient ways to do the same thing.

One alternative approach is to:

• Compute the distance transform of the inverse of your image. This is an image where the further away from your objects you get, the larger the grey value is. This image will have ridges exactly half-way between the objects. Let's call this image distance .

• Compute the watershed of distance . This is a binary image with lines that run along the ridges of distance . Let's call this separation .

• Threshold distance at the appropriate value to get objects grown by the amount you want them grown. If you want to grow objects by 10 pixels, threshold such that all pixels with a value less or equal to 10 are selected ( true ).

• Subtract separation from distance . The pixels in distance that are set, and also set in separation , should be reset. You now have your original objects grown by your chosen distance, but still separate.

I won't bore you width the dull code...