Shader: How can I find the coordinates of the outlining of a sprite?

So, to put into simple terms. This effect is similar to Blizzard’s Hearthstone game. Where there is a green outlining effect on the texture.

I was wondering if there was anyway to do that with sprites. To be more precise. How can I determine if a set of pixels are within the outline boundary, this way it isn’t just 1 pixel outlined, but rather a length of pixels.

I don’t know how given a set of texture coordinates to determine if that pixel is in a “outline area” when I am only allowed to do stuff at a per-pixel and per-vertex level.

Anyone have any insights?

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