Mikko Mononen's UI 'feathering' technique

FeatherUI.cpp

void CalculateEdgeNormal(float& nx, float& ny, float x0, float y0, float x1, float y1)
{
	const float x01 = x1 - x0;
	const float y01 = y1 - y0;

	const float length = Sqrt(x01 * x01 + y01 * y01);

	const float dx = x01 / length;
	const float dy = y01 / length;

	nx = dy;
	ny = -dx;
}

void FeatherConvexPolygon(Primitives& primitives, const Vertex* vertices, int count, float amount, const Texture* texture)
{
	Vertex* extruded = Memory::Allocate<Vertex>(Memory::Temp, sizeof(Vertex) * count);

	for (int i = 0; i < count; ++i)
	{
		const Vertex& previous = vertices[(i + count - 1) % count];
		const Vertex& current = vertices[i];
		const Vertex& next = vertices[(i + 1) % count];

		float nx0, ny0, nx1, ny1;

		CalculateEdgeNormal(nx0, ny0, previous.x, previous.y, current.x, current.y);
		CalculateEdgeNormal(nx1, ny1, current.x, current.y, next.x, next.y);

		float nx = (nx0 + nx1) * 0.5f;
		float ny = (ny0 + ny1) * 0.5f;

		extruded[i] = Vertex(current.x + nx * amount, current.y + ny * amount, Color(current.r, current.g, current.b, 0.0f));
	}

	for (int i = 0; i < count; ++i)
	{
		const int j = (i + 1) % count;
		AddQuad(primitives, vertices[i], extruded[i], extruded[j], vertices[j], texture);
	}

	Memory::Free(extruded);
}