Filling a circle with alpha blending will create visible stripes

[joshinils]

the problem seems to stem from the fact that in the FillCircle method the individual lines sometimes get draw multiple times:

here is my modified example code to generate the above image:

#define OLC_PGE_APPLICATION
#include "olcPixelGameEngine.h"

class Example : public olc::PixelGameEngine
{
public:
	Example()
	{
		sAppName = "Example";
	}

public:
	bool OnUserCreate() override
	{
		// Called once at the start, so create things here
		olc::PixelGameEngine::SetPixelMode(olc::Pixel::ALPHA);
		this->Clear(olc::BLACK);
		FillCircle(100, 100, 60, olc::Pixel(255,0,0,200));
		return true;
	}

	bool OnUserUpdate(float fElapsedTime) override
	{
		// called once per frame
		return true;
	}
};


int main()
{
	Example demo;
	if (demo.Construct(256, 256, 2, 2))
		demo.Start();

	return 0;
}

and here the modified FillCircle to show that is has overdrawn:

	void PixelGameEngine::FillCircle(int32_t x, int32_t y, int32_t radius, Pixel p)
	{
		// Taken from wikipedia
		int x0 = 0;
		int y0 = radius;
		int d = 3 - 2 * radius;
		if (!radius) return;

		static std::vector<std::vector<int>> hits(260, std::vector<int>(260,0));
		auto drawline = [&](int sx, int ex, int ny)
		{
			for (int i = sx; i <= ex; i++)
			{
				hits[i][ny]++;
				Draw(i, ny, p);
			}
		};

		while (y0 >= x0)
		{
			// Modified to draw scan-lines instead of edges
			drawline(x - x0, x + x0, y - y0);
			drawline(x - y0, x + y0, y - x0);
			drawline(x - x0, x + x0, y + y0);
			drawline(x - y0, x + y0, y + x0);
			if (d < 0) d += 4 * x0++ + 6;
			else d += 4 * (x0++ - y0--) + 10;
		}

		for(auto v : hits)
		{
			for(int i : v)
				std::cout << i;
			std::cout << std::endl;
		}
	}

on the console (background in the image) one can clearly see multiple calls to draw for the same position, because alphablending is enabled this leads to brighter reds than at other positions, hence the stripes.