Main.cpp

# include <Siv3D.hpp>

//
//	参考
//	『詳解 画像処理プログラミング』
//	http://www.sbcr.jp/products/4797344370.html
//
int Solve2(double a, double b, double c, double& x1, double& x2)
{
	if (a == 0.0)
	{
		if (c == 0.0)
		{
			return false;
		}
		
		x1 = -b / c;
		x2 = -b / c;
		
		return true;
	}

	double dd = b*b - 4.0*a*c;
	x1 = 0.0;
	x2 = 0.0;

	if (dd < 0.0)
	{
		return false;
	}
	
	x1 = (-b + sqrt(dd)) / (2.0*a);
	x2 = (-b - sqrt(dd)) / (2.0*a);
	
	return true;
}

void Solve(
	double a, double b, double c, double d,
	double e, double f, double g, double h,
	double& xa, double& ya, double& xb, double& yb
	)
{
	double p1 = a*h - f*c;
	double p2 = d*f - c*g + b*h - a*e;
	double p3 = d*g - b*e;

	if (p1 == 0.0)
	{
		xa = d / b;
		ya = (b*e - d*g) / (b*h + d*f);
		xb = -1000;
		yb = -1000;
		return;
	}
	
	if (!Solve2(p1, p2, p3, ya, yb))
	{
		xa = -1000;
		xb = -1000;
		return;
	}

	double tm = a* ya + b;

	if (tm)
	{
		xa = (d - c * ya) / tm;
	}
	else
	{
		xa = -1000;
	}

	tm = a* yb + b;
	
	if (tm)
	{
		xb = (d - c * yb) / tm;
	}
	else
	{
		xb = -1000.0;
	}
}

void Transform(const Image& src, Image& dst, const double(&px)[4], const double(&py)[4])
{
	const int rx1 = 0;
	const int ry1 = 0;
	const int rx2 = src.height - 1;
	const int ry2 = src.width - 1;
	double aa = px[2] - px[3] - px[1] + px[0];
	double bb = px[1] - px[0];
	double cc = px[3] - px[0];
	double ff = py[2] - py[3] - py[1] + py[0];
	double gg = py[1] - py[0];
	double hh = py[3] - py[0];

	if (!aa) aa = 0.001;
	if (!ff) ff = 0.001;

	for (int y = 0; y < dst.height; ++y)
	{
		for (int x = 0; x < dst.width; ++x)
		{
			const double xx = x + 0.5;
			const double yy = y + 0.5;

			if (!Quad(px[0], py[0], px[3], py[3], px[2], py[2], px[1], py[1]).intersects(Vec2(xx,yy)))
			{
				continue;
			}

			double s1, t1, s2, t2, tx, ty;
			const double dd = xx - px[0];
			const double ee = yy - py[0];

			Solve(aa, bb, cc, dd, ee, ff, gg, hh, s1, t1, s2, t2);

			if (s1 >= 0 && t1 >= 0 && s1 <= 1.0 && t1 <= 1.0)
			{
				tx = s1*(rx2 - rx1) + rx1;
				ty = t1*(ry2 - ry1) + ry1;
			}
			else
			{
				tx = s2*(rx2 - rx1) + rx1;
				ty = t2*(ry2 - ry1) + ry1;
			}

			if (tx < 0 || ty < 0)
			{
				continue;
			}

			dst[y][x] = src.sample(tx-0.5, ty-0.5);
		}
	}
}

void Transform(const Image& src, Image& dst, const Quad& to)
{
	if (&src == &dst)
	{
		return;
	}

	const double xs[4] = { to.tl.x, to.bl.x, to.br.x, to.tr.x };
	const double ys[4] = { to.tl.y, to.bl.y, to.br.y, to.tr.y };

	Transform(src, dst, xs, ys);
}

void Main()
{	
	Image image(L"Example/Windmill.png");
	Image canvas(640, 480, Color(0, 0));

	Quad quad({ 0, 0 }, { image.width, 0 }, { image.width, image.height }, { 0, image.height });
	Transform(image, canvas, quad);

	DynamicTexture texture(canvas);
	optional<int> grabbed;

	while (System::Update())
	{
		if (Input::MouseL.released)
		{
			grabbed = none;

			canvas.fill(Color(0, 0));

			Transform(image, canvas, quad);

			texture.fill(canvas);
		}

		if (grabbed)
		{
			quad.p[grabbed.get()].moveBy(Mouse::Delta());
		}

		if (Input::MouseL.clicked)
		{
			for (int i = 0; i < 4; ++i)
			{
				if (Circle(quad.p[i], 12).leftClicked)
				{
					grabbed = i;

					break;
				}
			}
		}

		texture.draw();

		for (const auto& pt : quad.p)
		{
			Circle(pt, 12).drawFrame(4, 0, Palette::Orange);
		}
	}
}