siv_trail_polygon_highlight.cpp

# include <Siv3D.hpp> // OpenSiv3D v0.6.11

struct PolygonHighlightEffect : IEffect
{
	Array<TrailMotion> trails;

	Polygon polygon;

	PolygonHighlightEffect(const Polygon& targetPolygon)
		: polygon{ targetPolygon }
	{
		constexpr std::array<ColorF, 3> colors = { Palette::White, Palette::Magenta, Palette::Cyan };

		for (int iTrail : step(3))
		{
			trails << TrailMotion{}
				.setLifeTime(DisplayTime + 0.5 * iTrail)
				.setColor(ColorF{ colors[iTrail], 1 - 0.2 * iTrail })
				.setSizeFunction([=](double timeSec) {
					const double size = 2 + 3 * iTrail;
					const double sizeScale1 = timeSec < LifeTime ? 1 : 0;
					const double sizeScale2 = timeSec < 0.1 ? timeSec / 0.1 : 1;
					const double displayTimeScaled = DisplayTime * 0.7;
					const double sizeScale3 = timeSec < displayTimeScaled ? 1 : EaseInExpo(1 - ((timeSec - displayTimeScaled) / (LifeTime - displayTimeScaled)));
					return size * sizeScale1 * sizeScale2 * sizeScale3;
				})
				.setPositionFunction([&](double timeSec) {
					const double t = Min(timeSec, DisplayTime);
					const double t0_1 = t / (DisplayTime * 0.8);
					const auto& vertices = polygon.outer();
					const int index1 = ((int)(t0_1 * vertices.size())) % vertices.size();
					const int index2 = (index1 + 1) % vertices.size();
					double tmp;
					const double lerpFactor = modf(t0_1 * vertices.size(), &tmp);
					const auto v = vertices[index1].lerp(vertices[index2], EaseInExpo(lerpFactor));
					return v;
				});
		}
	}

	bool update(double timeSec) override
	{
		for (auto& trail : trails)
		{
			trail.update();
		}

		{
			const ScopedRenderStates2D blend{ BlendState::Additive };

			for (const auto& trail : trails)
			{
				trail.draw();
			}
		}

		return timeSec < 3.0;
	}

private:
	inline static constexpr double DisplayTime = 0.3;
	inline static constexpr double LifeTime = 1.0;
};

void Main()
{
	Scene::SetBackground(ColorF{ 0 });

	const RectF rect{ Arg::center = Scene::CenterF().movedBy(-160, -140), SizeF{ 200, 180 }};
	const Circle circle{ Scene::CenterF().movedBy(160, 140), 100 };
	const Polygon star = Shape2D::Star(128, Scene::CenterF().movedBy(-160, 140));
	const Polygon nstar = Shape2D::NStar(12, 128, 96, Scene::CenterF().movedBy(160, -140));


	Effect effect;

	while (System::Update())
	{
		rect.drawFrame(1, 0, Palette::Silver);
		circle.drawFrame(1, 0, Palette::Silver);
		star.drawFrame(1, Palette::Silver);
		nstar.drawFrame(1, Palette::Silver);

		if (rect.leftClicked())
		{
			effect.add<PolygonHighlightEffect>(rect.asPolygon());
		}
		else if (circle.leftClicked())
		{
			effect.add<PolygonHighlightEffect>(circle.asPolygon());
		}
		else if (star.leftClicked())
		{
			effect.add<PolygonHighlightEffect>(star);
		}
		else if (nstar.leftClicked())
		{
			effect.add<PolygonHighlightEffect>(nstar);
		}
		else if (Scene::Rect().leftClicked())
		{
			effect.add<PolygonHighlightEffect>(Scene::Rect().stretched(-1).asPolygon());
		}

		effect.update();
	}
}

result_polygon_highlight.mp4