谢尔平斯基三角形分形 - 产生随机性的最简方法

1.什么是谢尔平斯基三角形？ 

谢尔平斯基三角形是一组多个（或无限）三角形。看看下面的谢尔平斯基三角形，看看它看起来有多无限。

这里的概念是，填充的三角形由中心的空等边三角形填充，使得这个三角形空间与围绕它形成的三个三角形一致。

如下图所示 ，我们将要使用的概念很简单。如果有一个或多个图块，而不是在图块空间上方的三个图块，那么，我们将在该空间中放置一个图块，否则，就没有图块！

也就是说我们只在黑三角里面放置白三角，其余地方不放。

2.如何用代码实现谢尔平斯基三角形？

文件有三个，分别是：main.cpp、SierpinskiTile.h、SierpinskiTile.cpp

在 SierpinskiTile.h 中：
我们需要包含 SDL.h 用于绘图和列表，以便包含SDL_Rect* 或图块的列表。 

#include <SDL.h>
#include <list>

#ifndef _SIERPINSKI_TILE_
#define _SIERPINSKI_TILE_

class SierpinskiTile
{
public:
	SierpinskiTile(int scrW, int scrH, int w, int h)
		: scrW(scrW), scrH(scrH), tileW(w), tileH(h) {};
	~SierpinskiTile();
	void setTile(int x_index, int y_index);
	bool isThereTile(int x_index, int y_index);
	void calculate(int y_index = -1);
	void draw(SDL_Renderer*& renderer, int r, int g, int b, int y_index);

private:
	int scrW, scrH;
	int tileW, tileH;
	std::list<SDL_Rect*> rects;
};

#endif 

 SierpinskiTile.cpp文件内容如下：

#include "SierpinskiTile.h"

SierpinskiTile::~SierpinskiTile() //Deleting all resources in destructor
{
	for (auto itr : rects)
		delete itr;
}

//setTile（） 方法在磁贴索引位置上设置小三角片：
void SierpinskiTile::setTile(int x_index, int y_index) //Setting tile on the tile index position
{
	SDL_Rect* rectToAdd = new SDL_Rect;
	rectToAdd->x = x_index * tileW;
	rectToAdd->y = y_index * tileH;
	rectToAdd->w = tileW;
	rectToAdd->h = tileH;

	rects.push_back(rectToAdd);
}

//isThereTile（） 方法，用于找出给定的磁贴索引位置是否具有小三角片：
bool SierpinskiTile::isThereTile(int x_index, int y_index) //Finding out whether a tile is here or not
{
	for (auto itr : rects)
		if (itr->x == tileW * x_index
			&& itr->y == tileH * y_index)
			return true;

	return false;
}



//compute（） 方法找出下一行的小三角片排列，默认参数为 -1，这将导致 calculate（）方法计算所有行的磁贴排列：
void SierpinskiTile::calculate(int y_index) //Calculating where to put tiles in the next row
//by the tile arrangement present in the previous row
{
	/
	//Conditions for putting a tile below the upper tile (or tile space):
	// 1- Tile is at that spot, 0- Tile is not at that spot, X- Unknown (can be 0 or 1)

	/
	// Case 1: 0 1 0, Case 2: 1 0 0, Case 3: 0 0 1,
	// Case 4: 1 1 0, Case 5: 1 0 1, Case 6: 0 1 1

	// Output for Cases 1-6: X 1 X

	/
	// Case 7: 0 0 0, Case 8: 1 1 1

	// Output for Cases 7-8: X 0 X

	int y = 0;
	if (y_index > -1)
	{
		y = y_index;

		for (int x = 0; x < scrW / tileW; x++)
		{
			if ((isThereTile(x, y) || isThereTile(x + 1, y) || isThereTile(x - 1, y))
				&& !(isThereTile(x, y) && isThereTile(x + 1, y) && isThereTile(x - 1, y))
				)
				setTile(x, y + 1);
		}
	}
	else
	{
		for (; y < scrH / tileH; y++)
			for (int x = 0; x < scrW / tileW; x++)
			{
				if ((isThereTile(x, y) || isThereTile(x + 1, y) || isThereTile(x - 1, y))
					&& !(isThereTile(x, y) && isThereTile(x + 1, y) && isThereTile(x - 1, y))
					)
					setTile(x, y + 1);
			}
	}
}


// draw（） 方法实际上只绘制一行并删除前面所有行中的所有小三角片：
void SierpinskiTile::draw(SDL_Renderer*& renderer, int r, int g, int b, int y_index)
{
	SDL_SetRenderDrawColor(renderer, r, g, b, 255); //Setting renderer's color

	std::list<SDL_Rect*> deleteRects; //For getting a list of rectangles/tiles to be deleted
	for (auto itr : rects)
	{
		SDL_RenderFillRect(renderer, itr); //Draw all tiles present in the rects which
		//will be just all tiles in the particular row

		if (itr->y <= tileH * y_index) //Put all tiles of rows before the given row
			//to deleteRects for deleting
			deleteRects.push_back(itr);
	}

	for (auto itr : deleteRects) //Delete all collected tiles and clear them
	{
		rects.remove(itr);
		delete itr;
	}
	deleteRects.clear();

	SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255); //Resetting renderer's color
}

 main.cpp文件，首先，我们需要一些必要的常量以及SDL_Window*，SDL_Renderer*和SDL_Event，此外我们还需要一个布尔值。

#include <SDL.h>
#include "SierpinskiTile.h"

#undef main //Solution to the problem: No entry point defined.

const int SCR_W = 640;
const int SCR_H = 480;
const int TILE_W = 5; //Each tile's width
const int TILE_H = 5; //Each tile's height

SDL_Window* window = NULL;
SDL_Renderer* renderer = NULL;
SDL_Event event;

bool quit = false;

SierpinskiTile* generator = NULL;

int main(int argc, char** args)
{
	SDL_Init(SDL_INIT_VIDEO); //Initializing SDL2

	window = SDL_CreateWindow("Koch Fractal", SDL_WINDOWPOS_UNDEFINED,
		SDL_WINDOWPOS_UNDEFINED, SCR_W, SCR_H, SDL_WINDOW_SHOWN); //Creating window
	renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED); //Creating renderer

	SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255); //Setting default screen color

	generator = new SierpinskiTile(SCR_W, SCR_H, TILE_W, TILE_H); //Creating fractal generator
	generator->setTile((SCR_W / TILE_W) / 2, 0); //Setting a tile at the top middle of the screen

	int row = 0;
	while (!quit)
	{
		while (SDL_PollEvent(&event) > 0) //Minimal event polling for proper quitting
			if (event.type == SDL_QUIT)
				quit = true;

		//***NOTE: Screen must not be cleaned as the draw() method draws a row only
		//and deletes all tiles of the previous rows***
		//SDL_RenderClear(renderer);

		if (row < SCR_H / TILE_H) //Draw and calculate until the last row
		{
			generator->draw(renderer, 0, 255, 0, row-1); //Drawing the row in green color

			SDL_RenderPresent(renderer); //Updating screen

			generator->calculate(row++); //Calculating the next row
		}
	}

	delete generator; //Deallocating fractal generator

	SDL_DestroyRenderer(renderer);
	SDL_DestroyWindow(window);
	SDL_Quit(); //Clearing all SDL resources

	return 0;
}

3. 运行结果如何？

下图显示了结果，细节取决于常量TILE_W和TILE_H。值越小，分形显示的细节就越详细。