實驗一：Bresenham’s Line Algorithm
不同象限不同斜率畫法思路如下：
（為濾清思路臨時打的草稿）

// ====== Computer Graphics Experiment #1 ======
// |     Bresenham's Line Drawing Algorithm    |
// |                                           |
// =============================================
//
// Requirement:
// Implement a general Bresenham's Line Drawing Algorithm.
 

// Only use GL_POINTS drawing mode. 
// Do not use OpenGL line drawing capability.

#include <windows.h>
#include <GL/glut.h>
#include <math.h>

#define PI 3.14159265359
int width, height;
// Bresenham line algorithm with line style and line width
void MyLine(int xs, int ys, int xe, int ye)
 

{
	int dx, dy, p, i;
	int x, y;
	dx = xe - xs; dy = ye - ys;
	if(dx==0)
	{
		if(dy>0)
		{
			x = xs; y = ys;
//			p = 2 * dy - dx;
        glBegin(GL_POINTS);
			for (i = 0; y < ye; ++i)
			{
                glVertex2i(x, y);
				y++;
			}

			glEnd();
		}
		else
        {
            x = xs; y = ys;
 

//			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; y > ye; ++i)
			{
				glVertex2i(x, y);
				y--;
			}
			glEnd();
		}
	}
	else if(dy==0)
	{
		if(dx>0)
		{
			x = xs; y = ys;
//			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; x < xe; ++i)
			{
				glVertex2i(x, y);
				x++;
			}
			glEnd();
		}
		else
        {
            x = xs; y = ys;
//			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; x > xe; ++i)
			{
				glVertex2i(x, y);
				x--;
			}
			glEnd();
		}
	}
	else if (dx > 0 && dy > 0)//1
	{
		if (dx > dy)
		{
			x = xs; y = ys;
			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; i < dx; ++i)
			{
				glVertex2i(x, y);
				++x; if (p < 0) { p += 2 * dy; }
				else { ++y; p += 2 * dy - 2 * dx; }
			}
			glEnd();
		}
		else
		{
			x = xs; y = ys;
			p = 2 * dx - dy;
			glBegin(GL_POINTS);
			for (i = 0; i < dy; ++i)
			{
				glVertex2i(x, y);
				++y; if (p < 0) { p += 2 * dx; }
				else { ++x; p += 2 * dx - 2 * dy; }
			}
			glEnd();
		}
	}
	else if (dx < 0 && dy < 0)//3
	{
		if (dx < dy)  //-1  >-2
		{
//			dx = -dx; dy = -dy;
			dx = dx; dy = dy;
			x = xs; y = ys;

			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; i < -dx; ++i)
			{
				glVertex2i(x, y);
				--x; if (p > 0) { p += 2 * dy; }
				else { --y; p += 2 * dy - 2 * dx; }
			}
			glEnd();
		}
		else
		{
//			dy=-dy;
			x = xs; y = ys;
			p = 2 * dx - dy;
			glBegin(GL_POINTS);
			for (i = 0; i < -dy; ++i)
			{
				glVertex2i(x, y);
				--y; if (p > 0) { p += 2 * dx; }
				else { --x; p += 2 * dx - 2 * dy; }
			}
			glEnd();
		}
	}
	else if (dx > 0 && dy < 0)//4
	{
		if (abs(dy) < dx)
		{
			dx = dx; dy = -dy; x = xs; y = ys;
			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; i < dx; ++i)
			{
				glVertex2i(x, y);
				++x; if (p < 0) { p += 2 * dy; }
				else { --y; p += 2 * dy - 2 * dx; }
			}
			glEnd();
		}
		else
		{
			dx = dx; dy = -dy;
			x = xs; y = ys;
			p = 2 * dx - dy;
			glBegin(GL_POINTS);
			for (i = 0; i < dy; ++i)
			{
				glVertex2i(x, y);
				--y; if (p < 0) { p += 2 * dx; }
				else { ++x; p += 2 * dx - 2 * dy; }
			}
			glEnd();
		}
	}
	else if (dx < 0 && dy > 0)//2
	{
		if (dy < abs(dx))
		{
			dx = -dx; dy = dy;
			 x = xs; y = ys;
			p = 2 * dy - dx;
			glBegin(GL_POINTS);
			for (i = 0; i < dx; ++i)
			{
				glVertex2i(x, y);
				--x; if (p < 0) { p += 2 * dy; }
				else { ++y; p += 2 * dy - 2 * dx; }
			}
			glEnd();
		}
		else
		{
			dx = -dx; dy = dy;
			x = xs; y = ys;
			p = 2 * dx - dy;
			glBegin(GL_POINTS);
			for (i = 0; i < dy; ++i)
			{
				glVertex2i(x, y);
				++y; if (p > 0) { p -= 2 * dx; }
				else { --x; p -= 2 * dx - 2 * dy; }
			}
			glEnd();
		}
	}

}

// Initialization function
void init(void)
{
	glClearColor (0.0, 0.0, 0.0, 0.0);
}

// Display callback function
void display(void)
{
	int i, x0, y0, x, y;
	double a;

	glClear (GL_COLOR_BUFFER_BIT);

	x0=width/2;
	y0=height/2;

	glColor3f(1.0, 1.0, 1.0);

	// Draw lines
	for (i=0; i<360; i+=15)
	{
		a=(double)i/180.0*PI;
		x=0.45*width*cos(a);
		y=0.45*height*sin(a);
		if (i==0 || i==180) y=0;
		if (i==90 || i==270) x=0;
		MyLine(x0, y0, x0+x, y0+y);
	}

	glFlush();
}

// Reshape callback function
void reshape(int w, int h)
{
	// Record width and height of program window
	width=w;
	height=h;

	// Set clipping window and viewport equal to
	// view area of program window
	glViewport(0, 0, w, h);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluOrtho2D (0.0, w, 0.0, h);
}

// Keyboard callback function
void keyboard(unsigned char key, int x, int y)
{
	switch (key) {
		case 27:
			exit(0);

		// Write your code for line style switching and line width switching
	}
}

// Main program entrance
int main(int argc, char* argv[])
{
	// Create window
	glutInit(&argc, argv);
	glutInitDisplayMode (GLUT_RGB);
	glutInitWindowSize(500, 500);
	glutCreateWindow("Lines");

	// Initialization
	init();

	// Define callback functions
	glutReshapeFunc(reshape);
	glutKeyboardFunc(keyboard);
	glutDisplayFunc(display);

	// Main program loop. (Event loop)
	glutMainLoop();

	return 0;
}