OpenGL: 平面圆绘制与贴图

两种画圆的方法，DrawCircle2()为一般的做法，利用GL_LINE_STRIP实现，

void DrawCircle2(float cx, float cy, float r, int num_segments)
{
	glBegin(GL_LINE_STRIP);
	for (int i = 0; i <= num_segments; i++)
	{
		glVertex2f(cx + r * cos((2 * M_PI * i) / num_segments), 
			   cy + r * sin((2 * M_PI * i) / num_segments));
	}
	glEnd();
}

DrawCircle1()则是利用GL_LINES_LOOP实现，

void DrawCircle1(float cx, float cy, float r, int num_segments) 
{ 
	float theta = 2 * M_PI / float(num_segments); 
	float tangetial_factor = tanf(theta);//calculate the tangential factor 
	float radial_factor = cosf(theta);//calculate the radial factor 
	float x = r;//we start at angle = 0 
	float y = 0; 

	glBegin(GL_LINE_LOOP); 
	for(int ii = 0; ii < num_segments; ii++) 
	{ 
		glVertex2f(x + cx, y + cy);//output vertex 
		//calculate the tangential vector 
		//remember, the radial vector is (x, y) 
		//to get the tangential vector we flip those coordinates and negate one of them 
		float tx = -y; 
		float ty = x; 
		//add the tangential vector 
		x += tx * tangetial_factor; 
		y += ty * tangetial_factor; 
		//correct using the radial factor 
		x *= radial_factor; 
		y *= radial_factor; 
	} 
	glEnd(); 
}

但是上面两个函数都只是画出了两个圆圈，想要给circle贴图，必须画出的是一个区域，所以可以利用GL_TRIANGLE_FAN绘制，要实现纹理映射，关键是纹理坐标的分配：

圆心纹理坐标为：(0.5, 0.5)选取图片的中心。

圆圈上的点的分配：

纹理坐标必须在0，1之间，而且这些纹理坐标和圆的半径没有关系，只和圆心角有关。

因为-1< cos(delta_angle*i) <1,则==> 0 <= (cos(delta_angle*i) + 1.0)*0.5 <= 1

同理：0 <= (sin(delta_angle*i) + 1.0)*0.5 <= 1

GLvoid draw_circle(const GLfloat radius,const GLuint num_vertex)
{
	GLfloat vertex[4]; 
	GLfloat texcoord[2];

	const GLfloat delta_angle = 2.0*M_PI/num_vertex;

	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D,texName);
	glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_REPLACE);
	glBegin(GL_TRIANGLE_FAN);

	//draw the vertex at the center of the circle
	texcoord[0] = 0.5;
	texcoord[1] = 0.5;
	glTexCoord2fv(texcoord);
	vertex[0] = vertex[1] = vertex[2] = 0.0;
	vertex[3] = 1.0;        
	glVertex4fv(vertex);

	//draw the vertex on the contour of the circle
	for(int i = 0; i < num_vertex ; i++)
	{
		texcoord[0] = (std::cos(delta_angle*i) + 1.0)*0.5;
		texcoord[1] = (std::sin(delta_angle*i) + 1.0)*0.5;
		glTexCoord2fv(texcoord);

		vertex[0] = std::cos(delta_angle*i) * radius;
		vertex[1] = std::sin(delta_angle*i) * radius;
		vertex[2] = 0.0;
		vertex[3] = 1.0;
		glVertex4fv(vertex);
	}

	texcoord[0] = (1.0 + 1.0)*0.5;
	texcoord[1] = (0.0 + 1.0)*0.5;
	glTexCoord2fv(texcoord);

	vertex[0] = 1.0 * radius;
	vertex[1] = 0.0 * radius;
	vertex[2] = 0.0;
	vertex[3] = 1.0;
	glVertex4fv(vertex);
	glEnd();

	glDisable(GL_TEXTURE_2D);
}

最终的效果见下图：

 

完整代码如下:

#include 
#include 
#include 

#pragma comment(lib,"glut32.lib")
#pragma comment(lib,"glu32.lib")
#pragma comment(lib,"opengl32.lib")

GLfloat spin = 0.0;

#define M_PI 3.1415926

#define checkImageWidth  64
#define checkImageHeight 64
GLubyte checkImage [checkImageWidth][checkImageHeight][4];
GLuint texName;

void DrawCircle2(float cx, float cy, float r, int num_segments)
{
	glBegin(GL_LINE_STRIP);
	for (int i = 0; i <= num_segments; i++)
	{
		glVertex2f(cx + r * cos((2 * M_PI * i) / num_segments), 
			cy + r * sin((2 * M_PI * i) / num_segments));
	}
	glEnd();
}
void DrawCircle1(float cx, float cy, float r, int num_segments) 
{ 
	float theta = 2 * M_PI / float(num_segments); 
	float tangetial_factor = tanf(theta);//calculate the tangential factor 
	float radial_factor = cosf(theta);//calculate the radial factor 
	float x = r;//we start at angle = 0 
	float y = 0; 

	glBegin(GL_LINE_LOOP); 
	for(int ii = 0; ii < num_segments; ii++) 
	{ 
		glVertex2f(x + cx, y + cy);//output vertex 
		//calculate the tangential vector 
		//remember, the radial vector is (x, y) 
		//to get the tangential vector we flip those coordinates and negate one of them 
		float tx = -y; 
		float ty = x; 
		//add the tangential vector 
		x += tx * tangetial_factor; 
		y += ty * tangetial_factor; 
		//correct using the radial factor 
		x *= radial_factor; 
		y *= radial_factor; 
	} 
	glEnd(); 
}
GLvoid draw_circle(const GLfloat radius,const GLuint num_vertex)
{
	GLfloat vertex[4]; 
	GLfloat texcoord[2];

	const GLfloat delta_angle = 2.0 * M_PI / num_vertex;

	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D, texName);
	glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

	glBegin(GL_TRIANGLE_FAN);
	{
		//draw the vertex at the center of the circle
		texcoord[0] = 0.5;
		texcoord[1] = 0.5;
		glTexCoord2fv(texcoord);
		vertex[0] = vertex[1] = vertex[2] = 0.0;
		vertex[3] = 1.0;        
		glVertex4fv(vertex);

		//draw the vertex on the contour of the circle
		for(GLuint i = 0; i < num_vertex ; i++)
		{
			texcoord[0] = (cos(delta_angle * i) + 1.0) * 0.5;
			texcoord[1] = (sin(delta_angle * i) + 1.0) * 0.5;
			glTexCoord2fv(texcoord);

			vertex[0] = cos(delta_angle * i) * radius;
			vertex[1] = sin(delta_angle * i) * radius;
			vertex[2] = 0.0;
			vertex[3] = 1.0;
			glVertex4fv(vertex);
		}

		texcoord[0] = (1.0 + 1.0) * 0.5;
		texcoord[1] = (0.0 + 1.0) * 0.5;
		glTexCoord2fv(texcoord);

		vertex[0] = 1.0 * radius;
		vertex[1] = 0.0 * radius;
		vertex[2] = 0.0;
		vertex[3] = 1.0;
		glVertex4fv(vertex);
	}
	glEnd();

	glDisable(GL_TEXTURE_2D);
}
void makeCheckImage()
{
	int i, j, c;
	for (i = 0; i < checkImageHeight; i++)
	{
		for (j = 0; j < checkImageWidth; j++)
		{
			c = (((i & 0x8) == 0) ^ ((j & 0x8)) == 0) * 255;
			checkImage[i][j][0] = (GLubyte)c;
			checkImage[i][j][1] = (GLubyte)c;
			checkImage[i][j][2] = (GLubyte)c;
			checkImage[i][j][3] = (GLubyte)255;
		}
	}
}
void init()
{
	glClearColor(0.0, 0.0, 0.0, 0.0);
	glShadeModel(GL_SMOOTH);

// 	glEnable(GL_DEPTH_TEST);
// 	glClearDepth(1.0);
	makeCheckImage();
	glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

	glGenTextures(1, &texName);
	glBindTexture(GL_TEXTURE_2D, texName);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, checkImage);
}
void display(void)
{
	glLineWidth(3.0);
	glColor3f(1.0, 1.0, 1.0);
	DrawCircle2(0.0, 0.0, 2.0, 20);
	glColor3f(1.0, 1.0, 0.0);
	DrawCircle1(0.0, 0.0, 1.5, 20);
	glColor3f(1.0, 0.0, 1.0);
	draw_circle(1.0, 20);

    glFlush ();

    glutSwapBuffers();
}

void reshape (int w, int h)
{
	if (h == 0)
	{
		h = 1;
	}
    glViewport (0, 0, (GLsizei) w, (GLsizei) h);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0, (GLfloat)w / (GLfloat)h, 1.0, 20.0);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	glTranslatef(0.0, 0.0, -5.0);
}
void spinDisplay(void)
{
    spin = spin + 1.0;
    if (spin > 360.0)
	{
		spin = spin - 360.0;
	}
    glutPostRedisplay();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize (500, 500); 
    glutInitWindowPosition (100, 100);
    glutCreateWindow (argv[0]);
    init ();
    glutDisplayFunc(display); 
    glutReshapeFunc(reshape);
    glutIdleFunc(spinDisplay);
    glutMainLoop();
    return 0;
}

运行结果如下:

http://blog.csdn.net/ryfdizuo/article/details/3968697