计算机图形学第四章练习——画饼图

计算机图形学第四章后边示例代码里有一段画饼图的练习，画出来是这样的

中间有一段中心画圆法未实现，搜集了网上资料补全并执行了这段代码，作为openGL的学习练习

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

GLsizei winWidth = 400, winHeight = 300;

const GLdouble twoPi = 6.283185;

class scrPt
{
public:
    GLint x, y;
};

class screenPt
{
public:
    screenPt()
    {
        x = y = 0;
    }

    GLint x, y;
    void setCoords(GLint xCoordValue, GLint yCorrdValue)
    {
        x = xCoordValue;
        y = yCorrdValue;
    }

    GLint getx() const
    {
        return x;
    }

    GLint gety() const
    {
        return y;
    }

    void incrementx()
    {
        x++;
    }

    void decrementy()
    {
        y--;
    }
};

void init(void)
{
    glClearColor(1.0, 1.0, 1.0, 1.0);            // 将窗口北京初始化为白色
    glMatrixMode(GL_PROJECTION);                 // 对投影矩阵操作
    gluOrtho2D(0.0, 200.0, 0.0, 150.0);          // 建立一个2D坐标系
}

void setPixel(GLint xCoord, GLint yCoord)
{
    glBegin(GL_POINTS);
    glVertex2i(xCoord, yCoord);
    glEnd();
}

void circlePlotPoints(GLint xc, GLint yc, screenPt circPt)
{
    setPixel(xc + circPt.getx(), yc + circPt.gety());
    setPixel(xc - circPt.getx(), yc + circPt.gety());
    setPixel(xc + circPt.getx(), yc - circPt.gety());
    setPixel(xc - circPt.getx(), yc - circPt.gety());
    setPixel(xc + circPt.gety(), yc + circPt.getx());
    setPixel(xc - circPt.gety(), yc + circPt.getx());
    setPixel(xc + circPt.gety(), yc - circPt.getx());
    setPixel(xc - circPt.gety(), yc - circPt.getx());
}

// 中心画圆法
void circleMidpoint(scrPt circCtr, GLint radius){
    screenPt circPt;
    GLint p = 1 - radius;
    circPt.setCoords(0, radius);

    circlePlotPoints(circCtr.x, circCtr.y, circPt);
    while (circPt.getx() < circPt.gety()){
        circPt.incrementx();
        if (p < 0)
            p += 2 * circPt.getx() + 1;
        else{
            circPt.decrementy();
            p += 2 * (circPt.getx() - circPt.gety()) + 1;
        }
        circlePlotPoints(circCtr.x, circCtr.y, circPt);
    }
}

template<class T>
int length(T& arr)
{
    return sizeof(arr) / sizeof(arr[0]);
}

// 画饼图
void pieChart(void)
{
    scrPt circCtr, piePt;
    GLint radius = winHeight / 4;       // 圆的半径
    GLdouble sliceAngle, previousSliceAngle = 0.0;

    GLfloat dataValues[12] = {10.0, 7.0, 13.0, 5.0, 13.0, 14.0, 3.0, 16.0, 5.0, 3.0, 17.0, 8.0};        // 饼图数据占比

    GLfloat dataSum = 0.0;

    circCtr.x = winWidth / 2;
    circCtr.y = winHeight / 2;
    circleMidpoint(circCtr, radius);            //中心画圆法，画出饼图外圆

    for(GLint k = 0; k < length(dataValues); k++)
    {
        dataSum += dataValues[k];                        // 计算数据总量
    }

    for(GLint k = 0; k < length(dataValues); k++)
    {
        sliceAngle = twoPi * dataValues[k] / dataSum + previousSliceAngle;          // 计算角度
        piePt.x = circCtr.x + radius * cos(sliceAngle);                            // 计算落点x
        piePt.y = circCtr.y + radius * sin(sliceAngle);                            // 计算落点y

        glBegin(GL_LINES);                                                         // 划线从原点到数据落点的
            glVertex2i(circCtr.x, circCtr.y);
            glVertex2i(piePt.x, piePt.y);
        glEnd();
        previousSliceAngle = sliceAngle;
    }
}

void displayFunc(void)
{
    glClear(GL_COLOR_BUFFER_BIT);            //显示缓存背景色
    glColor3f(1.0, 0.0, 1.0);                // 初始化画笔颜色
    pieChart();
    glFlush();
}

void winReshapeFunc(GLint newWidth, GLint newHeight)
{
    glMatrixMode(GL_PROJECTION);                        // 对投影矩阵操作
    glLoadIdentity();                                   // 重置当前指定的矩阵为单位矩阵
    gluOrtho2D(0.0, GLdouble(newWidth), 0.0, GLdouble(newHeight));    // 重新建立坐标系
    glClear(GL_COLOR_BUFFER_BIT);                                    

    winWidth = newWidth;
    winHeight = newHeight;
}

int main(int argc, char ** argv)
{
    glutInit(&argc, argv);                          // 初始化
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);    // 单一缓存RGB模式
    glutInitWindowPosition(100, 100);               // 初始化窗口位置
    glutInitWindowSize(winWidth, winHeight);        // 初始化窗口大小
    glutCreateWindow("en?~");                       // 创建窗口

    init();
    glutDisplayFunc(displayFunc);           // 注册显示回调函数
    glutReshapeFunc(winReshapeFunc);        // 改变窗口大小回调函数
    glutMainLoop();
    return 0;
}

原文地址：https://www.cnblogs.com/ziyoulu/p/10040950.html