通過前面幾節的學習，我們已經有能力實現一些高階的圖形繪製功能。這一節我們給大家示例折線圖、直方圖和餅狀圖的繪製程式。
我們先看看要實現的效果。

下面我們預設大家已經掌握了前面的知識，並會把講解的重點放在高階功能上。

#include <gl/glut.h>

GLsizei winWidth = 600, winHeight = 500;

GLubyte label[36] = { 'J', 'a', 'n', 'F', 'e', 'b', 'M', 'a', 'r',
'A', 'p', 'r', 'M', 'a', 'y', 'J', 'u', 'n',
'J', 'u', 'l'
 
, 'A', 'u', 'g', 'S', 'e', 'p', 'O', 'c', 't', 'N', 'o', 'v', 'D', 'e', 'c' };//月份標籤陣列
GLint dataValue[12] = { 420, 342, 324, 310, 262, 185, 190, 196, 217, 240, 312, 438 };//折線圖數值項，對應12個月
void init(void)
{
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(0.0, 600.0, 0.0, 500.0);
}
void
 
 linegraph(void)
{
    GLint xRaster = 25, yRaster = 150;//初始游標位置
    GLint month, k;
    GLint x = 30;
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.0, 0.0, 1.0);
    glBegin(GL_LINE_STRIP);
    for (k = 0; k < 12; k++)
        glVertex2i(x + k * 50, dataValue[k]);//繪製折線
    glEnd();
    glColor3f(1.0, 0.0, 0.0
 
);
    for (k = 0; k < 12;k++)
    {
        glRasterPos2i(xRaster + k * 50, dataValue[k] - 4);游標跟蹤折線節點
        glutBitmapCharacter(GLUT_BITMAP_9_BY_15, '*');//折線節點上繪製的‘*’
    }
    glColor3f(0.0, 0.0, 0.0);
    xRaster = 20;//重置游標位置
    for (month = 0; month < 12;month++)
    {
        glRasterPos2i(xRaster, yRaster);
        for (k = 3 * month; k < 3 * month + 3;k++)
        {
            glutBitmapCharacter(GLUT_BITMAP_HELVETICA_12, label[k]);//標註月份

        }
        xRaster += 50;
    }
    glFlush();
}
void winReshapeFcn(GLint newWidth, GLint newHeight)
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, GLdouble(newWidth), 0.0, GLdouble(newHeight));
    glClear(GL_COLOR_BUFFER_BIT);
}
void main(int argc, char**argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(winWidth, winHeight);
    glutCreateWindow("Line Chart Data Plot");
    init();
    glutDisplayFunc(linegraph);
    glutReshapeFunc(winReshapeFcn);
    glutMainLoop();
}

大家可以看到，看似複雜的折線圖，我們通過將其元素分解為折線，‘*’節點，月份標註分別進行繪製，整個過程非常簡單。在這裡給大家提出一個小問題，如果將游標位置變數設定為全域性變數，程式結果會出現什麼變化呢？

直方圖的程式碼也是同樣非常簡單，下面給大家提供一個示例。

void barChart(void)
{
    GLint xRaster = 20, yRaster = 150;
    GLint month, k;
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 0.0, 0.0);
    for (k = 0; k < 12; k++)        
    {
        glRecti(20 + k * 50, 165, 40 + k * 50, dataValue[k]);//直方圖條狀圖
    }
    glColor3f(0.0, 0.0, 0.0);
    for (month = 0; month < 12;month++)
    {
        glRasterPos2i(xRaster, yRaster);
        for (k = 3 * month; k < 3 * month + 3;k++)
        {
            glutBitmapCharacter(GLUT_BITMAP_HELVETICA_12, label[k]);//標註

        }
        xRaster += 50;
    }
    glFlush();
}

最後再給大家畫一個簡單的餅狀圖，大家可以試試自己去添加註釋等資訊。

#include <gl/glut.h>
#include <stdlib.h>
#include <math.h>
const GLdouble twoPi = 6.283185;
class scrPt{
public:
    GLint x, y;
};
GLsizei winWidth = 400, winHeight = 300;
void init(void)
{
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(0.0, 200.0, 0.0, 150.0);
}
void circleMidpoint(scrPt mid, GLint r)//畫圓函式
{
    int i = 0;
    glBegin(GL_LINE_LOOP);//通過細分曲線趨近圓形
    int n = 120;
    for (i = 0; i < n; i++)
    {
        glVertex2f(r*cos(twoPi / n*i) + mid.x, r*sin(twoPi / n*i) + mid.y);//圓上的連續點
    }
    glEnd();
}
void pieChart(void)
{
    scrPt circCtr, piePt;
    GLint radius = winWidth / 4;
    GLdouble sliceAngle, previousSliceAngle = 0.0;
    GLint k, nSlice = 12;
    GLfloat dataValue[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 };//單月份數
    GLfloat dataSum = 0.0;
    circCtr.x = winWidth / 2;//圓心
    circCtr.y = winHeight / 2;
    circleMidpoint(circCtr, radius);//根據圓心和半徑畫圓
    for (k = 0; k < nSlice;k++)
    {
        dataSum += dataValue[k];//計算總份數
    }
    for (k = 0; k < nSlice;k++)
    {
        sliceAngle = twoPi*dataValue[k] / dataSum + previousSliceAngle;//扇形邊界線的角度
        piePt.x = circCtr.x + radius*cos(sliceAngle);//扇形邊界線端點的座標
        piePt.y = circCtr.y + radius*sin(sliceAngle);
        glBegin(GL_LINES);
        glVertex2i(circCtr.x, circCtr.y);
        glVertex2i(piePt.x, piePt.y);//繪製扇形邊界線
        glEnd();
        previousSliceAngle = sliceAngle;//更新扇形邊界線角度
    }
}
void displayFcn(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.0, 0.0, 1.0);
    pieChart();
    glFlush();
}
void winReshapeFcn(GLint newWidth, GLint newHeight)
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, GLdouble(newWidth), 0.0, GLdouble(newHeight));
    glClear(GL_COLOR_BUFFER_BIT);
    winWidth = newWidth;
    winHeight = newHeight;
}
void main(int argc, char**argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(winWidth, winHeight);
    glutCreateWindow("Line Chart Data Plot");
    init();
    glutDisplayFunc(displayFcn);
    glutReshapeFunc(winReshapeFcn);
    glutMainLoop();
}