qt opengl 绘制点、线、三角形、多边形（二）

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一、相关宏定义参数

#define GL_POINTS 0x0000        // 点
#define GL_LINES 0x0001         // 直线线段，按添加点的顺序，每2个点组成一条线段
#define GL_LINE_LOOP 0x0002     // 直线线段，按添加点的顺序，相邻的两个点连接，且第一个点与最后一个点也连接(多边形)
#define GL_LINE_STRIP 0x0003    // 与GL_LINE_LOOP类似，区别在于 第一个点与最后一个点 不 连接
#define GL_TRIANGLES 0x0004     // 按添加点的顺序， 每3个点组成三角(面)
#define GL_TRIANGLE_STRIP 0x0005// 三角(面) ，区别见后面代码演示
#define GL_TRIANGLE_FAN 0x0006  // 三角(面)，区别见后面代码演示
#define GL_QUADS 0x0007         // 
#define GL_QUAD_STRIP 0x0008    // 
#define GL_POLYGON 0x0009       // 

二、代码及示例图

1. 点 LG_POINTS 0x0000

void MOpenGLWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);

    //点大小
    glPointSize(6.0f);
    //启动绘制（点）
    glBegin(GL_POINTS);
    {
        //
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex3f(-1.0f, 0.0f, 0.0f);

        glVertex3f(-0.5f, 0.0f, 0.0f);

        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex3f(0.0f, 0.0f, 0.0f);

        glColor3f(0.0f, 0.0f, 1.0f);
        glVertex3f(0.5f, 0.0f, 0.0f);

        glVertex3f(1.0f, 0.0f, 0.0f);
    }
    //结束绘制
    glEnd();
}

2. 线 GL_LINES 0x0001

//启动绘制（线段）
glBegin(GL_LINES);
{
    glColor3f(0.0f, 0.0f, 0.0f);
    glVertex3f(-1.0f, 0.0f, 0.0f);
    glVertex3f(1.0f, 0.0f, 0.0f);
    glVertex3f(0.0f, -1.0f, 0.0f);
    glVertex3f(0.0f, 1.0f, 0.0f);
}
//结束绘制
glEnd();

3. 线 GL_LINE_LOOP 0x0002

//启动绘制（多边形直线）
glBegin(GL_LINE_LOOP);
{
    glColor3f(1,0,0);
    glVertex2f(-1.0f, 0.0f);

    glColor3f(0,1,0);
    glVertex2f(0.0f, -1.0f);

    glColor3f(0,0,1);
    glVertex2f(1.0f, 0.0f);

    glColor3f(1,1,1);
    glVertex2f(0.0f, 1.0f);
}
//结束绘制
glEnd();

4. GL_LINE_STRIP 0x0003

//启动绘制（连续的线段）
glBegin(GL_LINE_STRIP);
{
    glColor3f(1,0,0);
    glVertex2f(-0.5f, 0.0f);

    glColor3f(0,1,0);
    glVertex2f(0.0f, -0.5f);

    glColor3f(0,0,1);
    glVertex2f(0.5f, 0.0f);

    glColor3f(1,1,1);
    glVertex2f(0.0f, 0.5f);
}
//结束绘制
glEnd();

5. GL_TRIANGLES 0x0004

先将所用的点画出

	//点大小
    glPointSize(6.0f);
    //
    glBegin(GL_POINTS);
    {
        glColor3f(0, 0, 0);
        
        glVertex3f( -0.9f, 0.2f, 0.0f);
        glVertex3f( -0.7f, -0.2f, 0.0f);
        glVertex3f( -0.5f, 0.2f, 0.0f);
        
        glVertex3f( -0.3f, -0.2f, 0.0f);
        glVertex3f( -0.1f, 0.2f, 0.0f);
        glVertex3f( 0.1f, -0.2f, 0.0f);
        
        glVertex3f( 0.3f, 0.2f, 0.0f);
        glVertex3f( 0.5f, -0.2f, 0.0f);
        glVertex3f( 0.7f, 0.2f, 0.0f);
    }
    glEnd();

GL_TRIANGLS绘制三角形规则如下：
若添加的N个坐标点顺序为 { P₀, P₁, P₂, … , P _n-2, P _n-1, P ~n ~}
组成的三角形为：{ P₀, P₁, P₂ } 、{ P₃, P₄, P₅ } 、… 、{ P_n-5, P_n-4, P_n-3 } 、{ P_n-2, P_n-1, P_n }

// 启动绘制（三角形)
glBegin(GL_TRIANGLES);
{
    glColor3f(0, 0, 0);
    glVertex3f( -0.9f, 0.2f, 0.0f);
    glColor3f(1, 0, 0);
    glVertex3f( -0.7f, -0.2f, 0.0f);
    glColor3f(0, 1, 0);
    glVertex3f( -0.5f, 0.2f, 0.0f);
    
    glColor3f(1, 1, 0);
    glVertex3f( -0.3f, -0.2f, 0.0f);
    glColor3f(0, 0, 1);
    glVertex3f( -0.1f, 0.2f, 0.0f);
    glColor3f(1, 0, 1);
    glVertex3f( 0.1f, -0.2f, 0.0f);

    glColor3f(0, 1, 1);
    glVertex3f( 0.3f, 0.2f, 0.0f);
    glColor3f(1, 1, 1);
    glVertex3f( 0.5f, -0.2f, 0.0f);
    glColor3f(0, 0, 0);
    glVertex3f( 0.7f, 0.2f, 0.0f);
}
glEnd();

6. GL_TRIANGLE_STRIP 0x0005

与第5小节所用的代码不变，仅glBegin(GL_TRIANGLE);改为glBegin(GL_TRIANGL_STRIP);所得结果如下图。

GL_TRIANGL_STRIP绘制三角形规则如下：
若添加的N个坐标点顺序为 { P₀, P₁, P₂, … , P _n-2, P _n-1, P _n }
组成的三角形为：{ P₀, P₁, P₂ } 、{ P₁, P₂, P₃ } 、{ P₂, P₃, P₄} 、… 、{ P_n-3, P_n-2, P_n-1} 、{ P_n-2, P_n-1, P_n}

7. GL_TRIANGLE_FAN 0x0006

与第5小节所用的代码不变，仅glBegin(GL_TRIANGLE);改为glBegin(GL_TRIANGL_FAN);所得结果如下图。

GL_TRIANGL_STRIP绘制三角形规则如下：
若添加的N个坐标点顺序为 { P₀, P₁, P₂, … , P _n-2, P _n-1, P _n }
组成的三角形为：{ P₀, P₁, P₂ } 、{ P₀, P₂, P₃ } 、… 、{ P₀, P_n-2, P_n-1 } 、、{ P₀, P_n-1, P_n }

8. GL_QUADS 0x0007

标记所用点位

//点大小
glPointSize(6.0f);
//
glBegin(GL_POINTS);
{
	//这里画点添加顺序与画四边形顺序不一致，这里只是为了画出用到的点，方便理解，与画四边形无任何关联
    glColor3f(0, 0, 0);
    glVertex3f(-1.0f, -0.55f, 0.0f);
    glVertex3f(-1.0f, -0.95f, 0.0f);
    glVertex3f(-0.6f, -0.55f, 0.0f);
    glVertex3f(-0.6f, -0.95f, 0.0f);
    glVertex3f(-0.2f, -0.55f, 0.0f);
    glVertex3f(-0.2f, -0.95f, 0.0f);
    glVertex3f(0.2f, -0.95f, 0.0f);
    glVertex3f(0.2f, -0.55f, 0.0f);
    glVertex3f(0.6f, -0.55f, 0.0f);
    glVertex3f(0.6f, -0.95f, 0.0f);
    glVertex3f(1.0f, -0.95f, 0.0f);
    glVertex3f(1.0f, -0.55f, 0.0f);
}
glEnd();

//!GL_QUADS四边形
glBegin(GL_QUADS);
{
    glColor3f(0, 0, 0);
    glVertex3f(-1.0f, -0.55f, 0.0f);
    glColor3f(1, 0, 0);
    glVertex3f(-1.0f, -0.95f, 0.0f);

    glColor3f(1, 1, 0);
    glVertex3f(-0.6f, -0.55f, 0.0f);
    glColor3f(0, 1, 0);
    glVertex3f(-0.6f, -0.95f, 0.0f);


    glColor3f(0, 0, 1);
    glVertex3f(-0.2f, -0.55f, 0.0f);
    glColor3f(1, 0, 1);
    glVertex3f(-0.2f, -0.95f, 0.0f);

    glColor3f(0, 1, 1);
    glVertex3f(0.2f, -0.95f, 0.0f);
    glColor3f(1, 1, 1);
    glVertex3f(0.2f, -0.55f, 0.0f);


    glColor3f(0, 0, 0);
    glVertex3f(0.6f, -0.55f, 0.0f);
    glColor3f(0, 0, 1);
    glVertex3f(1.0f, -0.55f, 0.0f);

    glColor3f(0, 1, 0);
    glVertex3f(0.6f, -0.95f, 0.0f);
    glColor3f(0, 1, 0);
    glVertex3f(1.0f, -0.95f, 0.0f);

}
glEnd();

如上图和示例代码，绘制了12个点及由这12点分别组成的3个四边形，图中标注了点坐标添加顺序。
从上图可以分析出如下特征：

1. 添加N个顶点，每4个组成一个四边形。(若N%4不为0，那只能画出N/4个四边形，多出的N%4个顶点无用)
2. 四边形是按照顶点添加顺序绘制的
   (这部分是个人理解，不确定是否有错)

9. GL_QUAD_STRIP 0x0008

延用第8小节所用的代码，仅glBegin(GL_QUADS);改为glBegin(GL_QUAD_STRIP);所得结果如下图。

从图中可以发现，顶点是可以共用的，好比两栋楼相邻的一面共用一堵墙。
图中有 { P₀, P₁,P₂,P₃,P₄,P₅,P₆,P₇,P₈,P₉,P₁₀,P₁₁} 等12个顶点。
组成的四边形：{ P₀，P₁ ，P₂ ，P₃ }、{ P₂，P₃ ，P₄ ，P₅ }、{ P₄，P₅ ，P₆ ，P₇ }、{ P₆，P₇ ，P₈ ，P₉ }、{ P₈，P₉ ，P₁₀ ，P₁₁ }
绘制顺序：
… 1. 定义每个四边形中，第一个点是P₀，第二个点是 P₁，第三个点是 P₂，第四个点是 P₃
… 2. 绘制路线: P₀ --》 P₁ --》P₃ 、P₀ --》 P₂ --》P₃ ，形成的四边形就是最终结果
(这部分是个人理解，不确定是否有错)

10. GL_POLYGON 0x0009

   //多边形
    glBegin(GL_POLYGON);
    {
        //1
        glColor3f(0, 0, 0);
        glVertex3f(-0.8f, 0.6f, 0.0f);
        //2
        glColor3f(1, 0, 0);
        glVertex3f(-1.0f, 0.75f, 0.0f);
        //3
        glColor3f(0, 1, 0);
        glVertex3f(-0.8f, 0.90f, 0.0f);
        //3
        glColor3f(1, 1, 0);
        glVertex3f(-0.3f, 0.90f, 0.0f);
        //2
        glColor3f(0, 0, 1);
        glVertex3f(-0.1f, 0.75f, 0.0f);
        //1
        glColor3f(1, 0, 1);
        glVertex3f(-0.3f, 0.60f, 0.0f);
        //2
        glColor3f(1, 1, 1);
        glVertex3f(-0.4f, 0.75f, 0.0f);
        //1
        glColor3f(0.0f, 0.0f, 0.0f);
        glVertex3f(-0.5f, 0.6f, 0.0f);
        //1
        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex3f(-0.6f, 0.6f, 0.0f);
        //2
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex3f(-0.7f, 0.75f, 0.0f);

    }
    glEnd();

    glBegin(GL_POINTS);
    {
        //1
        glColor3f(0, 0, 0);
        glVertex3f(-0.8f, 0.6f, 0.0f);
        //2
        glColor3f(1, 0, 0);
        glVertex3f(-1.0f, 0.75f, 0.0f);
        //3
        glColor3f(0, 1, 0);
        glVertex3f(-0.8f, 0.90f, 0.0f);
        //3
        glColor3f(1, 1, 0);
        glVertex3f(-0.3f, 0.90f, 0.0f);
        //2
        glColor3f(0, 0, 1);
        glVertex3f(-0.1f, 0.75f, 0.0f);
        //1
        glColor3f(1, 0, 1);
        glVertex3f(-0.3f, 0.60f, 0.0f);
        //2
        glColor3f(1, 1, 1);
        glVertex3f(-0.4f, 0.75f, 0.0f);
        //1
        glColor3f(1.0f, 1.0f, 1.0f);
        glVertex3f(-0.5f, 0.6f, 0.0f);
        //1
        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex3f(-0.6f, 0.6f, 0.0f);
        //2
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex3f(-0.7f, 0.75f, 0.0f);
    }
    glEnd();

    //多边形
    glBegin(GL_POLYGON);
    {
        //1
        glColor3f(0, 0, 0);
        glVertex3f( 0.8f, 0.6f, 0.0f);
        //2
        glColor3f(1, 0, 0);
        glVertex3f( 1.0f, 0.75f, 0.0f);
        //3
        glColor3f(0, 1, 0);
        glVertex3f( 0.8f, 0.90f, 0.0f);
        //3
        glColor3f(1, 1, 0);
        glVertex3f( 0.3f, 0.90f, 0.0f);
        //2
        glColor3f(0, 0, 1);
        glVertex3f( 0.1f, 0.75f, 0.0f);
        //1
        glColor3f(1, 0, 1);
        glVertex3f( 0.3f, 0.60f, 0.0f);
        //2
        glColor3f(1, 1, 1);
        glVertex3f( 0.4f, 0.45f, 0.0f);
        //1
        glColor3f(0.0f, 0.0f, 0.0f);
        glVertex3f( 0.5f, 0.6f, 0.0f);
        //1
        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex3f( 0.6f, 0.6f, 0.0f);
        //2
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex3f( 0.7f, 0.45f, 0.0f);

    }
    glEnd();

    glBegin(GL_POINTS);
    {
        //1
        glColor3f(0, 0, 0);
        glVertex3f( 0.8f, 0.6f, 0.0f);
        //2
        glColor3f(1, 0, 0);
        glVertex3f( 1.0f, 0.75f, 0.0f);
        //3
        glColor3f(0, 1, 0);
        glVertex3f( 0.8f, 0.90f, 0.0f);
        //3
        glColor3f(1, 1, 0);
        glVertex3f( 0.3f, 0.90f, 0.0f);
        //2
        glColor3f(0, 0, 1);
        glVertex3f( 0.1f, 0.75f, 0.0f);
        //1
        glColor3f(1, 0, 1);
        glVertex3f( 0.3f, 0.60f, 0.0f);
        //2
        glColor3f(0.5f, 0.5f, 0.5f);
        glVertex3f( 0.4f, 0.45f, 0.0f);
        //1
        glColor3f(1.0f, 0.0f, 1.0f);
        glVertex3f( 0.5f, 0.6f, 0.0f);
        //1
        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex3f( 0.6f, 0.6f, 0.0f);
        //2
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex3f( 0.7f, 0.45f, 0.0f);
    }
    glEnd();

按顺序绘制，只能绘制凸多边形，不能绘制凹多边形

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总结

以上内容为个人学习理解，可能存在错误。