OpenGL（四）点、线、多边形

       像素是计算机屏幕上显示最小的单位。点是最简单的图元，我们可以理解为点的大小为一个像素，两个点可以确定一条线段，三条线段或三条以上可以组成为多边形，当然三角形为最简单的多边形。这篇我们一起来绘制点，线，多边形。

一、几何图元

GL_POINTS：点GL_LINES：线段，二个点确定线段GL_LINE_STRIP：第一个点依次连接的线段GL_LINE_LOOP：和GL_LINE_STRIP相同，但首尾连接，形成环状GL_POLYGON：多边形GL_QUADS：由四个点组成一个四边形GL_QUADS_STRIP：四边形带GL_TRIANGLES：三角形，三个点确定GL_TRIANGLE_STRIP：共用一个条带上的顶点的一组三角形GL_TRIANGLE_FAN：以一个原点为中心呈扇形排列，公共相邻顶点的一组三角形

二、点

1.点的大小：glPointSize(GLfloat size);

2.获得点的大小范围

定义点的大小范围数组：GLfloat  sizes[2];  //sizes[0]：最小值；sizes[1]：最大值获取点的大小范围：glGetFloatv(GL_POINT_SIZE_RANGE, sizes); //参数一：点范围枚举值；参数二：数组

3.获得点的步长（增量大小）

定义点步长：GLfloat step;获取点的步长：glGetFloatv(GL_POINT_SIZE_GRANULARITY，step); //参数一：点步长枚举值；参数二：步长值

二、线

1.线的宽度：glLineWidth(GLfloat  width);

三、三角形

在默认情况下，逆时针方向环绕的多边形是正面，顺时针方向环绕是背面。

glFrontFace(GL_CW); //GL_CW：设置顺时针环绕为正面；GL_CCW：设置逆时针环绕为正面。

四、例子代码

#include <GLTools.h> #include <GL/glut.h> #include <iostream> GLBatch pointsBatch; GLBatch linesBatch; GLBatch lineStripBatch; GLBatch lineLoopBatch; GLBatch polygonBatch; GLBatch guadsBatch; GLBatch trianglesBatch; GLBatch trianglesStripBatch; GLBatch guadStripBatch; GLBatch triangleFanBatch; GLShaderManager shaderManager; int iStep = 0; int iPelType = 9; //图元类型数量 void SetupRec(void){ //背景 glClearColor(0.0f, 0.0f, 0.0f, 1.0f); shaderManager.InitializeStockShaders(); GLfloat vVertex_1[4][3] = { { -0.5f, -0.5f, 0.0f }, { -0.5f, 0.5f, 0.0f }, { 0.5f, 0.5f, 0.0f }, { 0.5f, -0.5f, 0.0f } }; //GL_POINTS，点 pointsBatch.Begin(GL_POINTS, 4); //参数二：4个顶点 pointsBatch.CopyVertexData3f(vVertex_1); pointsBatch.End(); //GL_LINES，线 linesBatch.Begin(GL_LINES, 4); linesBatch.CopyVertexData3f(vVertex_1); linesBatch.End(); //GL_LINE_STRIP，线带 lineStripBatch.Begin(GL_LINE_STRIP, 4); lineStripBatch.CopyVertexData3f(vVertex_1); lineStripBatch.End(); //GL_LINE_LOOP，线环 lineLoopBatch.Begin(GL_LINE_LOOP, 4); lineLoopBatch.CopyVertexData3f(vVertex_1); lineLoopBatch.End(); //GL_POLYGON，多边形 polygonBatch.Begin(GL_POLYGON, 4); polygonBatch.CopyVertexData3f(vVertex_1); polygonBatch.End(); GLfloat vVertex_2[8][3] = { { -0.5f, -0.5f, 0.0f }, { -0.5f, 0.5f, 0.0f }, { -0.1f, 0.5f, 0.0f }, { -0.1f, -0.5f, 0.0f }, { 0.1f, -0.5f, 0.0f }, { 0.1f, 0.5f, 0.0f }, { 0.5f, 0.5f, 0.0f }, { 0.5f, -0.5f, 0.0f } }; //GL_QUADS，四边形 guadsBatch.Begin(GL_QUADS, 8); guadsBatch.CopyVertexData3f(vVertex_2); guadsBatch.End(); //GL_QUADS_STRIP，四边形带 guadStripBatch.Begin(GL_QUAD_STRIP, 8); guadStripBatch.CopyVertexData3f(vVertex_2); guadStripBatch.End(); //GL_TRIANGLES，三角形 trianglesBatch.Begin(GL_TRIANGLES, 8); trianglesBatch.CopyVertexData3f(vVertex_2); trianglesBatch.End(); //GL_TRIANGLE_STRIP，三角形带 trianglesStripBatch.Begin(GL_TRIANGLE_STRIP, 8); trianglesStripBatch.CopyVertexData3f(vVertex_2); trianglesStripBatch.End(); GLfloat vVertex_3[4][3] = { { 0.0f, 0.0f, 0.0f }, { 0.1f, 0.45f, 0.0f }, { 0.2f, 0.4f, 0.0f }, { 0.3f, 0.2f, 0.0f } }; //GL_TRIANGLE_FAN，扇形 triangleFanBatch.Begin(GL_TRIANGLE_FAN, 4); triangleFanBatch.CopyVertexData3f(vVertex_3); triangleFanBatch.End(); } void KeyPressFunc(unsigned char key, int x, int y) { //空格键 if (key == 32) { iStep += 1; iStep = iStep > iPelType ? 0 : iStep; } switch (iStep) { case 0: glutSetWindowTitle("GL_POINTS"); //设置窗口标题 break; case 1: glutSetWindowTitle("GL_LINES"); break; case 2: glutSetWindowTitle("GL_LINE_STRIP"); break; case 3: glutSetWindowTitle("GL_LINE_LOOP"); break; case 4: glutSetWindowTitle("GL_POLYGON"); break; case 5: glutSetWindowTitle("GL_QUADS"); break; case 6: glutSetWindowTitle("GL_QUADS_STRIP"); break; case 7: glutSetWindowTitle("GL_TRIANGLES"); break; case 8: glutSetWindowTitle("GL_TRIANGLE_STRIP"); break; case 9: glutSetWindowTitle("GL_TRIANGLE_FAN"); break; } glutPostRedisplay(); } void MyDisplay(void) { glClear(GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT | GL_COLOR_BUFFER_BIT); switch (iStep) { case 0: glPointSize(5.0f); //点的大小 pointsBatch.Draw(); glPointSize(1.0f); break; case 1: glLineWidth(2.0f); //线的宽度 linesBatch.Draw(); glLineWidth(1.0f); break; case 2: glLineWidth(2.0f); lineStripBatch.Draw(); glLineWidth(1.0f); break; case 3: glLineWidth(2.0f); lineLoopBatch.Draw(); glLineWidth(1.0f); break; case 4: glLineWidth(2.0f); polygonBatch.Draw(); glLineWidth(1.0f); break; case 5: guadStripBatch.Draw(); break; case 6: guadsBatch.Draw(); break; case 7: trianglesBatch.Draw(); break; case 8: trianglesStripBatch.Draw(); break; case 9: triangleFanBatch.Draw(); break; } glutSwapBuffers(); } int main(int argc, char *argv[]) { gltSetWorkingDirectory(argv[0]); glutInit(&argc, argv); glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE | GLUT_STENCIL); glutInitWindowSize(800, 600); glutCreateWindow("GL_POINTS"); glutKeyboardFunc(KeyPressFunc); glutDisplayFunc(MyDisplay); GLenum err = glewInit(); if (err != GLEW_OK) { std::cout << "init glew fail!" << std::endl; return 1; } SetupRec(); glutMainLoop(); return 0; }

五、运行效果