12.OpenGL--多边形偏移

• 理论基础 
  多边形偏移：有时候我们需要着重显示多边形的边缘，一般做法是先绘制实心的再在同一位置绘制空心的，这样就可以突出边缘。但是，由于直线与多边形的光栅化并不完全相同，即使在同一位置绘制它们的深度值也不一定相同，这样绘制的直线可能忽浓忽淡。这时我们就可以激活多边形偏移来解决这个问题，其原理就是给实体或线框的深度加上一个偏移值。即把实体推向远处或把线框拉近，具体偏移值多少是通过设置glPolygonOffset(factor,units)算出的,其底层公式是这样：offset = m * factor + r * units.

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• 实例代码

#include "GLTools.h"#include "GLShaderManager.h"#ifdef __APPLE__#include <glut/glut.h>#else#define FREEGLUT_STATIC#include <GL/glut.h>#endifGLuint list;GLint spinx = 0;GLint spiny = 0;GLfloat tdist = 0.0;GLfloat polyfactor = 1.0;GLfloat polyunits = 1.0;void display (void){    GLfloat gray[] = { 0.8, 0.8, 0.8, 1.0 };    GLfloat black[] = { 0.0, 0.0, 0.0, 1.0 };    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glPushMatrix ();    glTranslatef (0.0, 0.0, tdist);    glRotatef ((GLfloat) spinx, 1.0, 0.0, 0.0);    glRotatef ((GLfloat) spiny, 0.0, 1.0, 0.0);    //在开启光照与多边形偏移下，绘制一个实心球体    glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, gray);    glMaterialfv(GL_FRONT, GL_SPECULAR, black);    glMaterialf(GL_FRONT, GL_SHININESS, 0.0);    glEnable(GL_LIGHTING);    glEnable(GL_LIGHT0);    glEnable(GL_POLYGON_OFFSET_FILL);//激活多边形偏移    glPolygonOffset(polyfactor, polyunits);//计算出一个偏移值，以后每个片段的深度值都加上这个值    glCallList (list);    //在关闭光照与多边形偏移下，同一位置绘制一个线框球体    glDisable(GL_POLYGON_OFFSET_FILL);//关闭多边形偏移    glDisable(GL_LIGHTING);    glDisable(GL_LIGHT0);    glColor3f (1.0, 1.0, 1.0);    glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);    glCallList (list);    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);    glPopMatrix ();    glFlush ();}void gfxinit (void){    GLfloat light_ambient[] = { 0.0, 0.0, 0.0, 1.0 };    GLfloat light_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };    GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 };    GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };    GLfloat global_ambient[] = { 0.2, 0.2, 0.2, 1.0 };    glClearColor (0.0, 0.0, 0.0, 1.0);    list = glGenLists(1);    glNewList (list, GL_COMPILE);    glutSolidSphere(1.0, 20, 12);    glEndList ();    glEnable(GL_DEPTH_TEST);    glLightfv (GL_LIGHT0, GL_AMBIENT, light_ambient);    glLightfv (GL_LIGHT0, GL_DIFFUSE, light_diffuse);    glLightfv (GL_LIGHT0, GL_SPECULAR, light_specular);    glLightfv (GL_LIGHT0, GL_POSITION, light_position);    glLightModelfv (GL_LIGHT_MODEL_AMBIENT, global_ambient);}void reshape(int width, int height){    glViewport (0, 0, width, height);    glMatrixMode (GL_PROJECTION);    glLoadIdentity ();    gluPerspective(45.0, (GLdouble)width/(GLdouble)height,                   1.0, 10.0);    glMatrixMode (GL_MODELVIEW);    glLoadIdentity ();    gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);}void mouse(int button, int state, int x, int y) {    switch (button) {        case GLUT_LEFT_BUTTON:            switch (state) {                case GLUT_DOWN:                    spinx = (spinx + 5) % 360;                    glutPostRedisplay();                    break;                default:                    break;            }            break;        case GLUT_RIGHT_BUTTON:            switch (state) {                case GLUT_DOWN:                    spiny = (spiny + 5) % 360;                    glutPostRedisplay();                    break;                default:                    break;            }            break;        default:            break;    }}void keyboard (unsigned char key, int x, int y){    switch (key) {        //调节观察点距离        case 't':            if (tdist < 4.0) {                tdist = (tdist + 0.5);                glutPostRedisplay();            }            break;        case 'T':            if (tdist > -5.0) {                tdist = (tdist - 0.5);                glutPostRedisplay();            }            break;         //调节计算偏移值的两个参数        case 'F':            polyfactor = polyfactor + 0.1;            printf ("polyfactor is %f\n", polyfactor);            glutPostRedisplay();            break;        case 'f':            polyfactor = polyfactor - 0.1;            printf ("polyfactor is %f\n", polyfactor);            glutPostRedisplay();            break;        case 'U':            polyunits = polyunits + 1.0;            printf ("polyunits is %f\n", polyunits);            glutPostRedisplay();            break;        case 'u':            polyunits = polyunits - 1.0;            printf ("polyunits is %f\n", polyunits);            glutPostRedisplay();            break;        default:            break;    }}int main(int argc, char** argv){    glutInit(&argc, argv);    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);    glutCreateWindow(argv[0]);    glutReshapeFunc(reshape);    glutDisplayFunc(display);    glutMouseFunc(mouse);    glutKeyboardFunc(keyboard);    gfxinit();    glutMainLoop();    return 0;}
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下面是没有开启多边形偏移与开启了多边形偏移下的表现：