23.OpenGL--阴影

• 理论基础 
  阴影：就是我们所说的影子，其实现原理其实就是把场景渲染了两次，第一次是把相机放到光源位置渲染场景 ，然后存储渲染的深度信息到一张纹理上，即阴影图(只关心深度信息，所以此时可以关闭光照和纹理贴图提高效率)。第二次再从观察者的角度来渲染场景，在这次渲染时才渲染阴影，这次的渲染过程本质就是纹理贴图，只不过这个纹理是阴影图，贴图的方式根据我们设置的深度纹理过滤方式处理。生成阴影的过程是这样的：对于每个片元的深度信息和第一次从灯光角度渲染的深度信息比较，如果当前深度值大于第一次渲染的深度值，则肯定有物体在当前片元和灯光之间，那么当前片元在阴影区。

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• 代码示例

#include "GLTools.h"#ifdef __APPLE__#include <glut/glut.h>#else#define FREEGLUT_STATIC#include <GL/glut.h>#endif#ifdef GL_ARB_shadow#define GL_TEXTURE_COMPARE_MODE      GL_TEXTURE_COMPARE_MODE_ARB#define GL_TEXTURE_COMPARE_FUNC      GL_TEXTURE_COMPARE_FUNC_ARB#define GL_DEPTH_TEXTURE_MODE        GL_DEPTH_TEXTURE_MODE_ARB#define GL_COMPARE_R_TO_TEXTURE      GL_COMPARE_R_TO_TEXTURE_ARB#endif#define PI       3.14159265359GLdouble    fovy      = 60.0;GLdouble    nearPlane = 10.0;GLdouble    farPlane  = 100.0;GLfloat     angle = 0.0;GLfloat     torusAngle = 0.0;GLfloat     lightPos[] = { 25.0, 25.0, 25.0, 1.0 };GLfloat     lookat[] = { 0.0, 0.0, 0.0 };GLfloat     up[] = { 0.0, 0.0, 1.0 };GLboolean showShadow = GL_FALSE;/*初始化光源与阴影图*/void init( void ){    GLfloat  white[] = { 1.0, 1.0, 1.0, 1.0 };    //生成深度纹理(阴影图，只关心深度，对图像颜色信息并不关心，所以最后数据参数可以是NULL)    glTexImage2D( GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT,                 256, 256, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, NULL );//GL_DEPTH_COMPONENT:深度纹理格式，用于将深度值记录到这张图中    glLightfv( GL_LIGHT0, GL_POSITION, lightPos );    glLightfv( GL_LIGHT0, GL_SPECULAR, white );    glLightfv( GL_LIGHT0, GL_DIFFUSE, white );    //设置阴影图相关过滤方式    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );    /*深度纹理特有*/    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL );//比较规则    glTexParameteri( GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_LUMINANCE );//阴影图亮度    /*分为普通对比模式（GL_NONE）和引用到贴图对比模式（GL_COMPARE_REF_TO_TEXTURE）,后者使用的深度纹理贴图是线性过滤的，而前者是直接填充。*/    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE,GL_COMPARE_R_TO_TEXTURE );    //自动生成纹理坐标    glTexGeni( GL_S, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );    glTexGeni( GL_T, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );    glTexGeni( GL_R, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );    glTexGeni( GL_Q, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );    glColorMaterial( GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE );    glCullFace( GL_BACK );//剔除背面    glEnable( GL_DEPTH_TEST );    glEnable( GL_LIGHT0 );    glEnable( GL_LIGHTING );    glEnable( GL_TEXTURE_2D );    glEnable( GL_TEXTURE_GEN_S );    glEnable( GL_TEXTURE_GEN_T );    glEnable( GL_TEXTURE_GEN_R );    glEnable( GL_TEXTURE_GEN_Q );    glEnable( GL_COLOR_MATERIAL );//激活颜色材料模式    glEnable( GL_CULL_FACE );//激活表面剔除}void reshape( int width, int height ){    glViewport( 0, 0, width, height );    glMatrixMode( GL_PROJECTION );    glLoadIdentity();    gluPerspective( fovy, (GLdouble) width/height, nearPlane, farPlane );    glMatrixMode( GL_MODELVIEW );}void idle( void ){    angle += PI / 10000;    torusAngle += .1;    glutPostRedisplay();}void keyboard( unsigned char key, int x, int y ){    switch( key ) {        case 27:  /* Escape */            exit( 0 );            break;          //开启与关闭纹理贴图        case 't': {            static GLboolean textureOn = GL_TRUE;            textureOn = !textureOn;            if ( textureOn )                glEnable( GL_TEXTURE_2D );            else                glDisable( GL_TEXTURE_2D );        }            break;        //深度纹理贴图的两种模式对比        case 'm': {            static GLboolean compareMode = GL_TRUE;            compareMode = !compareMode;            printf( "Compare mode %s\n", compareMode ? "On" : "Off" );            glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE,                            compareMode ? GL_COMPARE_R_TO_TEXTURE : GL_NONE );        }            break;         //比较规则切换        case 'f': {            static GLboolean funcMode = GL_TRUE;            funcMode = !funcMode;            printf( "Operator %s\n", funcMode ? "GL_LEQUAL" : "GL_GEQUAL" );            glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC,funcMode ? GL_LEQUAL : GL_GEQUAL );//当前片段z与阴影图z比较规则(<=, >=)        }            break;        //是否显示阴影图        case 's':            showShadow = !showShadow;            break;         //运动与停止        case 'p': {            static GLboolean  animate = GL_TRUE;            animate = !animate;            glutIdleFunc( animate ? idle : NULL );        }            break;    }    glutPostRedisplay();}void transposeMatrix( GLfloat m[16] ){    GLfloat  tmp;#define Swap( a, b )    tmp = a; a = b; b = tmp    Swap( m[1],  m[4]  );    Swap( m[2],  m[8]  );    Swap( m[3],  m[12] );    Swap( m[6],  m[9]  );    Swap( m[7],  m[13] );    Swap( m[11], m[14] );#undef Swap}/*绘制场景上对象*/void drawObjects( GLboolean shadowRender ){    GLboolean textureOn = glIsEnabled( GL_TEXTURE_2D );    if ( shadowRender )        glDisable( GL_TEXTURE_2D );    //绘制矩形    if ( !shadowRender ) {        glNormal3f( 0, 0, 1 );        glColor3f( 1, 1, 1 );        glRectf( -20.0, -20.0, 20.0, 20.0 );    }    //红色圆环    glPushMatrix();    glTranslatef( 11, 11, 11 );    glRotatef( 54.73, -5, 5, 0 );    glRotatef( torusAngle, 1, 0, 0 );    glColor3f( 1, 0, 0 );    glutSolidTorus( 1, 4, 8, 36 );    glPopMatrix();    //蓝色立方体    glPushMatrix();    glTranslatef( 2, 2, 2 );    glColor3f( 0, 0, 1 );    glutSolidCube( 4 );    glPopMatrix();    //线框圆表光源    glPushMatrix();    glTranslatef( lightPos[0], lightPos[1], lightPos[2] );    glColor3f( 1, 1, 1 );    glutWireSphere( 0.5, 6, 6 );    glPopMatrix();    if ( shadowRender && textureOn )        glEnable( GL_TEXTURE_2D );}/*生成阴影图*/void generateShadowMap( void ){    GLint    viewport[4];    GLfloat  lightPos[4];    glGetLightfv( GL_LIGHT0, GL_POSITION, lightPos );//获得光源的位置    glGetIntegerv( GL_VIEWPORT, viewport );//获得视口信息    glViewport( 0, 0, 256, 256 );//设置视口与阴影图大小匹配    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );    glMatrixMode( GL_PROJECTION );    glPushMatrix();    glLoadIdentity();    gluPerspective( 80.0, 1.0, 10.0, 1000.0 );    glMatrixMode( GL_MODELVIEW );    glPushMatrix();    glLoadIdentity();    gluLookAt( lightPos[0], lightPos[1], lightPos[2],              lookat[0], lookat[1], lookat[2],              up[0], up[1], up[2] );//把摄像机放到光源处    drawObjects( GL_TRUE );//绘制场景    glPopMatrix();    glMatrixMode( GL_PROJECTION );    glPopMatrix();    glMatrixMode( GL_MODELVIEW );    glCopyTexImage2D( GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, 0, 0, 256, 256, 0 );//摄像机移到光源处渲染的阴影图    glViewport( viewport[0], viewport[1], viewport[2], viewport[3] );//恢复当前视口    //绘制阴影图    if ( showShadow ) {        GLfloat depthImage[256][256];        glReadPixels( 0, 0, 256, 256,GL_DEPTH_COMPONENT, GL_FLOAT, depthImage );        glWindowPos2f( viewport[2]/2, 0 );//设置阴影图绘制位置        glDrawPixels( 256, 256, GL_LUMINANCE,                     GL_FLOAT, depthImage );        glutSwapBuffers();    }}//自动生成阴影图的纹理坐标void generateTextureMatrix( void ){    GLfloat  tmpMatrix[16];    glPushMatrix();    glLoadIdentity();    glTranslatef( 0.5, 0.5, 0.0 );    glScalef( 0.5, 0.5, 1.0 );    gluPerspective( 60.0, 1.0, 1.0, 1000.0 );    gluLookAt( lightPos[0], lightPos[1], lightPos[2],              lookat[0], lookat[1], lookat[2],              up[0], up[1], up[2] );    glGetFloatv( GL_MODELVIEW_MATRIX, tmpMatrix );    glPopMatrix();    transposeMatrix( tmpMatrix );    glTexGenfv( GL_S, GL_OBJECT_PLANE, &tmpMatrix[0] );    glTexGenfv( GL_T, GL_OBJECT_PLANE, &tmpMatrix[4] );    glTexGenfv( GL_R, GL_OBJECT_PLANE, &tmpMatrix[8] );    glTexGenfv( GL_Q, GL_OBJECT_PLANE, &tmpMatrix[12] );}//渲染void display( void ){    GLfloat  radius = 30;    generateShadowMap();    generateTextureMatrix();    if ( showShadow )        return;    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );    glPushMatrix();    gluLookAt( radius*cos(angle), radius*sin(angle), 30,              lookat[0], lookat[1], lookat[2],              up[0], up[1], up[2] );    drawObjects( GL_FALSE );    glPopMatrix();    glutSwapBuffers();}int main( int argc, char** argv ){     glWindowPos2f   = (PFNGLWINDOWPOS2FARBPROC)glutGetProcAddress("glWindowPos2f");    glutInit( &argc, argv );    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );    glutInitWindowSize( 512, 512 );    glutInitWindowPosition( 100, 100 );    glutCreateWindow( argv[0] );    init();    glutDisplayFunc( display );    glutReshapeFunc( reshape );    glutKeyboardFunc( keyboard );    glutIdleFunc( idle );    glutMainLoop();    return 0;}
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