既然模拟出了光照,那么也少不了阴影,阴影的产生是因为距离光线较近的物体遮挡了距离较远的物体,导致被遮挡的物体接受的光照少于遮挡物的,因此阴影的产生与否与物体到光源的位置有关系,静态物体的阴影可以用光照贴图来模拟,而动态阴影要用阴影锥或者阴影贴图实现,阴影锥会引入许多额外的顶点为管线带来负担,目前比较流行的阴影模拟方法是用阴影贴图,它的好处在于只是用纹理存储物体的深度信息而并不会引入额外顶点.

要实现阴影贴图有以下几个步骤:

首先开辟一块纹理缓存以便之后保存世界的深度信息:

?//Create the shadow map texture
?glGenTextures(1, &shadowMapTextureLeft);
?glBindTexture(GL_TEXTURE_2D, shadowMapTextureLeft);
?glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
?glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
?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_COMPARE_MODE_ARB, GL_COMPARE_R_TO_TEXTURE_ARB);
?glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC_ARB, GL_LEQUAL);
?glTexImage2D( GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, shadowMapSize, shadowMapSize,
? ?0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, 0);
?glBindTexture(GL_TEXTURE_2D, 0);

我这边开了两张深度纹理shadowMapTextureLeft与shadowMapTextureRight.

然后新建渲染目标对象fbo,fbo类似于帧缓冲区,只不过把渲染的片段保存于其他的缓存而不是屏幕上的帧缓冲区:

?glGenFramebuffersEXT(1, &fboId);
?glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
?glDrawBuffer(GL_NONE);
?glReadBuffer(GL_NONE);
?glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

这边的glDrawBuffer(GL_NONE);与glReadBuffer(GL_NONE);是为了屏蔽颜色的输入输出,因为这个fbo用于写入深度信息.

缓冲区准备工作完成.

接着设置光源:

void changeLightPos(float lx,float ly,float lz) {
?lightPos.x=lx;
?lightPos.y=ly;
?lightPos.z=lz;
?lightPos.w=0;
}

然后设置渲染阴影贴图的投影矩阵:

?//Calculate light projection matrix
?float size=128*2;
?lightProjectionMatrixLeft = ortho(-size-size,size-size,-size*2,size*2,-size*2,size*2);

这边我用的是方向光源,所以设置的平行投影.

然后设置渲染阴影贴图的视图矩阵,将视点放在光源位置,将方向定位光向量:

?lightViewMatrixLeft=lookAt(lightPos.x, lightPos.y, lightPos.z,
? ? ? ?cx, cy, cz,
? ? ? ?0.0f, 1.0f, 0.0f);

好了,现在准备工作都完成了,开始渲染阴影贴图:

? ? glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,fboId);
?glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,
? ?GL_DEPTH_ATTACHMENT_EXT,GL_TEXTURE_2D, shadowMapTextureLeft, 0);
?//Use viewport the same size as the shadow map
?glViewport(0, 0, shadowMapSize, shadowMapSize);
?//First pass - from light's point of view
?glMatrixMode(GL_PROJECTION);
?glLoadMatrixf(lightProjectionMatrixLeft);
?glMatrixMode(GL_MODELVIEW);
?glLoadMatrixf(lightViewMatrixLeft);
?//Draw back faces into the shadow map
?glCullFace(GL_FRONT);
?glClear(GL_DEPTH_BUFFER_BIT);

?//Draw the scene
?callback();

给fbo挂上深度纹理,接下来渲染的内容都将保存到深度纹理当中,把遮挡物的正面剔除,因为遮挡物反面也会产生正确的阴影.

然后还原渲染目标,清除一下帧缓冲区内容

? ? glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,0);
?//restore states
?glCullFace(GL_BACK);
?glClear(GL_DEPTH_BUFFER_BIT|GL_COLOR_BUFFER_BIT);

接着进行一遍普通渲染,重置投影与视图矩阵:

?glViewport(0, 0, windowWidth, windowHeight);
?glMatrixMode(GL_PROJECTION);
?glLoadMatrixf(cameraProjectionMatrix);

?glMatrixMode(GL_MODELVIEW);
?glLoadMatrixf(cameraViewMatrix);

传入深度纹理进行一次普通渲染:

?glActiveTextureARB(GL_TEXTURE1_ARB);
?glBindTexture(GL_TEXTURE_2D, shadowMapTextureLeft);
?glActiveTextureARB(GL_TEXTURE2_ARB);
?glBindTexture(GL_TEXTURE_2D, shadowMapTextureRight);

?callback();

?glActiveTextureARB(GL_TEXTURE1_ARB);
?glBindTexture(GL_TEXTURE_2D, 0);
?glActiveTextureARB(GL_TEXTURE2_ARB);
?glBindTexture(GL_TEXTURE_2D, 0);