NeHe OpenGL Lesson27 – Volume Shadows(体积阴影)

This sample shows us how to create dynamic volume shadows with OpenGL. The main technology used here are, create the shadow volumes, figure out the shadow area in the stencil buffer.
The first problem is how to create the shadow volumes based on the light source position and cast shadow objects.
1) Calculate the plane equation for each triangle after the objects loaded. Later, we could use the plane parameters to determine whether a light source in the front or back of the plane.
2) Find the connectivity for each triangle edges. We could find which two triangles sharing this edge.
3) In the draw process, we will find out the out-lines of the shadow volume. The out-lines are just edges that one triangle lit by the light source and other one unlit or only one triangles own it. With the edge information and light source position, we could create a big quad that extend some distance along the light direction. Those quads come together is of the shadow volume geometry.
One thing need to mention is that, the shadow volume calculation happen in the object space, that means we need to convert light source position from the world space to the object space. The following is the code for doing the space converting:

glLoadIdentity();
glRotatef(-yrot, 0.0f, 1.0f, 0.0f);
glRotatef(-xrot, 1.0f, 0.0f, 0.0f);
glTranslatef(-ObjPos[0], -ObjPos[1], -ObjPos[2]);
glGetFloatv(GL_MODELVIEW_MATRIX,Minv);
lp[0] = LightPos[0];
lp[1] = LightPos[1];
lp[2] = LightPos[2];
lp[3] = LightPos[3];
VMatMult(Minv, lp);
 
// the following are the normal process of drawing objects
//glTranslatef(ObjPos[0], ObjPos[1], ObjPos[2]);
//glRotatef(xrot, 1.0f, 0.0f, 0.0f);
//glRotatef(yrot, 0.0f, 1.0f, 0.0f);
//DrawGLObject(obj);

As you see, you could place a ‘-’ and inverse the matrix operation function orders to get a inversed matrix.

Mask Screen Shadow Area in Stencil Buffer

To draw the shadow volumes with stencil buffer:
1) disable the color buffer & depth buffer writing, enable the surface culling feature, set up z-test function to less or equal;

glCullFace(GL_BACK);
glEnable(GL_CULL_FACE);    
 
glDepthMask(GL_FALSE);
glDepthFunc(GL_LEQUAL);
 
glEnable(GL_STENCIL_TEST);
glColorMask(0, 0, 0, 0);
glStencilFunc(GL_ALWAYS, 1, 0xffffffff);

2) draw the front faces of the shadow volumes (culling the back surfaces), and increase the stencil value when z-test success;

glFrontFace(GL_CCW);
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
// draw shadow volumes here

3) draw the back faces of the shadow volumes (culling the front surfaces), and decrease the stencil value when z-test success;

glFrontFace(GL_CW);
glStencilOp(GL_KEEP, GL_KEEP, GL_DECR);
// draw shadow volumes here

4) the area that stencil value not equal to ZERO will be the shadow area. Usually, we draw a black quad or alpah blend full screen quad with the stencil testing.

glFrontFace(GL_CCW);
glColorMask(1, 1, 1, 1);
 
//draw a shadowing rectangle covering the entire screen
glColor4f(0.0f, 0.0f, 0.0f, 0.4f);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glStencilFunc(GL_NOTEQUAL, 0, 0xffffffff);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
glPushMatrix();
glLoadIdentity();
glBegin(GL_TRIANGLE_STRIP);
    glVertex3f(-0.1f, 0.1f,-0.10f);
    glVertex3f(-0.1f,-0.1f,-0.10f);
    glVertex3f( 0.1f, 0.1f,-0.10f);
    glVertex3f( 0.1f,-0.1f,-0.10f);
glEnd();
glPopMatrix();
glDisable(GL_BLEND);