opengl: why the shadow disappear when i stand in the shadow?

Question

if i don't stand in the shadow,the shadow effect look perfect.but when i'm in the shadow,the shadow is gone.I don't why?

here is my code to realize the shadow effect.

void  CastShadow(SECTOR &sec, float *lp)
{//lp is the relative position to the sec
    unsigned int    i, j, k, jj;
    unsigned int    p1, p2;
    VECTOR          v1, v2;
    float           side;


    for (i=0;i<sec.numplanes;i++)
    {
        side =sec.planes[i].planeeq.a*lp[0]+sec.planes[i].planeeq.b*lp[1]+sec.planes[i].planeeq.c*lp[2]+sec.planes[i].planeeq.d*lp[3];
        if (side>0) 
            sec.planes[i].visible = TRUE;
        else
            sec.planes[i].visible = FALSE;
    }

    glDisable(GL_LIGHTING);
    glDepthMask(GL_FALSE);
    glDepthFunc(GL_LEQUAL);
    glEnable(GL_STENCIL_TEST);
    glColorMask(0, 0, 0, 0);
    glStencilFunc(GL_ALWAYS, 1, 0xffffffff);

    // first pass, stencil operation decreases stencil value
    glFrontFace(GL_CCW);
    glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
    for (i=0; i<sec.numplanes;i++)
    {
        if (sec.planes[i].visible)
            for (j=0;j<3;j++){
                k = sec.planes[i].neigh[j];
                if ((!k) || (!sec.planes[k-1].visible))//如果以第k个点开始的邻边没有相邻平面或者相邻平面不可见
                {
                    // here we have an edge, we must draw a polygon
                    p1 = sec.planes[i].p[j]-1;//邻边的起点
                    jj = (j+1)%3;           
                    p2 = sec.planes[i].p[jj]-1;//邻边的终点

                    //calculate the length of the vector
                    v1.x = (sec.points[p1].vec.x - lp[0])*100;
                    v1.y = (sec.points[p1].vec.y - lp[1])*100;
                    v1.z = (sec.points[p1].vec.z - lp[2])*100;

                    v2.x = (sec.points[p2].vec.x - lp[0])*100;
                    v2.y = (sec.points[p2].vec.y - lp[1])*100;
                    v2.z = (sec.points[p2].vec.z - lp[2])*100;

                    glBegin(GL_TRIANGLE_STRIP);//将光源连到邻边的起点并延长,将光源连到邻边的终点的并延长,最后延长出来的梯形,画了过后模板缓冲区的值加1
                        glVertex3f(sec.points[p1].vec.x,sec.points[p1].vec.y,sec.points[p1].vec.z);
                        glVertex3f(sec.points[p1].vec.x + v1.x,sec.points[p1].vec.y + v1.y,sec.points[p1].vec.z + v1.z);
                        glVertex3f(sec.points[p2].vec.x,sec.points[p2].vec.y,sec.points[p2].vec.z);
                        glVertex3f(sec.points[p2].vec.x + v2.x,sec.points[p2].vec.y + v2.y,sec.points[p2].vec.z + v2.z);
                    glEnd();
                }
            }
    }

    // second pass, stencil operation increases stencil value
    glFrontFace(GL_CW);//这句表示画的多边形,只看到里面
    glStencilOp(GL_KEEP, GL_KEEP, GL_DECR);
    for (i=0; i<sec.numplanes;i++){
        if (sec.planes[i].visible)
            for (j=0;j<3;j++){
                k = sec.planes[i].neigh[j];
                if ((!k) || (!sec.planes[k-1].visible))//如果以第k个点开始的邻边没有相邻平面或者相邻平面不可见
                {
                    // here we have an edge, we must draw a polygon
                    p1 = sec.planes[i].p[j]-1;//邻边的起点
                    jj = (j+1)%3;           
                    p2 = sec.planes[i].p[jj]-1;//邻边的终点

                    //calculate the length of the vector
                    v1.x = (sec.points[p1].vec.x - lp[0])*100;
                    v1.y = (sec.points[p1].vec.y - lp[1])*100;
                    v1.z = (sec.points[p1].vec.z - lp[2])*100;

                    v2.x = (sec.points[p2].vec.x - lp[0])*100;
                    v2.y = (sec.points[p2].vec.y - lp[1])*100;
                    v2.z = (sec.points[p2].vec.z - lp[2])*100;

                    glBegin(GL_TRIANGLE_STRIP);//将光源连到邻边的起点并延长,将光源连到邻边的终点的并延长,最后延长出来的梯形,画了过后模板缓冲区的值加1
                        glVertex3f(sec.points[p1].vec.x,sec.points[p1].vec.y,sec.points[p1].vec.z);
                        glVertex3f(sec.points[p1].vec.x + v1.x,sec.points[p1].vec.y + v1.y,sec.points[p1].vec.z + v1.z);
                        glVertex3f(sec.points[p2].vec.x,sec.points[p2].vec.y,sec.points[p2].vec.z);
                        glVertex3f(sec.points[p2].vec.x + v2.x,sec.points[p2].vec.y + v2.y,sec.points[p2].vec.z + v2.z);
                    glEnd();
                }
            }
    }

    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,-0.0010f);
        glVertex3f(-0.1f,-0.1f,-0.0010f);
        glVertex3f( 0.1f, 0.1f,-0.0010f);
        glVertex3f( 0.1f,-0.1f,-0.0010f);
    glEnd();
    glPopMatrix();
    glDisable(GL_BLEND);

    glDepthFunc(GL_LEQUAL);
    glDepthMask(GL_TRUE);
    glEnable(GL_LIGHTING);
    glDisable(GL_STENCIL_TEST);
    glShadeModel(GL_SMOOTH);

}

and the class SECTOR is like this:

class SECTOR
{
public:
    int numpoints;
    int numplanes;
    vector<VERTEX> points;
    vector<PLANE> planes;
    MATERIAL material;
    bool read();
    bool loadtexture();
    bool build();
    bool plane_calc();
    void SetConnectivity();
    SECTOR& SECTOR::subdivide(long depth);
    SECTOR(string str1,string str2):modelfilename(str1),texturefilename(str2)
    {
        numpoints=0;
        numplanes=0;

    }
    SECTOR()
    {
        numpoints=0;
        numplanes=0;

    }

private:
    FILE *modelfilein,*texturefilein;
    string modelfilename,texturefilename;
    char oneline[255];
    UINT texturename;
    AUX_RGBImageRec *TextureImage;
};

Answer 1

Since you have already handled 'capping' the shadow volume at the near clip plane, you probably have a problem with the camera being inside a shadow volume cast by an object which is behind the camera. You can test this out by moving your camera so that it is inside the shadow volume (where the problem occurs) and then rotate the camera such that the object is infront of the near clip plane.

It is easier to use the 'depth fail' algorithm to handle such special cases, although you must be able to compute and render caps for the far ends of shadow volumes. You can get an overview of the differences between shadow volume algorithms on wikipedia.