Computer Graphics
Børre Stenseth
JOGL>Shadow

A Simple Shadow

What

The mathematics are described in module:Planes

More on shadows in module: Shadows

screen

The idea is based on a situation where we position a light source in origo.

sketch

The plane (P!,P2,P3) is described with the equation: Ax+By+Cz=D.

private void CalculatePlane()
 {
   double[] v1={floor[3]-floor[0],  //x
               floor[4]-floor[1],   //y
               floor[5]-floor[2]};  //z
   double[] v2={floor[6]-floor[0],  //x
               floor[7]-floor[1],   //y
               floor[8]-floor[2]};  //z
     // crossproduct: (ay*bz-az*by, az*bx-ax*bz, ax*by-ay*bx)
     A = v1[1] * v2[2] - v1[2] * v2[1];
     B = v1[2] * v2[0] - v1[0] * v2[2];
     C = v1[0] * v2[1] - v1[1] * v2[0];
     // using a point on the floor to find D
     D = -(A * floor[0] + B * floor[1] + C * floor[2]);
 }

Based on this we render the scene with;

public void display(GLAutoDrawable drawable)
{
    GL gl = drawable.getGL();
    GLU glu=new GLU();
    // Clear the drawing area
   gl.glClear(GL.GL_COLOR_BUFFER_BIT | 
              GL.GL_DEPTH_BUFFER_BIT|
              GL.GL_ACCUM_BUFFER_BIT);
   gl.glLoadIdentity();
    // watch this
    glu.gluLookAt(25.0,25.0,20.0,
                 0.0,0.0,-10.0,
                 0.0,0.0,1.0);
    //gl.glRotatef(view_rotx,1.0f,0.0f,0.0f);
    //gl.glRotatef(view_roty,0.0f,1.0f,0.0f);
    // draw axes
    gl.glBegin(GL.GL_LINES);
    gl.glVertex3d(0.0, 0.0, 0.0); gl.glVertex3d(20.0, 0.0, 0.0);
    gl.glVertex3d(0.0, 0.0, 0.0); gl.glVertex3d(0.0, 20.0, 0.0);
    gl.glVertex3d(0.0, 0.0, -20.0); gl.glVertex3d(0.0, 0.0, 0.0);
    gl.glEnd();
    // draw the light as a sphere
    // set bulb material
    stdMaterials.setMaterial(gl,stdMaterials.MAT_YELLOW_PLASTIC,GL.GL_FRONT);
    GLUquadric q=glu.gluNewQuadric();
    q=glu.gluNewQuadric();
    glu.gluSphere(q,0.5f,20,20);
    glu.gluDeleteQuadric(q);
    // draw the floor
    //set floormaterial
    stdMaterials.setMaterial(gl,stdMaterials.MAT_GREEN_RUBBER,GL.GL_FRONT);
    gl.glBegin(GL.GL_POLYGON);
    gl.glNormal3d(0.0, 0.0, 1.0);
    for (int ix = 0; ix < 12; ix += 3)
        gl.glVertex3d(floor[ix], floor[ix + 1], floor[ix + 2] - 0.1);
    gl.glEnd();
    // draw the form that casts the shadow
    gl.glRotatef(ZRotation, 0.0f, 0.0f, 1.0f);
    // set form material
    stdMaterials.setMaterial(gl,stdMaterials.MAT_RUBY,GL.GL_FRONT);
    DrawForm(gl);
    // since light is in origo
    double[] mpx = {D,  0.0f,0.0f,-A,
                   0.0f,D,   0.0f,-B,
                   0.0f,0.0f,D,   -C,
                   0.0f,0.0f,0.0f,0.0f};
    gl.glPushMatrix();
    gl.glMultMatrixd(mpx,0);
    // set shadow material
    stdMaterials.setMaterial(gl,stdMaterials.MAT_BLACK_RUBBER,GL.GL_FRONT);
    // draw the shadow
    DrawForm(gl);
    gl.glPopMatrix();
    // Flush all drawing operations to the graphics card
    gl.glFlush();
}

The form casting the shadow is rendered like this;

private void DrawForm(GL gl)
 {
     double[] triangle ={5.0,0.0,0.0,
                 0.0,5.0,0.0,
                 5.0,5.0,0.0};
     int Radius = 7;
     GLU glu=new GLU();
     GLUT glut=new GLUT();
     GLUquadric glpQ=glu.gluNewQuadric();
     
     gl.glTranslatef(0.0f,0.0f,ZElevation);
     switch (SelectedForm)
     {
         case 1:glu.gluSphere(glpQ,Radius/3.0f,20,20);break;
         case 2:theTorus.drawTorus(Radius, Radius/10.0f, 50, 50);break;
         case 3:glut.glutSolidTeapot(1.0);break;
         default:
            gl.glBegin(GL.GL_POLYGON);
             gl.glNormal3d(A, B, C);
             for (int ix = 0; ix < 9; ix += 3)
                 gl.glVertex3d(triangle[ix], 
                               triangle[ix + 1],
                               triangle[ix + 2]);
            gl.glEnd();
            break;
     }
     gl.glTranslatef(0.0f,0.0f,-ZElevation);
     glu.gluDeleteQuadric(glpQ);
 }
  • Simple shadow project: https://svnit.hiof.no/svn/psource/JOGL/simpleshadow
  • The module Planes describes the basic mathematics.
  • The module Shadows is description of shadows in OpenGL and two examples.
Maintainance
B.Stenseth, june 2009
JOGL>Shadow
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