Jeff Molofee - NeHe's OpenGL Tutorials

glEnable(GL_DEPTH_TEST); // Enable Depth Testing

glColorMask(1,1,1,1); // Set Color Mask to TRUE, TRUE, TRUE, TRUE

glStencilFunc(GL_EQUAL, 1, 1); // We Draw Only Where The Stencil Is 1

// (I.E. Where The Floor Was Drawn)

glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); // Don't Change The Stencil Buffer

Now we enable the mirrored clipping plane. This plane is defined by eqr, and only allows object to be drawn from the center of the screen (where the floor is) down to the bottom of the screen (any negative value on the y-axis). That way the reflected ball that we draw can't come up through the center of the floor. That would look pretty bad if it did. If you don't understand what I mean, remove the first line below from the source code, and move the real ball (non reflected) through the floor. If clipping is not enabled, you will see the reflected ball pop out of the floor as the real ball goes into the floor.

After we enable clipping plane0 (usually you can have from 0-5 clipping planes), we define the plane by telling it to use the parameters stored in eqr.

We push the matrix (which basically saves the position of everything on the screen) and use glScalef(1.0f,-1.0f,1.0f) to flip the object upside down (creating a real looking reflection). Setting the y value of glScalef({x},{y},{z}) to a negative value forces OpenGL to render opposite on the y-axis. It's almost like flipping the entire screen upside down. When position an object at a positive value on the y-axis, it will appear at the bottom of the screen instead of at the top. When you rotate an object towards yourself, it will rotate away from you. Everything will be mirrored on the y-axis until you pop the matrix or set the y value back to 1.0f instead of –1.0f using glScalef({x},{y},{z}).

glEnable(GL_CLIP_PLANE0); // Enable Clip Plane For Removing Artifacts

// (When The Object Crosses The Floor)

glClipPlane(GL_CLIP_PLANE0, eqr); // Equation For Reflected Objects

glPushMatrix(); // Push The Matrix Onto The Stack

glScalef(1.0f, –1.0f, 1.0f); // Mirror Y Axis

The first line below positions our light to the location specified by LightPos. The light should shine on the bottom right of the reflected ball creating a very real looking light source. The position of the light is also mirrored. On the real ball (ball above the floor) the light is positioned at the top right of your screen, and shines on the top right of the real ball. When drawing the reflected ball, the light is positioned at the bottom right of your screen.

We then move up or down on the y-axis to the value specified by height. Translations are mirrored, so if the value of height is 5.0f, the position we translate to will be mirrored (-5.0f). Positioning the reflected image under the floor, instead of above the floor!

After position our reflected ball, we rotate the ball on both the x axis and y axis, based on the values of xrot and yrot. Keep in mind that any rotations on the x axis will also be mirrored. So if the real ball (ball above the floor) is rolling towards you on the x-axis, it will be rolling away from you in the reflection.

After positioning the reflected ball and doing our rotations we draw the ball by calling DrawObject(), and pop the matrix (restoring things to how they were before we drew the ball). Popping the matrix all cancels mirroring on the y-axis.

We then disable our clipping plane (plane0) so that we are not stuck drawing only to the bottom half of the screen, and last, we disable stencil testing so that we can draw to other spots on the screen other than where the floor should be.

Note that we draw the reflected ball before we draw the floor. I'll explain why later on.

glLightfv(GL_LIGHT0, GL_POSITION, LightPos); // Set Up Light0

glTranslatef(0.0f, height, 0.0f); // Position The Object

glRotatef(xrot, 1.0f, 0.0f, 0.0f); // Rotate Local Coordinate System On X Axis

glRotatef(yrot, 0.0f, 1.0f, 0.0f); // Rotate Local Coordinate System On Y Axis

DrawObject(); // Draw The Sphere (Reflection)

glPopMatrix(); // Pop The Matrix Off The Stack

glDisable(GL_CLIP_PLANE0); // Disable Clip Plane For Drawing The Floor

glDisable(GL_STENCIL_TEST); // We Don't Need The Stencil Buffer Any More (Disable)

We start off this section of code by positioning our light. The y-axis is no longer being mirrored so drawing the light this time around will position it at the top of the screen instead of the bottom right of the screen.

We enable blending, disable lighting, and set the alpha value to 80% using the command glColor4f(1.0f,1.0f,1.0f,0.8f). The blending mode is set up using glBlendFunc(), and the semi transparent floor is drawn over top of the reflected ball.

If we drew the floor first and then the reflected ball, the effect wouldn't look very good. By drawing the ball and then the floor, you can see a small amount of coloring from the floor mixed into the coloring of the ball. If I was looking into a BLUE mirror, I would expect the reflection to look a little blue. By rendering the ball first, the reflected image looks like it's tinted the color of the floor.

glLightfv(GL_LIGHT0, GL_POSITION, LightPos); // Set Up Light0 Position

glEnable(GL_BLEND); // Enable Blending (Otherwise The Reflected Object Wont Show)

glDisable(GL_LIGHTING); // Since We Use Blending, We Disable Lighting

glColor4f(1.0f, 1.0f, 1.0f, 0.8f); // Set Color To White With 80% Alpha

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // Blending Based On Source Alpha And 1 Minus Dest Alpha

DrawFloor(); // Draw The Floor To The Screen

Now we draw the 'real' ball (the one that floats above the floor). We disabled lighting when we drew the floor, but now it's time to draw another ball so we will turn lighting back on.

We don't need blending anymore so we disable blending. If we didn't disable blending, the colors from the floor would mix with the colors of our 'real' ball when it was floating over top of the floor. We don't want the 'real' ball to look like the reflection so we disable blending.

We are not going to clip the actual ball. If the real ball goes through the floor, we should see it come out the bottom. If we were using clipping the ball wouldn't show up after it went through the floor. If you didn't want to see the ball come through the floor, you would set up a clipping equation that set the Y value to +1.0f, then when the ball went through the floor, you wouldn't see it (you would only see the ball when it was drawn on at a positive value on the y-axis. For this demo, there's no reason we shouldn't see it come through the floor.

We then translate up or down on the y-axis to the position specified by height. Only this time the y-axis is not mirrored, so the ball travels the opposite direction that the reflected image travels. If we move the 'real' ball down the reflected ball will move up. If we move the 'real' ball up, the reflected ball will move down.

We rotate the 'real' ball, and again, because the y-axis is not mirrored, the ball will spin the opposite direction of the reflected ball. If the reflected ball is rolling towards you the 'real' ball will be rolling away from you. This creates the illusion of a real reflection.

After positioning and rotating the ball, we draw the 'real' ball by calling DrawObject().

glEnable(GL_LIGHTING); // Enable Lighting

glDisable(GL_BLEND); // Disable Blending

glTranslatef(0.0f, height, 0.0f); // Position The Ball At Proper Height