# Real Time Reflections Han-Wei Shen

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1 Real Time Reflections Han-Wei Shen

2 Reflections One of the most noticeable effect of inter-object lighting Direct calculation of the physics (ray tracing) is too expensive Our focus is to capture the most significant ifi reflection while minimizing the overhead via rendering the virtual object virtual object Clipped to reflector reflector object

3 Image vs. Object Space Methods Image space methods: create a texture from a view of the reflected objects and apply it to the reflector Advantage: does not depend d on the object geometry Disadvantage: sampling issue and also only an approximation (environment mapping as an example) Object space methods: create the actual geometry of the object s reflection and render it to the reflector Disadvantage: accuracy of the geometry Advantage: more accurate reflection (for nearby objects) Both methods need to create the virtual objects

4 Planar Reflections The most common reflection flat mirror, floor, wall, etc Creating virtual objects for the reflected objects is much easier A view independent operation only consider the relative position of the object and the reflector The virtual object is created by transforming the object across the reflector plan

5 Reflection Transformation Can be broken into three stages: 1. Transform the objects into the reflector s local coordinate system Translate the reflector to the world origin and rotate so the reflector plane will coincident with the world s XY plane 2. Scale by -1 in Z 3. Transform the reflected object s local coordinates of the reflector back to the world coordinates Rotate the world s XY plane to the reflector s plane and then translate the origin to the reflector s local origin Overall - A translation of the mirror plane to the origin, a Overall A translation of the mirror plane to the origin, a rotation embedding the mirror to x-y plane, a scale of -1 in Z, inverse rotation, and a translation back to the mirror location

6 Reflection Matrix Given a point P on the reflector plane, vector V perpendicular to the plane 1-2Vx^2-2VxVy -2VxVz 2(P.V)Vx R = -2VxVy 1-2Vy^2-2VyVz 2(P.V)Vy -2VxVz -2VyVz 1-2Vz^2 2(P.V)Vz Multiply R to the object vertices produce an reflected object on the opposite side of the reflector plane The entire scene is duplicated, simulating a reflector of infinite extent

7 Render the Reflected Geometry An important task: clip the reflected geometry so that it is only visible on the reflector surface Beyond the reflector boundaries and in front of reflector side front

8 Clipping using the stencil You only want the reflected geometry to appear on the reflector surface Use stencil buffer: Clear the stencil buffer Render the reflector and set the stencil Render the reflected geometry only at where the stencil pixels have been set The above algorithm is to use the stencil buffer to control where to draw the reflection

9 Clipping using the stencil Another method: render the reflected object first, and then render the reflector to set the stencil buffer, then clear the color buffer everywhere except where the stencil being set This method is to use the stencil buffer to control where to erased the incorrect reflection Advantage: when it is faster to use stencil to control clearing the scene than drawing the entire scene with stenfil tests

10 The stencil erase algorithm Draw the reflected objects Clear the stencil and depth buffers Configure the stencil so that t 1 will be set where polygons are rendered Disable draw to color buffer Draw the reflector Reconfigure the stencil Clear the color and depth buffer to the background color (except where the stencil being set) Disable stencil test Draw the rest of scene (except reflected objects and reflector)

11 The stencil erase algorithm

12 Clipping using texture mapping Render the reflected geometry Store the image into a texture (using FBO or glcopyteximage2d) Clear the color and depth buffer Redraw the entire scene, with the reflector textured with the previous texture (the process of texturing reflector will automatically clip the reflected geometry outside the boundary) Note that reflected geometry in front of the reflector still needs to be clipped before the texture map is created (using the reflector as the clipping plane supported by OpenGL s application clipping plane)

13 Clipping using texture mapping The key is to assign correct texture coordinates to the reflector Basic idea s and t coordinates correlate to x and y window coordinates of the reflector Project the reflector vertex to the clip space Bias and scale the texture coordinates from [-1,1] to [0,1] Perform perspective division and use the first two components to access the texture

14 Clipping using texture mapping

15 Interreflection Goal: let the reflections to bounce between reflectors The number of bounces needs to be limited to preserve interactivity The reflection transformations need to be concatenated Render the deepest interreflection first

16 Interreflection Clear the stencil buffer Set the stencil operation to increment the stencil values where pixels are rendered Render each reflector involved in interreflection into stencil buffer Set the stencil test to pass where the stencil value equal the number of interreflection Apply planar transformation Draw the reflected scene Draw the reflector, blending if desired

17 Refraction Snell s law refraction angel is based on the medium s refraction index n1 sin 1 = n2 sin 2 U (eye) N n1 n1 R (refraction vector) n2 n2

18 Refraction Vector n = n1/n2 R = nu N( n(n.u) + 1-n^2(1-(N.U)^2) (N ) or R = U (1-n)N(N.U) (a simplified form) U (eye) N n1 R n2

19 Planar Refraction Move the eye point to match the refraction vector 1/n * d d away from the interface rotate the eye (when not (when eye ray is perpendicualr) Perpendicular) When the refraction ray bent toward the normal direction (a common case)

20 Environment Map Refraction Take 6 shots from the refractor to its environment Based on the eye rays, calculate the refracted rays using snell s s law at each refractor s vertex Using the refracted rays as texture coordinate (s,t,r) to look up the environment cube map

21 Environment Map Refraction Create Environment Cube Map Calculate the refractor ray and Use it as the texture coordinates to the environment cube map result

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