Stefan Habel
Visual Effects Software Developer
Frog In The Throat (2008)
3D-animated short film -- Trailer for the 2008 Stuttgart Festival of Animated Film (ITFS)
Directed by Wolfram Kampffmeyer
My role: Scripting
Software used: XSI
Wolfram needed an expression in XSI to calculate the intensity
of a point light over a surface based on the position of the main
point light source to create a fake caustics effect for the pearl
necklace of his main character.
I used vector maths and the dot product to calculate
the fake caustics light's intensity. Due to lack of variables and
vector functions in the XSI expression language, the resulting expression
turned out to be rather lengthy (see below).
ITFS 2008
took place May 1-6 in
Stuttgart, Germany.
You can watch the short on YouTube (link below).
youtube.com/watch?v=HiWTJEi7v1I
The Expression
The expression could have been given as:
l = Hauptlicht - Caustics_Fake0
n = cross( grid0CornerA - grid0Origin, grid0CornerB - grid0Origin )
intensity = ( 90 - acos( dot(l, n) / (length(l) * length(n) ) ) / 90 * 2
- Hauptlicht is the main light source
- Caustics_Fake0 is the fake caustics light
- l is the direction to the main light, and
- n is the normal of the surface below the fake caustics light
As XSI didn't support functions or variables, the resulting expression
became rather lengthy:
intensity = ( 90 - acos( ( ( Hauptlicht.kine.global.posx - Caustics_Fake0.kine.global.posx ) * ( ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) - ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) ) + ( Hauptlicht.kine.global.posy - Caustics_Fake0.kine.global.posy ) * ( ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) - ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) ) + ( Hauptlicht.kine.global.posz - Caustics_Fake0.kine.global.posz ) * ( ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) - ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) ) ) / ( sqrt( ( Hauptlicht.kine.global.posx - Caustics_Fake0.kine.global.posx ) * ( Hauptlicht.kine.global.posx - Caustics_Fake0.kine.global.posx ) + ( Hauptlicht.kine.global.posy - Caustics_Fake0.kine.global.posy ) * ( Hauptlicht.kine.global.posy - Caustics_Fake0.kine.global.posy ) + ( Hauptlicht.kine.global.posz - Caustics_Fake0.kine.global.posz ) * ( Hauptlicht.kine.global.posz - Caustics_Fake0.kine.global.posz ) ) * sqrt( ( ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) - ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) ) * ( ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) - ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) ) + ( ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) - ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) ) * ( ( grid0CornerB.kine.global.posz - grid0Origin.kine.global.posz ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) - ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posz - grid0Origin.kine.global.posz ) ) + ( ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) - ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) ) * ( ( grid0CornerB.kine.global.posx - grid0Origin.kine.global.posx ) * ( grid0CornerA.kine.global.posy - grid0Origin.kine.global.posy ) - ( grid0CornerB.kine.global.posy - grid0Origin.kine.global.posy ) * ( grid0CornerA.kine.global.posx - grid0Origin.kine.global.posx ) ) ) ) ) ) / 90 * 2
There were a number of pearls in the necklace that needed the fake caustics effect, so a lot of copying and pasting were done, but in the end, everything worked out just fine.
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