Allow tweaking z buffer when rendering polygon, improve explosion render

src/Dodge/RandomHelp.hs

@@ -3,6 +3,7 @@ Helpers for random generation.
 -}
 module Dodge.RandomHelp where
 import Geometry
+import Geometry.Vector3D
 import Geometry.Data
 
 import System.Random
@@ -68,6 +69,26 @@ randInCirc maxRad = do rad <- state $ randomR (0,maxRad)
                        ang <- state $ randomR (0,2*pi)
                        return $ rad *.* unitVectorAtAngle ang
 
+randOnUnitSphere :: RandomGen g => State g Point3
+randOnUnitSphere = do
+    z <- state $ randomR (negate 1,1)
+    longitude <- state $ randomR (0, 2*pi)
+    let (x,y) = sqrt (1 - z^(2::Int)) *.* unitVectorAtAngle longitude
+    return (x,y,z)
+
+randOnHemisphere :: RandomGen g => State g Point3
+randOnHemisphere = do
+    z <- state $ randomR (0,1)
+    longitude <- state $ randomR (0, 2*pi)
+    let (x,y) = sqrt (1 - z^(2::Int)) *.* unitVectorAtAngle longitude
+    return (x,y,z)
+
+randInHemisphere :: RandomGen g => State g Point3
+randInHemisphere = do
+    p <- randOnHemisphere
+    r <- state $ randomR (0,1)
+    return $ r *.*.* p
+
 randInRect :: RandomGen g => Float -> Float -> State g Point2
 randInRect w h = do x <- state $ randomR (0,w)
                     y <- state $ randomR (0,h)