Once again, into the foray of trying to understand matrices better…

I have created a matrix that creates “shadows” of an object with this code:

```
function CreateShadowMatrix(groundplane, lightpos)
local dot
shadowMat = matrix() --I overwrite all values so this is fast to declare
-- Find dot product between light position vector and ground plane normal.
dot = groundplane.x* lightpos.x +
groundplane.y * lightpos.y +
groundplane.z * lightpos.z +
groundplane.w * lightpos.w
shadowMat[1] = dot - lightpos.x * groundplane.x
shadowMat[5] = 0. - lightpos.x * groundplane.y
shadowMat[9] = 0.- lightpos.x * groundplane.z
shadowMat[13] = 0.- lightpos.x * groundplane.w
shadowMat[2] = 0.- lightpos.y * groundplane.x
shadowMat[6] = dot - lightpos.y * groundplane.y
shadowMat[10] = 0.- lightpos.y * groundplane.z
shadowMat[14] = 0.- lightpos.y * groundplane.w
shadowMat[3] = 0.- lightpos.z * groundplane.x
shadowMat[7] = 0.- lightpos.z * groundplane.y
shadowMat[11] = dot - lightpos.z * groundplane.z
shadowMat[15] = 0.- lightpos.z * groundplane.w
shadowMat[4] = 0.- lightpos.w * groundplane.x
shadowMat[8] = 0.- lightpos.w * groundplane.y
shadowMat[12] = 0.- lightpos.w * groundplane.z
shadowMat[16] = dot - lightpos.w * groundplane.w
return shadowMat
end
```

In draw() I render my array of cubes with this code:

```
lightVec.x = lightVec.x +.5 -- declared in setup(), below. Move the list source...
groundPlane = vec4(0,1,0,0)
shadowMatrix=CreateShadowMatrix(groundPlane, lightVec)
--other code here
.
.--- you know, the exploding stuff in the video...
.
---render the cubes
for k,v in pairs(tm) do
pushMatrix()
translate(v.x,0,v.z)
applyMatrix(shadowMatrix)
Cubes.mshadow:draw() -- copy of the Cubes:mw cube with black color, 32 alpha, no textures.
popMatrix()
pushMatrix()
translate(v.x,v.y,v.z)
Cubes.mw:draw()
if v.y > .5 then v.y = v.y - .25 end --create falling effect
popMatrix()
end
```

In setup() I have this code:

```
--unused, but I think I need it for this to work!
bias = matrix()
bias[1]=.5 bias[6]=.5 bias[11]=.5
bias[4]=.5 bias[8]=.5 bias[12]=.5
bias[16]=1
lightVec = vec4(0,193,0,1)
groundPlane = vec4(.20,1,0,0)
shadowMatrix=CreateShadowMatrix(groundPlane, lightVec)
```

However, for some reason I don’t understand, by the time my object “touches the ground”, the shadow sems to be “centered” on the object x and z axes, whilke sitting correctly on the y=0 (ground) plane. Moving the light source and recalc’in the shadow shows this situation; the shadow stays centered on the object instead of being attached to the “proper” place on the object.

Sample video:

http://www.youtube.com/watch?v=pmvE_fCU5fI