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XNA Constrained Billboard Particles

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I'm using the particles sample from create.msdn.com/en-US/education/catalog/sample/particle_3d which by default, has all the particles look straight at the camera. I'm trying to modify it to use constrained billboard particles.

I took the code for the XNA CreateConstrainedBillboard method and rewrote it in HLSL. This is what I have in the shader now, but its not working right. What did I do wrong?

float4x4 billboardRotation = CreateConstrainedBillboard(output.Position, CameraPosition, float3(0, 1, 0));

output.Position.xy += mul(input.Corner, rotation) * size * ViewportScale;
output.Position = mul(output.Position, billboardRotation);
output.Position = mul(mul(output.Position, View), Projection);

This is the CreateConstrainedBillboard code in HLSL

float4x4 CreateConstrainedBillboard(float3 objectPosition, float3 cameraPosition, float3 rotateAxis)
float3 forwardVector = float3(0, 0, -1);
float3 rightVector = float3(1, 0, 0);
float3 result1 = objectPosition - cameraPosition;
float num = length(result1);

if (num < 0.0099999)
result1 = forwardVector;
result1 = mul(result1, 1.0f / num);

float3 vector2 = rotateAxis;
float result2 = dot(rotateAxis, result1);
float3 result3;
float3 result4;

if (abs(result2) > 0.998254656791687)

if(abs((rotateAxis.x * forwardVector.x + rotateAxis.y * forwardVector.y + rotateAxis.z * forwardVector.z)) > 0.998254656791687)
result3 = rightVector;
result3 = forwardVector;

result4 = normalize(cross(rotateAxis, result3));
result3 = normalize(cross(result4, rotateAxis));
result4 = normalize(cross(rotateAxis, result1));
result3 = normalize(cross(result4, vector2));

float4x4 outputValue = float4x4(
float4(result4, 0),
float4(vector2, 0),
float4(result3, 0),
float4(objectPosition, 1)

return outputValue;

And since I can't really describe in words what the particles are doing, I recorded it: http://dl.dropbox.co...e/particles.mkv. The particles are created in a square area and are supposed to fall straight down with no movement on the XZ axis.

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