XNA: How to make particles face the camera but not alter the pitch

Question

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 so that the particles will still look at the camera but won't rotate up and down. I think these are called billboard particles but I'm not really sure.

Edit:

This is what I have in the shader now. It seems like its working but the particles are really tiny. What did I do wrong?

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

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

Answer 1

Sprite billboards can be of two types:

• Spherical: they look always at the camera.
• Cylindrical/constrained: you define a constraint axis and they always try to face the camera, but rotating along that axis. This is the kind you are aiming for.

So, instead of using Matrix.CreateBillboard when creating the world matrix for your particles, use Matrix.CreateConstrainedBillboard. The only difference is that you need to specify the object's rotation axis instead of the camera up vector, and the optional cameraForwardVector and objectForwardVector to smooth possible deformations when the camera is too close.

Answer 2

I haven't had the time to test this, but after looking through the sample's code I believe you'll need to do a few changes for it to work. Here's what I have in mind, although I'm not sure it will work as it is:

The Sample

For starters, the way the particles are being billboarded at the moment doesn't allow for much variation. Each vertex basically carries the particle position and a 2D corner offset such as (-1,-1) or (1,1).

The GPU does all of the particle animation, applies the view and projection matrices to get it into clip space, and then adds the corner offset multiplied by the particle's size to the clip space XY position. Since this sum is done in clip space, the resulting quad will always be facing you. No billboard matrix anywhere.

The Solution

You'll need to change your shader in order to apply the billboards in a more traditional way using a matrix. Using the method r2d2rigo mentioned (Matrix.CreateConstrainedBillboard) you'll need to calculate a billboard matrix for each particle and pass it to the shader when drawing.

Then you'll need to change the method in the shader that is currently applying the View and Projection matrices and leave that part out, because you still need to do a few other things before converting to clip space.

With the position still in world space, that's when you will need to apply the corner offset, along with the billboard matrix you created before to make it face the camera.

Finally just apply the View and Projection matrices as it was done originally in the sample.

Or...

Start from the billboard sample instead and adapt it to your particle needs. That might turn out to be easier.