Billboard shader transform/move object to the center of the screen

Question

I am trying to get a textured plane object to be always in the center of the screen when being looked at (some sort of overlay/hud).
At the moment I use a billboard shader to keep it rotated to the camera

 o.pos = mul(UNITY_MATRIX_P, mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0)) - float4(v.vertex.y, v.vertex.x, 0.0, 0.0) * float4(0.1, 0.1, 1.0, 1.0));

How would I go about getting it to move to the center of the screen/camera when the object is being rendered? Which transformations do I have to do? Is it even possible to do?

Answer 1

Answer by JonnyHilly · May 09, 2020 at 09:58 PM

keep a reference to your camera... say public Transform camTran; or get it from Camera.main

then in your billboard code... just point it's forward vector towards the camera....

transform.forward = (camTran.position - transform.forward.position).normalized; or transform.LookAt(camTran.position);

in order to keep it always in front of the camera... you need the distance you want to maintain from the camera... either calculate current distance first... float currDistance = (camTran.position - transform.posion).magnitude; or just keep it in a public float desiredCamDistance = 20f;

then set the position of the object along the camera's forward looking vector

transform.position = camTran.forward * desiredCamDistance;

if you want to do both... set the position first, then point it at the camera second.

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farfelu · May 11, 2020 at 09:38 AM 0

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Thanks for the reply, but this looks like a C# script and not Unitys shader language.
I realize I can get the camera position with the inbuilt _WorldSpaceCameraPos variable, but how would I go about transforming the vertices to that position? I am having troubles understanding which matrixes I need to multiply it to get there.

JonnyHilly farfelu · May 12, 2020 at 08:12 AM 0

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sorry, i totally didn't notice this was a shader question, I wouldn't know the answer off the top of my head, but have done some similar work... my gut feeling says you might need to send extra information into the shader as a parameter, e.g. position of your object.. or the distance from the camera that you want to position your object... e.g. positional info that you normally can't easily access from within the shader. try this thread, https://forum.unity.com/threads/does-unity_matrix_mv-unity_matrix_it_mv-identity.199032/ and also this one on the various matrices (if you haven't seen it already. https://docs.unity3d.com/$$anonymous$$anual/SL-UnityShaderVariables.html

Billboard shader transform/move object to the center of the screen

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