How to see position relative to another gameobject in fragment shader?

Unity Engine
1

My goal is a custom shader applied to any object which clips pixels based on the position of a shared bounding volume. That shared bounding volume is simply a cube, and I upload the min/max corners to the shader whenever it changes.

I thought it would be sufficient to get the world position within the shader, and then multiply that by the shared bounding volume’s worldToLocalMatrix (I upload this as a float4x4 matrix). However, this is incorrect – it works if all rotations are 0, but the clipping area moves around depending on the rotation, so I know that the coordinate space conversion is wrong.

This is what I do:

float4 posWorld = mul(unity_ObjectToWorld, v.vertex);
o.objectPosInBoundingSpace = mul(_FadeWorldToLocal, posWorld);

Then in the fragment, I discard pixels based on the z axis of the Min/Max (right now I only care about the z axis)

if(_FadeMaxWorld.z < i.objectPosInBoundingSpace.z || _FadeMinWorld.z > i.objectPosInBoundingSpace.z)
{
discard;
}

What am I missing? Is the “worldspace” unique per gameobject at least where the shader is concerned? I thought that I could rely on worldspace for all conversion.

2

Do you have batching enabled?

3

Yes. I just disabled batching and am having the same problem.

For what it’s worth – the clipping area seems to have the correct orientation and scale, but not position. It moves along the z axis as I rotate an object which contains both the shared transform and the individual gameobjects.

4

World space should be consistent regardless of batching or not. In fact batched geometry’s vertex locations are already in world space.

I’m going to suggest you try something a little odd. Do this:

mul(worldPos, _CustomWorldToLocal);

5

Instead of mul(_CustomWorldToLocal, worldPos); ?

I just tried that – now I have the same type of position alignment issue, but also an orientation issue.