Getting RGBA value of a particular voxel "colliding" with a Game Object

[virtualdissection]

Hi everyone,

I am trying to get the RGBA value of a particular voxel that my probe touches or crosses in the air.

Since it is being generated by a shader, I am failing at using this:

void OnCollisionEnter(Collision other)
    {
        Debug.Log(other.gameObject.GetComponent<Renderer>().material.color); // this line will tell you the object's color which collide to the gameObject you attached this script to
    }

Here is a video of my attempt:

UnityVolumeRendering

I have attached the shader that is generating the volume rendering I would like to probe.

Thank you for your help!

8007173–1030082–DirectVolumeRenderingShader.shader (12.5 KB)

[mgear]

random ideas:
could pass probe position into shader, and then sample that position from 3d texture,
and get values back with appendbuffer or something similar.

or if can somehow calculate local position for probe (in that “3d chunk”) and do getpixel (x,y,z) there…maybe from bounds can interpolate it.

[virtualdissection]

@mgear

random ideas:
could pass probe position into shader, and then sample that position from 3d texture,
and get values back with appendbuffer or something similar.

or if can somehow calculate local position for probe (in that “3d chunk”) and do getpixel (x,y,z) there…maybe from bounds can interpolate it.

Thank you! Do you think that I should do it with CollisionEnter or CollisionStay? I would like to probe it as often as possible as it’s for haptics, which should run at 1kHz, but I guess FixedUpdate wouldn’t have the position info, right?

[mgear]

could add box collider on that cube area, to get overall collision on the bounds,
but it won’t give per voxel hit position, need to calculate the position and get value manually,
so at that point could just do it inside FixedUpdate().

i’m thinking something like this,
if you know that volumetric gameobject bounds, and if its aligned bounding box then easier, then interpolate to get probe position in X,Y,Z 3d texture coordinates

[virtualdissection]

Thank

@mgear

could add box collider on that cube area, to get overall collision on the bounds,
but it won’t give per voxel hit position, need to calculate the position and get value manually,
so at that point could just do it inside FixedUpdate().

i’m thinking something like this,
if you know that volumetric gameobject bounds, and if its aligned bounding box then easier, then interpolate to get probe position in X,Y,Z 3d texture coordinates

Thank you again

[virtualdissection]

@mgear

could add box collider on that cube area, to get overall collision on the bounds,
but it won’t give per voxel hit position, need to calculate the position and get value manually,
so at that point could just do it inside FixedUpdate().

i’m thinking something like this,
if you know that volumetric gameobject bounds, and if its aligned bounding box then easier, then interpolate to get probe position in X,Y,Z 3d texture coordinates

I managed to pass in my probe position by adding a script to the same GameObject that has the shader

material.SetVector("_probePosition", probe.transform.position);

Inside the shader, I declared this Property:

_probePosition("Probe Position", Vector) = (0, 0, 0, 1)

I thought about declaring another property to hold the voxel color, but I don’t understand the shader lifecycle and how to make it update the property’s value so that I can access it with my script, so these didn’t work:

_voxelDensity("Voxel Density", Float) = 0.0
...
_voxelDensity = getDensity(_probePosition);

Do you know how to invoke an update on this method call to update _voxelDensity when _probePosition changes? Thank you!

[mgear]

this is whats happening:

How to store and read data strictly in GPU

The order of operations here is: CPU side c# script tells GPU it needs to create a compute buffer of a specific size, and to bind it to a read/write register. Bind same buffer to global property as a read only buffer. GPU side shader can then write to said register with a set of data that's equal in size to the buffer that's been bound. GPU side shader can read from register, or structured buffer. Unless you call GetData() on the buffer, it's only ever on the GPU. But you do have to tell th…

so cannot directly read values from shader.

some options

• AppendStructuredBuffer, some examples at, see Shader “Custom/AppendBufferCg” (which appends data into buffer, which can be read with GetData) https://cheneyshen.com/directcompute-tutorial-for-unity-append-buffers/
• Unity - Scripting API: ComputeBuffer.GetData
• examples for AsyncGPUReadback GitHub - cinight/MinimalCompute
• compute shaders can also access those values in shader, but i dont know much about them

also need to check if that passed position is correct, local space vs worldspace. (you could debug it by setting some vertex or pixel colors based on distance to that passed position)

i’d also explore the other way around, reading directly from 3d texture, by mapping the probe position into local coordinates inside bounds. (no need shader work there)

[virtualdissection]

Thank you again, I will try those options!