Can a compute shader incrementally change a texture?

I'm trying to use a compute shader to modify a 3D render texture. I want to keep the existing contents, but additively paint into the texture in a specific location. However, while I could use a compute shader to fill everything with a constant colour, I couldn't get it to do any modifications on top of existing state. I've been banging my head against this for a while and can't find any useful documentation on the subject.

My compute shader:

Code (CSharp):

1.  
2. #pragma kernel CSMain
3.  
4. Texture3D<float4> texIn;
5. RWTexture3D<float4> texOut;
6.  
7. float3 boundsDimensions;
8. float3 boundsCenter;
9.  
10. float3 paintCenter;
11. float paintRadius;
12. float paintMult;
13. float paintPower;
14. float4 paintColor;
15.  
16. [numthreads(8,8,8)]
17. void CSMain (uint3 id : SV_DispatchThreadID)
18. {
19.     float3 texSize;
20.     texIn.GetDimensions(texSize.x, texSize.y, texSize.z);
21.  
22.     float3 unitsPerTexel = (boundsDimensions/texSize);
23.     float3 diff = (id.xyz*unitsPerTexel + boundsCenter - paintCenter);
24.     float dist = length(diff);
25.  
26.     float amount = saturate(1-dist/paintRadius);
27.     amount = pow(amount,paintPower) * paintMult;
28.      
29.     float4 finalPaint = saturate(texIn[id.xyz] + amount * paintColor);
30.  
31.     texOut[id.xyz] = finalPaint;
32. }
33.  

The calling code:

Code (CSharp):

1.         ComputeShader paintShader = Resources.Load<ComputeShader>("ComputeShaders/PaintSphere");
2.      
3.         paintShader.SetTexture(0, "texIn", paintVolume.Volume);
4.         paintShader.SetTexture(0, "texOut", paintVolume.Volume);
5.  
6.         paintShader.SetVector("boundsDimensions", paintVolume.BoundsSize);
7.         paintShader.SetVector("boundsCenter", paintVolume.BoundsCenter);
8.         paintShader.SetVector("paintCenter", transform.InverseTransformPoint(worldPosition));
9.         paintShader.SetFloat("paintRadius", radius);
10.         paintShader.SetFloat("paintMult", mult);
11.         paintShader.SetFloat("paintPower", power);
12.         paintShader.SetVector("paintColor", color);
13.  
14.         paintShader.Dispatch(0, paintVolume.Volume.width / 8, paintVolume.Volume.height / 8, paintVolume.Volume.volumeDepth / 8);

...And, for good measure, the render texture creation:

Code (CSharp):

1.             volume = new RenderTexture(volumeDimensions, volumeDimensions, 0, RenderTextureFormat.ARGB32, RenderTextureReadWrite.Linear);
2.             volume.isVolume = true;
3.             volume.volumeDepth = volumeDimensions;
4.             volume.enableRandomWrite = true;

As far as I can tell, the questionable bit is that I'm feeding the same texture into the shader as both an input and an output variable. However, I don't know another way to approach this. Does anyone know what I'm doing wrong, or what I could do as an alternative approach?

 

I was able to work around this by using two render textures and swapping between them; a bit circuitous, but it seems to get the job done. Some strange issues when I try to do too many paint events at once, though. I haven't been able to sort them out, but I suspect they have to do with execution order or possibly some sort of concurrency issue. It'd be much better if I could change the texture in-place with some sort of additive scheme...