How render 3D render texture in a Unity3D custom shader - unity3d

Question

Hi is it possible to render a 3d render texture on a custom shader raymarching like a 3dtexture?
I use a 3D render texture because I calculate and set the color of the 3D volume in a compute shader. I set the rendertexture 3D as shown below:
output3DRenderTexture= new RenderTexture(m_CubeDim.x, m_CubeDim.y, 0, thisTexFormat);
outpuoutput3DRenderTextureRendTex.enableRandomWrite = true;
output3DRenderTexture.dimension = UnityEngine.Rendering.TextureDimension.Tex3D;
output3DRenderTexture.volumeDepth = m_CubeDim.z;
output3DRenderTexture.Create();
I populate the 3D RenderTexture data in a a compute shader and GetData helps me confirm the 3d render texture has all the correct color data.
I can successfully render if I replace the 3D renderTex in the custom shader's sampler3D for a 3DTexture I create with the Tex2D slices.
cubeRenderer.material.SetTexture("_MainTex", output3DRenderTexture);//this does not render
versus
cubeRenderer.material.SetTexture("_MainTex", outputTexture3D);//this renders
This post in 2016 seems to suggest it's possible to render 3d render textures in custom shaders but it may now be outdated, it doesn't work for me and no error shows either.
It seems to me there maybe a significant performance hit if create the Textures2D slices in GPU, carry on with the creation of the Texture3D on CPU and re-send this Tex3D to GPU for the custom shader to consume it. After all the 3D volume already existed in GPU except as RenderTexture set as Tex3D. Thank you!
Shader:
#include "UnityCG.cginc"
#define ITERATIONS 100
#define PI2 6.28318530718
half4 _Color;
sampler3D _MainTex;
half _Intensity, _Threshold;
half3 _SliceMin, _SliceMax;
float4x4 _AxisRotationMatrix;
float _Angle;
struct Ray {
float3 origin;
float3 dir;
};
struct AABB {
float3 min;
float3 max;
};
// https http.download.nvidia.com/developer/presentations/2005/GDC/Audio_and_Slides/VolumeRendering_files/GDC_2_files/GDC_2005_VolumeRenderingForGames_files/Slide0073.htm
bool intersect(Ray r, AABB aabb, out float t0, out float t1)
{
float3 invR = 1.0 / r.dir;
float3 tbot = invR * (aabb.min - r.origin);
float3 ttop = invR * (aabb.max - r.origin);
float3 tmin = min(ttop, tbot);
float3 tmax = max(ttop, tbot);
float2 t = max(tmin.xx, tmin.yz);
t0 = max(t.x, t.y);
t = min(tmax.xx, tmax.yz);
t1 = min(t.x, t.y);
return t0 <= t1;
}
float3 get_uv(float3 p) {
return (p + 0.5);
}
float sample_volume(float3 uv, float3 p)
{
float v = tex3D(_MainTex, uv).r * _Intensity;
return v;
}
struct appdata
{
float4 vertex : POSITION;
float2 uv : TEXCOORD0;
};
struct v2f
{
float4 vertex : SV_POSITION;
float2 uv : TEXCOORD0;
float3 world : TEXCOORD1;
float3 local : TEXCOORD2;
};
v2f vert(appdata v)
{
v2f o;
o.vertex = UnityObjectToClipPos(v.vertex);
o.uv = v.uv;
o.world = mul(unity_ObjectToWorld, v.vertex).xyz;
o.local = v.vertex.xyz;
return o;
}
fixed4 frag(v2f i) : SV_Target
{
Ray ray;
ray.origin = i.local;
// world space direction to object space
float3 dir = (i.world - _WorldSpaceCameraPos);
ray.dir = normalize(mul(unity_WorldToObject, dir));
AABB aabb;
aabb.min = float3(-0.5, -0.5, -0.5);
aabb.max = float3(0.5, 0.5, 0.5);
float tnear;
float tfar;
intersect(ray, aabb, tnear, tfar);
tnear = max(0.0, tnear);
// float3 start = ray.origin + ray.dir * tnear;
float3 start = ray.origin;
float3 end = ray.origin + ray.dir * tfar;
float dist = abs(tfar - tnear);
float step_size = dist / float(ITERATIONS);
float3 ds = normalize(end - start) * step_size;
float4 dst = float4(0, 0, 0, 0);
float3 p = start;
[unroll]
for (int iter = 0; iter < ITERATIONS; iter++)
{
float3 uv = get_uv(p);
float v = sample_volume(uv, p);
float4 src = float4(v, v, v, v);
src.a *= 0.5;
src.rgb *= src.a;
// blend
dst = (1.0 - dst.a) * src + dst;
p += ds;
if (dst.a > _Threshold) break;
}
return saturate(dst) * _Color;
}
#endif