Help Reconstructing a Normal Map from a Swizzled map using a compute shader

I'm trying to reconstruct a normal map texture from a swizzled normal map. These normal maps are generated at runtime (it's UMA). I've attempted to make a compute shader, copying the "UnpackNormalAGorRG" macro.

Code (CSharp):

1. void NormalConverter(uint3 id : SV_DispatchThreadID)
2. {
3. #pragma kernel NormalConverter
4. Texture2D<float4> Input;
5. RWTexture2D<float3> Result;
6. [numthreads(1, 1, 1)]
7. void NormalConverter(uint3 id : SV_DispatchThreadID)
8. {
9.     float2 src = float2(id.x, id.y);
10.     float4 packednormal = Input[src];
11.     float3 normal;
12.     // This do the trick
13.     packednormal.x *= packednormal.w;
14.     normal.xy = packednormal.xy * 2 - 1;
15.     normal.z = sqrt(1 - saturate(dot(normal.xy, normal.xy)));
16.     Result[id.xy] = (normal * 0.5) + 0.5;
17. }
18. }

The only problem is... it doesn't work. It returns back a normal map that is one shade of blue for everything. I know the texture is being written to (I generated a gradient in it just to verify). I've also verified the texture going in is correct (I've saved it to disk -- it even works as a normal map, if you don't set it to "normal map" in the import settings).

I'm a newbie at compute shaders, so obviously I've done something completely wrong.

Here's how I'm calling it (normalMap is a standard Texture2D that contains the swizzled normal map).

Code (CSharp):

1.     RenderTexture normalMapRenderTex = new RenderTexture(normalMap.width, normalMap.height, 24);
2.             normalMapRenderTex.enableRandomWrite = true;
3.             normalMapRenderTex.Create();
4.             normalMapConverter.SetTexture(kernel, "Input", normalMap);
5.             normalMapConverter.SetTexture(kernel, "Result", normalMapRenderTex);
6.             normalMapConverter.Dispatch(kernel, normalMap.width, normalMap.height, 1);

 

in short, this works if you pass it a RenderTexture for input instead of a Texture2D. No idea why that is required.