Animated UVs work in frag but not in vert...

I use the following shader code to animate uvs :

void vert (inout appdata_full v, out Input o) {
o.uv_BumpMap = v.texcoord;
}
void surf (Input IN, inout EditorSurfaceOutput o) {
fixed tm = _Time.y * 30;
fixed ty = floor(tm / 8);
fixed2 uv = IN.uv_BumpMap + fixed2(floor(tm - ty * 8), ty) * 0.125;
o.Normal = UnpackNormal( tex2D(_BumpMap,uv));
}

and this works nicely, so I wanted to move the coordinate math part into the vert function like so:

void vert (inout appdata_full v, out Input o) {
fixed tm = _Time.y * 30;
fixed ty = floor(tm / 8);
fixed tx = floor(tm - ty * 8);
o.uv_BumpMap = v.texcoord + fixed2(tx * 0.125, ty * 0.125);
}
void surf (Input IN, inout EditorSurfaceOutput o) {
o.Normal = UnpackNormal( tex2D(_BumpMap,IN.uv_BumpMap));
}

but this doesn't work and I really can't see why not. Does _Time work differently in the vert program as opposed to frag?
Can anyone help?

 

I'm guessing that uv_BumpMap is being overwritten by Unity's own UV coordinate handling. You might want to rename the variable?

 

@Farfarer Thanks for answering.
I had tried changing the variable name before but it had the result of eliminating the bump effect though no errors were thrown.
For now, I got it to work by passing both the unaltered uv_BumpMap, and an offset variable from the vert program, then adding them together in frag. Not a pretty solution but it works.
I'd still like to understand why the first solution didn't work though.

 

When you ask Unity for the uv_BumpMap variable, it takes the default UV coordintes of your mesh and then scales/offsets them to account for the tiling/offset values in the texture. It will do this even if they are left at the default values (1 tile, 0 offset in both U and V).

But you're trying to write to that same variable, which is probably getting overwritten by Unity itself.

 

Got it! You're absolutely right. uv_BumpMap gets set automatically post vert program as part of the surface shader "magic" so my changes were indeed being overwritten.
Many thanks again for answering.