problem moving vertexes in shader

Have torus model indicating a landing zone on a tile in my game. Animates upward to indicate the destination of the player.

Managed to make a shader that moves the vertices of my model from zero in the y to a limit (making if lift off the ground) then it's suppose to reset to 0 on the y and repeat the process. The raising up part works fine but after it resets the vertex.y to 0 it just seems to stop instead of adding the _scrollpos value to the vertex.y as it did initially. I have stared at this logic and for the life of me think it should work but I guess I'm missing something here.

_ScrollY is range for the Limit to scroll vertexes

_ScrollS is the amount to move the vertex each pass

_scrollpos stores the value to be applied to vertex.y

I've also tried to omit _scrollpos and used vertex.y += t and same result. Help!?

void vert(inout appdata v, out Input o) {

UNITY_INITIALIZE_OUTPUT(Input, o);

if (_ScrollY != 0 && _ScrollS != 0) {

float t = _Time * _ScrollS;

_scrollpos += t;

if (_ScrollY > 0 && _scrollpos > _ScrollY) {

_scrollpos = 0;

}

else if (_ScrollY < 0 && _scrollpos < _ScrollY) {

_scrollpos = 0;

}

}

v.vertex.y = _scrollpos;

}

void surf(Input IN, inout SurfaceOutput o) {

o.Albedo = tex2D(_MainTex, IN.uv_MainTex);

}

 

Shaders don't work like that. Outside of the local scope of each invocation of a shader (each vertex or pixel each frame) no data is retained apart from what's passed to the next shader stage (which is thrown out after that) or the final output color (which is also thrown out after being displayed on screen).

To put it another way, the vertex data and shader properties that the vertex shader gets is a copy of that data, not a reference, and is thrown out as soon as the function returns. You can't write back to the original mesh, nor accumulate values in shader properties over multiple runs of the shader.

Well, technically you can with creative usage of compute buffers and DX11, but then each vertex is accumulating to the same variable, so the rate then depends on the number of visible meshes using the shader, and their vertex counts, as to how fast the value accumulates.

Also _Time is a float4, not a single float. It's also not the time delta, but total time since the last level load, divided by 20, unmodified, multiplied by two, and multiplied by three.
https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html

Instead you should be using a modulo (%) or frac(), like this:

v.vertex.y = frac(_Time.y * _ScrollS) * _ScrollY;

If you want it to always start at zero when it starts to animate you'll need to pass in the current time to the shader as an offset, like this:
// c#
myMaterial.SetFloat("_ScrollStartTime", Time.timeSinceLevelLoad);

// shader
v.vertex.y = frac((_Time.y - _ScrollStartTime) * _ScrollS) * _ScrollY;

 

Thanks @bgolus, this helped me fix a glitch in my shader when using frac() to scroll textures.