Using the surf function to lerp a value by DeltaTime

Hi All,
I want to lerp towards a value, using unity_DeltaTime. I put the following line in the surf function of my shader.

Code (CSharp):

1. theValue= lerp(theValue, targetValue, unity_DeltaTime );

However, the result seems to be as if surf gets called once only. Is that right?
I cannot find any documentation on the surf function.

 

In case it helps, this is is the full Custom shader I expected would lerp the value.

Code (Boo):

1. Shader "Custom/TempTestShader" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,1)
4.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
5.         _Glossiness ("Smoothness", Range(0,1)) = 0.5
6.         _Metallic ("Metallic", Range(0,1)) = 0.0
7.  
8.         _LerpTest ("Test Lerp Target", Range(0,1)) = 1
9.     }
10.     SubShader {
11.         Tags { "RenderType"="Opaque" }
12.         LOD 200
13.  
14.         CGPROGRAM
15.         // Physically based Standard lighting model, and enable shadows on all light types
16.         #pragma surface surf Standard fullforwardshadows
17.  
18.         // Use shader model 3.0 target, to get nicer looking lighting
19.         #pragma target 3.0
20.  
21.         sampler2D _MainTex;
22.  
23.         struct Input {
24.             float2 uv_MainTex;
25.         };
26.  
27.         half _Glossiness;
28.         half _Metallic;
29.         fixed4 _Color;
30.  
31.         half _LerpTest;
32.         half _LerpTest_Current = 0;
33.  
34.         // Add instancing support for this shader. You need to check 'Enable Instancing' on materials that use the shader.
35.         // See https://docs.unity3d.com/Manual/GPUInstancing.html for more information about instancing.
36.         // #pragma instancing_options assumeuniformscaling
37.         UNITY_INSTANCING_BUFFER_START(Props)
38.             // put more per-instance properties here
39.         UNITY_INSTANCING_BUFFER_END(Props)
40.  
41.         void surf (Input IN, inout SurfaceOutputStandard o) {
42.             // Albedo comes from a texture tinted by color
43.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
44.             o.Albedo = c.rgb;
45.             // Metallic and smoothness come from slider variables
46.             o.Metallic = _Metallic;
47.             o.Smoothness = _Glossiness;
48.  
49.             _LerpTest_Current = lerp (_LerpTest_Current, _LerpTest, unity_DeltaTime.x);
50.             o.Alpha = _LerpTest_Current;
51.         }
52.         ENDCG
53.     }
54.     FallBack "Diffuse"
55. }
56.  

I also tried adding my own Global Property (using a uniform float) and setting it via C# script using

 Shader.SetGlobalFloat ( "Unity_Delta_Time", Time.deltaTime );

 

You cannot accumulate values in a shader across frames. All of those variables are temporary per-frame, and per-vertex / per-pixel and are thrown out as soon as each fragment shader draws to screen.

 

Thank-you, that would explain it!

So is the only way to achieve the effect is to Lerp in C#, and set a shader Property every frame?

When I tested that I found my shader was very slow with Device.present when on a mobile platform.

Is setting Shader properties every frame generally a done thing? Or is too costly for mobile platforms?

 

It is a way, and it works perfectly fine. The alternative is to pass a start time to the shader. The _Time.y value is equivalent to Time.timeSinceLevelLoad, so you can pass a begin time along with the duration, start, and end values. Then you only have to set it the one time when it starts.

o.Alpha = lerp(_StartAlpha, _EndAlpha, saturate((_Time.y - _BeginTime) / _LerpDuration));