Question RenderTexture Mip Map Bias not working in WebGL

Hi there!

Setting RenderTexture.mipMapBias does not work at all in a WebGL build.

In my scene I am trying to make a presentation screen look a bit sharper from a distance by setting the mipMapBias to -0.5. I see the desired result for this in the Unity editor, but there is no difference in a WebGL build. To verify, I set the mipMapBias to 2.0, which makes the texture look very blurry in the Unity editor, but in a WebGL build the texture is not affected at all.

How can I enable support for mip map bias in a WebGL build? Do I need to use a custom shader of some kind? If so, where could I find an example of such a shader?

 

Figured it out, with a little help from a friend of mine.

Rather than using

RenderTexture.mipMapBias

I had to make a custom shader that samples the texture with mipmap bias via

tex2Dbias

. This should also work for other OpenGL ES based platforms such as Android (which could be useful particularly for standalone Android VR devices like the Meta Quest).

Here's my shader. It's an Unlit shader, but the mipmap bias parts should be able to be incorporated into other types of shaders that need it:

Code (csharp):

1. Shader "Unlit/UnlitMipMapBias"
2. {
3.     Properties
4.     {
5.         _MainTex ("Texture", 2D) = "white" {}
6.         // Mipmap bias Unity ref: https://docs.unity3d.com/ScriptReference/Texture-mipMapBias.html
7.         // ^Says we need to use a custom shader for WebGL (OpenGL ES), so here we are.
8.         _MainTexBias("Mipmap Bias", Range(-1.0,1.0)) = -0.5
9.     }
10.     SubShader
11.     {
12.         Tags { "RenderType"="Opaque" }
13.         LOD 100
14.  
15.         Pass
16.         {
17.             CGPROGRAM
18.             #pragma vertex vert
19.             #pragma fragment frag
20.             // Commenting out fog because it isn't needed for this shader.
21.             // #pragma multi_compile_fog
22.  
23.             #include "UnityCG.cginc"
24.  
25.             struct appdata
26.             {
27.                 float4 vertex : POSITION;
28.                 float2 uv : TEXCOORD0;
29.             };
30.  
31.             struct v2f
32.             {
33.                 float2 uv : TEXCOORD0;
34.                 UNITY_FOG_COORDS(1)
35.                 float4 vertex : SV_POSITION;
36.             };
37.  
38.             sampler2D _MainTex;
39.             float4 _MainTex_ST;
40.             half _MainTexBias;
41.  
42.             v2f vert (appdata v)
43.             {
44.                 v2f o;
45.                 o.vertex = UnityObjectToClipPos(v.vertex);
46.                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
47.                 UNITY_TRANSFER_FOG(o,o.vertex);
48.                 return o;
49.             }
50.  
51.             fixed4 frag (v2f i) : SV_Target
52.             {
53.                 // Sample the texture with mipmap bias via tex2Dbias. Ref: https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-tex2dbias
54.                 fixed4 col = tex2Dbias(_MainTex, half4(i.uv.x, i.uv.y, 0.0, _MainTexBias));
55.                 // Commenting out fog because it isn't needed for this shader.
56.                 // UNITY_APPLY_FOG(i.fogCoord, col);
57.                 return col;
58.             }
59.             ENDCG
60.         }
61.     }
62. }