unity_ObjectToWorld does not contain translation?

Me again,

now this might only be me being blind again, but i have the suspicion that unity_ObjectToWorld does not contain any translation information.

so here is a minimal shader to test that: (also great that the forum does not have a codetag for shaders)

Code (csharp):

1. Shader "Custom/TranslationTest"
2. {
3.     Properties
4.     {
5.     }
6.     SubShader
7.     {
8.         Tags { "RenderType"="Opaque" }
9.         LOD 200
10.  
11.         CGPROGRAM
12.         #pragma surface surf Standard fullforwardshadows
13.  
14.         struct Input
15.         {
16.             float3 worldPos;
17.         };
18.  
19.         void surf (Input IN, inout SurfaceOutputStandard o)
20.         {
21.             o.Albedo = mul(unity_ObjectToWorld, float3(0.1, 0.1, 0.1));
22.         }
23.         ENDCG
24.     }
25.     FallBack "Diffuse"
26. }
27.  

so you see all i do is transform a small vector with the worldmatrix. i would expect the translation to also come to play here, but nope, results look like this


the two colored ones have scale 2 on x and y, so that channel gets scaled, rotation also gets applied (color changes with rotation when scale is applied). but with the front cube u see that the x part of the vector does not get translated (or any other axis for that matter). now what am i doing wrong? where can i get the translation info? im running 2018.3.0f2

 

That's because unity_ObjectToWorld is a 4x4 matrix, and you're multiplying a float3 which ignores the position. You're only seeing scale and rotation have any effect because that's all you're extracting from it.

Try:
o.Albedo = mul(unity_ObjectToWorld, float4(0.1, 0.1, 0.1, 1.0)).xyz;

Note you may still not see any color change if you view this in the game view. That's because of batching.

 

thanks it worked, i knew it had to be a small detail.

 

could you help me to explain, why multiplying a float 3 is ignores the position? Thank you

 

Because in a 4x4 3D transform matrix the first 3x3 section holds the rotation and scale. The last column of the 4x4 holds the translation. If you multiply a float4x4 by a float3 it only applies the 3x3 (or rather 3x4 and multiplies the last column by zero). If you want to apply the full transform you want to use a float4 with a w of 1.0.

 

i got it, thank you very much