Shader: actual view direction/normal?

Question

Hiho.

I only have basic knowledge about shaders, ShaderLab, and CG, and I didn't find my answer in any docs, examples, or by trial&error, so my hope is one of you could help me out.

I am looking for the actual view direction, i.e. the vector from the camera position through the currently rendered pixel, as I am trying to find the angle between that and the normal of the object at that point/along that ray. Or to put it differently, I am looking for the actual orientation of an object/vertex/pixel with respect to an arbitrary screen location, not with respect to the camera plane (see the following example).

I can use this which gives me some view-dependent object space(?) normal:

 float3 viewN = mul ((float3x3)UNITY_MATRIX_IT_MV, v.normal);

but this vector remains constant, no matter where on the screen the object is (i.e., it uses the constant normal of the camera plane (=the equivalent of transform.forward) for all pixels). And there is something called "viewDir" in surface shaders, if I understand it correctly, but I am not sure what it does precisely.

I don't care if I need to use a fragment shader or a surface shader for that, I am happy with any solution that works.

Thanks! Wolfram

Answer 1

Answer by Julien-Lynge · Jan 31, 2012 at 05:22 PM

I believe that you can get the view direction vector within the surface shader function like so:

struct Input { float3 viewDir; //whatever else you need to define for Input }

void surf (Input IN, inout EditorSurfaceOutput o) {
  float3 temp = IN.viewDir;
  //gives the view direction as a float3, so you can access e.g. IN.viewDir.x
}

Note that you may have to use a lighting model that takes into account view direction - that's beyond my expertise.

half4 LightingName (SurfaceOutput s, half3 lightDir, half3 viewDir, half atten);

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equalsequals · Jan 31, 2012 at 07:23 PM 0

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getting the view direction in a surface shader is straight-forward like that, yes. But we, or at least I am talking about vertex-to-fragment programs and NOT surface shaders. Surface Shaders do unnecessary calculations for a lot of effects and is often more effective and more economical to write the v2f functions yourself instead of letting the surface shader generate it for you.

Julien-Lynge · Feb 01, 2012 at 04:57 PM 4

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I certainly understand that you don't want a surface shader, but that's what the initial question asked for, so that's what I gave. I don't appreciate you giving me -1 karma for answering the question as asked - you may want to start your own post if your question is markedly different.

Answer 2

Answer by equalsequals · Jan 31, 2012 at 07:28 PM

I was able to figure it out here:

 struct v2f {
     float4 pos : SV_POSITION;
     float3 normal : TEXCOORD0; //you don't need these semantics except for XBox360
     float3 viewT : TEXCOORD1; //you don't need these semantics except for XBox360
 };

 v2f vert (appdata_base v)
 {
     v2f o;
     o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
     o.normal = normalize(v.normal);
     o.viewT = normalize(WorldSpaceViewDir(v.vertex));//ObjSpaceViewDir is similar, but localspace.
     return o;
 }

Just make sure you have #include "UnityCG.cginc" in there as well.

Cheers

==

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Shader: actual view direction/normal?

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