CG: Specify a variable not to be interpolated between vertex and fragment shader

Question

I'm using GC for writing shaders inside Unity3D.

I'm using vertex colors attributes for passing some parameters to the shader. They won't be used so for defining colors, and should be forwarded from vertex shader to pixel shader without modifyng them.

This is the structure I'm taking as input from Unity3D to the vertex shader:

struct appdata_full {
    float4 vertex : POSITION;
    float4 tangent : TANGENT;
    float3 normal : NORMAL;
    float4 texcoord : TEXCOORD0;
    float4 texcoord1 : TEXCOORD1;
    fixed4 color : COLOR;
#if defined(SHADER_API_XBOX360)
    half4 texcoord2 : TEXCOORD2;
    half4 texcoord3 : TEXCOORD3;
    half4 texcoord4 : TEXCOORD4;
    half4 texcoord5 : TEXCOORD5;
#endif
};

This is the structure returned by vertex shader as input to the fragment:

struct v2f {
  float4 pos : SV_POSITION;
  float2  uv : TEXCOORD0;
  fixed4 col: COLOR;           
};

If I simply forward the parameter to the fragment shader, of course it will be interpolated:

v2f vert (appdata_full v)
{

  v2f output;
  //....
  output.col = v.color;
}

I'd like to pass v.color parameter not interpolated to the fragment shader. Is this possible?if yes how?

EDIT

like Tim pointed out, this is the expected behavior, because of the shader can't do anything else than interpolating colors if those are passed out from vertex shader to fragment. I'll try to explain better what I'm trying to achieve. I'm using per vertex colors to store other kind of information than colors. Without telling all details on what I'm doing with that, let's say you can consider each color vertex as an id(each vertex of the same triangle, will have the same color. Actually each vertex of the same mesh).

So I used the color trick to mask some parameters because I have no other way to do this. Now this piece of information must be available at the fragment shader in some way. If a pass as an out parameter of the vertex shader, this information encoded into a color will arrive interpolated at the fragment, that can't no longer use it.

I'm looking for a way of propagating this information unchanged till the fragment shader (maybe is possible to use a global variable or something like that?if yes how?).

How are you going to reconcile three inputs (vertices) per pixel if you don't interpolate the values? What do you expect to happen if each vertex of the triangle has a different value? — Tim, Dec 16 '12 at 21:59
@Tim: ok..you are right. I understand your objection. The point is that actually colors are not colors in my shader. I'm using color values to store other kind of information. I need to forward this per vertex information from vertex to fragment shader, eventually not passing it as out parameter of vertex shader, but using some "gloabal variable" if something like that is possible. I hope to be clear. — Heisenbug, Dec 16 '12 at 22:08
It doesn't really matter if it's a color or an index or anything else. Lets say you're drawing a single triangle. Based on some arbitrary per-vertex computation; vertex 0 outputs "5", vertex 1 outputs "10", and vertex 2 outputs "15". Which of these three numbers do you want to show up in your fragment shader? If it's really a global value than use a uniform, but otherwise I think you're not fully thinking this through. Unless you just say "I always want vertex 0's value and ignore the other two", this doesn't make much sense. — Tim, Dec 16 '12 at 22:21
Unless you just say "I always want vertex 0's value and ignore the other two", this doesn't make much sense". This is more ore less what I need. Accordingly to cg doc: "The uniform qualifier indicates the source of a variable's initial value. When a Cg program declares a variable as uniform , it conveys that the variable's initial value comes from an environment that is external to the specified Cg program.". Actually my value doesn't come from external program. It comes from external, but encoded in per vertex color. — Heisenbug, Dec 16 '12 at 22:28

score 10 · Accepted Answer · answered Dec 18 '12 at 22:07

I'm not sure this counts for an answer but it's a little much for a comment. As Bjorke points out, the fragment shader will always receive an interpolated value. If/when Unity supports Opengl 4.0 you might have access to Interpolation qualifiers, namely 'flat' that disables interpolation, deriving all values from a provoking vertex.

That said, the problem with trying to assign the same "color" value to all vertices of a triangle is that the vertex shader iterates over the vertices once, not per triangle. There will always be a "boundary" region where some vertex shares multiple edges with other vertices of a different "color" or "id", see my dumb example below. When applied to a box at (0,0,0), the top will be red, the bottom green, and the middle blue.

Shader "custom/colorbyheight" {
Properties {
 _Unique_ID ("Unique Identifier", float) = 1.0
}
SubShader {
Pass {
  CGPROGRAM
  #pragma vertex vert
  #pragma fragment frag
  #include "UnityCG.cginc"
  struct v2f {
      float4 pos : SV_POSITION;
      fixed4 color : COLOR;
  };
  uniform float _Unique_ID;
  v2f vert (appdata_base v)
  {
      v2f o;
      o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
      float3 worldpos = mul(_Object2World, v.vertex).xyz;
      if(worldpos[1] >= 0.0)
        o.color.xyz = 0.35;  //unique_id = 0.35
      else
        o.color.xyz = 0.1;   //unique_id = 0.1
      o.color.w = 1.0;
      return o;
  }


  fixed4 frag (v2f i) : COLOR0 { 
    // local unique_id's set by the vertex shader and stored in the color
    if(i.color.x >= 0.349 && i.color.x <=0.351)
        return float4(1.0,0.0,0.0,1.0); //red
    else if(i.color.x >= 0.099 && i.color.x <=0.11)
        return float4(0.0,1.0,0.0,1.0); //green

    // global unique_id set by a Unity script
    if(_Unique_ID == 42.0)
        return float4(1.0,1.0,1.0,1.0); //white

    // Fallback color = blue
    return float4(0.0,0.0,1.0,1.0);
  }
  ENDCG
}
} 
}

In your addendum note you say "Actually each vertex of the same mesh." If that's the case, why not use a modifiable property, like I have included above. Each mesh just needs a script then to change the unique_id.

public class ModifyShader : MonoBehaviour {
public float unique_id = 1;
// Use this for initialization
void Start () {

}

// Update is called once per frame
void Update () {
    renderer.material.SetFloat( "_Unique_ID", unique_id );
}
}

well, thanks for your answer too. I can't use SetFloat because I'm batching draw call through StaticBatchingUtility. If I modify a value of a Material, Unity will instantiate a new Material instance for each object, preventing it from using a sharedMaterial and consequently it won't be batched anymore. So I'm forced to use color parameters assigned per vertex to assing custom shader value to each mesh. — Heisenbug, Dec 18 '12 at 22:15
Actually I'm using per color vertex as id and texture indirection indices. Unfortunately these values should be used inside the fragment shader. Using 3 color global variable shared between fragment and vertex programs (or some out parameters specified not to be interpolated)could help me in realize a more efficient implementation of some indirections. — Heisenbug, Dec 18 '12 at 22:19
Bugger, yep that wouldn't work for you. One "solution", and I dislike this because it's hacky and slow, is to make all triangles of your mesh unique by only allowing each vertex to form 2 edges. In this way the linear interpolation happens only between verts of a single triangle giving you back the correct "id". — Jerdak, Dec 18 '12 at 22:22
Yes, I'm using unique vertices. It's not so slowly because my mesh are very simple (the whole rendering effor in upon layered textures into the fragment). Anyway, If I use the same color for each vertex I can forward only 32 bit of information to the fragment. Specify some output variable not to be interpolated could led me to more information transfer between vertex and fragment. Btw it seems not to be possible. — Heisenbug, Dec 18 '12 at 22:27
How much info do you need because V2f supports something like 10 channels x 32 bits. — Jerdak, Dec 18 '12 at 22:31
mmm..well. 10 channels could be sufficient. maybe I didn't realize that since now. thanks for the information. — Heisenbug, Dec 18 '12 at 22:35

Rupti · Answer 2 · 2018-11-26T14:59:32.930

5

I know this is an old thread, but it's worth answering anyway since this is one of the top google results. You can now use the nointerpolation option for your variables in regular CG shaders. i.e.

nointerpolation fixed3 diff : COLOR0;

edited Nov 26 '18 at 14:59

answered Nov 22 '18 at 12:37

Rupti

125
1
9

mle · Answer 3 · 2014-02-14T11:25:21.153

This is a pretty old thread, but I recently had a similar issue and I found a super simple answer. OSX Mavericks now supports OpenGL 4.1 so soon it won't be an issue at all, but it still may take a while before Unity3d picks it up. Anyway, there is a neat way to enable flat shading in Unity even on earlier OSX (e.g. Mountain Lion) !

The shader below will do the job (the crucial part is the line with #extension, otherwise you'd get a compilation error for using a keyword flat"

Shader "GLSL flat shader" {
SubShader {
  Pass {
     GLSLPROGRAM

     #extension GL_EXT_gpu_shader4 : require
     flat varying vec4 color;

     #ifdef VERTEX
     void main()
     {
        color = gl_Color;
        gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
     }
     #endif

     #ifdef FRAGMENT
     void main()
     {
        gl_FragColor = color; // set the output fragment color
     }
     #endif

     ENDGLSL
  }
  }
}

Got to it by combining things from: http://poniesandlight.co.uk/notes/flat_shading_on_osx/ http://en.wikibooks.org/wiki/GLSL_Programming/Unity/Debugging_of_Shaders

score 2 · Answer 4 · answered Dec 18 '12 at 06:19

2

The GPU will always interpolated between values. If you want a constant value for a triangle, you need to set the same value for all vertices of that triangle. This can at times be inefficient, but it's how OpenGL (and DirectX) works. There is no inherent notion of a "face" value.

answered Dec 18 '12 at 06:19

bjorke

3,295
1
16
20

can't I use some global variable shared between a vertex shader instance and all the related fragment shaders? – Heisenbug Dec 18 '12 at 09:05
that's not how the hardware's laid out. the constants are per-mesh, not per-triangle, and each mesh gets only one vertex and one fragment shader. The "connector" registers, which are interpolated, are the only channel of communication between the two. – bjorke Dec 20 '12 at 04:04

score 1 · Answer 5 · answered Dec 20 '12 at 19:47

You might do this: glShadeModel(GL_FLAT). This turns off interpolation for all fragment shader inputs, and is available in older OpenGL also (pre 4.0).

If you have some inputs you want to interpolate and some you don't, render once with GL_FLAT to a texture of the same resolution as your output, and then render again with GL_SMOOTH and sample the texture to read the flat values for each pixel (while also getting interpolated values in the usual way).

If you could use DirectX9 instead, you can use the nointerpolation modifier on individual fragment shader inputs (shader model 4 or later).

glShadeModel is not exposed through the Unity API. – Jerdak Dec 21 '12 at 16:31 — Jerdak, Dec 21 '12 at 16:31

score 1 · Answer 6 · edited Jul 17 '19 at 21:45

The following steps works for me.

Unfortunately, DX uses vertex 0 as the provoking vertex while GL by default uses 2.

You can change this in GL but glProvokingVertex does not seem to be exposed.

We are doing flat shading and this reduces our vertex count significantly.

We have to reorder the triangles and compute normals in a special way (if anyone is interested I can post example source).

The problem is that we have to have different meshes on GL vs DX as the triangle indexes need to be rotated in order for the triangles to use the appropriate provoking vertex.

Maybe there is some way to execute a GL command via a plugin.

CG: Specify a variable not to be interpolated between vertex and fragment shader

6 Answers6