How to calculate 2D viewport coordinates in a shader?

Question

In Unity, I use the following shader to calculate 2D viewport coordinates :

Shader "Foo"
{
    SubShader
    {
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            struct vertInput {
                float4 pos : POSITION;
            }; 

            struct vertOutput {
                float4 pos : SV_POSITION;
                float4 uv : TEXCOORD0;
            };

            vertOutput vert (vertInput input)
            {
                vertOutput o;
                o.pos = mul(UNITY_MATRIX_MVP, input.pos);
                o.uv = mul(UNITY_MATRIX_MVP, input.pos);
                return o;
            }

            fixed4 frag (vertOutput output) : SV_Target
            {   
                float2 viewport = output.uv.xy / output.uv.z;
                return float4(sin(viewport.x*10.0), sin(viewport.y*10.0), 0.0, 1.0);
            }
            ENDCG
        }
    }
}

At first, it seems to work great. Here is results :

However if I move camera very close to the surface, the following unexpected glitch occurs :

Answer 1

When the distance to the camera is really small, you can end up dealing with very small numbers which create floating point rounding errors in your calculations. If you change

o.uv = mul(UNITY_MATRIX_MVP, input.pos);

to

o.uv = mul(UNITY_MATRIX_MV, input.pos);

then the projection portion of the matrix will no longer be included in your calculations (for the uv component at least), which has the effect of making the distance to the camera irrelevant and removing that warping.