How is vertex data used in pixel Shader

Question

A triangle has 3 vertices. A pixel shader receives data from the vertex shader. If each vertex has a different color, the transition is calculated between the colors.

This calculation of such a transition, does it do this for all the data the pixel shader receives from the vertex shader?

For example, take a textured triangle, and a pointlight. The distance from the pointlight to each vertex is calculated in vertex shader. This is passed to the pixel shader. In the pixelshader, it takes a color from the texture and does some light calculations. It does this 3 times for a pixel, and then takes the average color?

Thank you,

Answer 1

Common per-pixel lighting effects are done by passing a world-position to the pixel shader. Here's an example pipeline:

struct VSInput
{
    float4 Position : SV_Position;
    float3 Normal   : NORMAL;
    float2 TexCoord : TEXCOORD0;
};

struct VSOutput
{
    float2 TexCoord   : TEXCOORD0;
    float4 PositionWS : TEXCOORD1;
    float3 NormalWS   : TEXCOORD2;
    float4 Diffuse    : COLOR0;
    float4 PositionPS : SV_Position;
};

// Vertex shader
VSOutput VertexShader(VSInput vin)
{
    VSOutput vout;

    vout.TexCoord = vin.TexCoord;
    vout.PositionPS = mul(position, WorldViewProj);
    vout.PositionWS = mul(position, World).xyz;
    vout.NormalWS = normalize(mul(normal, WorldInverseTranspose));
    vout.Diffuse = float4(1, 1, 1, DiffuseColor.a);

    return vout;
}

struct PSInput
{
    float2 TexCoord   : TEXCOORD0;
    float4 PositionWS : TEXCOORD1;
    float3 NormalWS   : TEXCOORD2;
    float4 Diffuse    : COLOR0;
};

float4 PixelShader(PSInput pin) : SV_Target0
{
    float4 color = Texture.Sample(Sampler, pin.TexCoord) * pin.Diffuse;

    float3 eyeVector = normalize(EyePosition - pin.PositionWS.xyz);
    float3 worldNormal = normalize(pin.NormalWS);

    ColorPair lightResult = ComputeLights(eyeVector, worldNormal, 3);

    color.rgb *= lightResult.Diffuse;

    AddSpecular(color, lightResult.Specular);
    ApplyFog(color, pin.PositionWS.w);

    return color;
}

This is cribbed from the DirectX Tool Kit shaders.