Hey so I recently decided to start playing around with DirectX 11. I have some previous experience with fixed-function OpenGL. Right now I'm having some trouble getting DirectX to interpolate my model's (a low poly sphere) normals. In a fixed function pipeline i remember you just called ShadeMode(smooth) and it would automatically interpolate your normals. I read around and saw that DirectX is able to automatically interpolate your normals between the vertex and pixel shaders using interpolation modifiers. However despite hours of messing around and searching i have yet to find a solution.

My set up is pretty simple:

A low poly sphere lit with a basic directional light with only an ambient and diffuse component. I have set up some buffers to include specular light but i don't want to set that up yet without figuring out how to interpolate the model's normals.

Here is my pixel shader

Texture2D shaderTexture;
SamplerState sampleType;

cbuffer LightBuffer
{
float4 ambientColor;
float4 diffuseColor;
float4 specularColor;
float specularPower;
float3 lightDirection;
};

struct PixelInput
{
float4 position: SV_POSITION;
float2 tex: TEXCOORD0;
float3 viewDirection: TEXCOORD1;
float3 normal: NORMAL;
};

float4 LightPixelShader(PixelInput input):SV_TARGET
{
float4 finalColor;
float4 textureColor;
float3 lightDir;
float lightIntensity;


textureColor = shaderTexture.Sample(sampleType, input.tex);

textureColor = textureColor * textureColor;

finalColor = ambientColor;

lightDir = -lightDirection;
lightIntensity = saturate(dot(input.normal, lightDir));

if(lightIntensity > 0.0f)
{
finalColor += (diffuseColor * lightIntensity);
}

return finalColor;
}


Thanks for the quick replies, PIX revealed the problem almost instantly. The issue appears when parsing a .obj model to the engine's model format. Every vertex of a given face is assigned the FACE normal and not it's own normal. So if every vertex of a face has the same normal...........that means interpolating the normals has no effect. This would also explain some very weird bug I'm having creating a simple silhouette edge.