osx - Metal normal doesn't interpolate -

i've been learning how metal works using swift , targeting macos. thing's have been going okay, now, close getting stuff done, i've hit problem cannot possibly understand ... hope guys me :)

i'm loading , displaying obj teapot, i'm lighting using ambiant+diffuse+specular light. lighting in works well, problem : normal vector not interpolated when going fragment shader, results in having flat lighting on supposedly curved surface ... not ...

i don't understand why normal not interpolated while other values (position + eye) ... here shader , image show result :

thanks in advance :)

struct vertex {     float4 position;     float4 normal; };  struct projectedvertex {     float4 position [[position]];     float3 eye;     float3 normal; };  vertex projectedvertex vertex_project(device vertex *vertices [[buffer(0)]],                                       constant uniforms &uniforms [[buffer(1)]],                                       uint vid [[vertex_id]]) {     projectedvertex outvert;     outvert.position = uniforms.modelviewprojectionmatrix * vertices[vid].position;     outvert.eye =  -(uniforms.modelviewprojectionmatrix * vertices[vid].position).xyz;     outvert.normal = (uniforms.modelviewprojectionmatrix * float4(vertices[vid].normal)).xyz;      return outvert; }  fragment float4 fragment_light(projectedvertex vert [[stage_in]],                                constant uniforms &uniforms [[buffer(0)]]) {     float3 ambientterm = light.ambientcolor * material.ambientcolor;      float3 normal = normalize(vert.normal);     float diffuseintensity = saturate(dot(normal, light.direction));     float3 diffuseterm = light.diffusecolor * material.diffusecolor * diffuseintensity;      float3 specularterm(0);     if (diffuseintensity > 0)     {         float3 eyedirection = normalize(vert.eye);         float3 halfway = normalize(light.direction + eyedirection);         float specularfactor = pow(saturate(dot(normal, halfway)), material.specularpower);         specularterm = light.specularcolor * material.specularcolor * specularfactor;     }      return float4(ambientterm + diffuseterm + specularterm, 1); } 

screenshot

so problem using obj-c, when indexed vertices obj file, generated 1 vertex shared vertices between surfaces, kept 1 normal.

when translating swift, hash value used check if vertex @ same place 1 have wrong , couldn't detect shared vertices, resulted in keeping of normals, each surface flat.

i don't know if i'm clear enough that's happened, future reference, question making swift version of "metalbyexample" book obj-c only.