Fragment shader and Vertex Shader in Metal

Question

Fragment Shader

vertex VertexOutBezier bezier_vertex(constant BezierParameters *allParams[[buffer(0)]],
                                    //  constant GlobalParameters& globalParams[[buffer(1)]],
                                     uint vertexId [[vertex_id]],
                                     uint instanceId [[instance_id]])
{

    float t = (float) vertexId / 300;
    rint(t);
    BezierParameters params = allParams[instanceId];

    float lineWidth = (1 - (((float) (vertexId % 2)) * 2.0)) * params.lineThickness;

    float2 a = params.a;
    float2 b = params.b;
    float nt = 1.0f - t;
    float nt_2 = nt * nt;
    float nt_3 = nt_2 * nt;
    float t_2 = t * t;
    float t_3 = t_2 * t;

    float2 point = a * nt_3 + params.p1 * nt_2 * t + params.p2 * nt * t_2 + b * t_3;

    float2 tangent = -3.0 * a * nt_2 + params.p1 * (1.0 - 4.0 * t + 3.0 * t_2) + params.p2 * (2.0 * t - 3.0 * t_2) + 3 * b * t_2;

    tangent = normalize(float2(-tangent.y, tangent.x));
    VertexOutBezier vo;   
    vo.pos.xy = point   + (tangent * (lineWidth / 3.0f));
    vo.pos.zw = float2(0, 1);
    vo.color = params.color ;
    return vo;
}

My Fragment shader is

fragment float4 bezier_fragment(VertexOutBezier params[[stage_in]],
                                texture2d<float> texture [[texture(0)]]
                                )
{
    constexpr sampler defaultSampler;
   float4 canvasColor = texture.sample(defaultSampler, params.pos.xy);
    return canvasColor;   
}

Here i expect to get the pixel color of the texture. But here it is only getting single color. It is not getting the color of the texture according to its position.

Even when I do this in fragment I am getting the single color it is not varying with coordinates

fragment float4 bezier_fragment(VertexOutBezier params[[stage_in]],
                                texture2d<float> texture [[texture(0)]]
                                )
{
    constexpr sampler defaultSampler;
   float4 canvasColor = params.color * params.pos.x;
    return canvasColor;
}

If I do this in Vertex Shader I got color varying according position of x

vo.pos.xy = point   + (tangent * (lineWidth / 3.0f));
vo.pos.zw = float2(0, 1);
vo.color = params.color * vo.pos.x;

What is the Issue in fragment Shader. I cannot get the coordinates from Vertex Shader

Answer 1

Please make sure the VertexOutBezier.pos.xy value is normalization ( 0 ~ 1.0) ,due to the defaultSampler only receive normalization position value, if always return a single may be the position is beyond the bounds.