What is the difference between [[textureCoordinates]] and [[position]] in Metal Shading Language

Question

I am processing videos with metal and swift. I have a vertex shader that scales the frames to aspect fit inside my view. In my fragment shader it works well if I sample with textureCoordinates like so: inputTexture.sample(sampler, inputFragment.textureCoordinates);

But if I use read with position instead, it returns the unscaled frame: inputTexture.read(uint2(inputFragment.position.xy));

I am assuming that this is usual behavior, I just don't know why so I was wondering if someone could explain the difference between the two. Thanks!

score 1 · Answer 1 · answered Jan 16 '22 at 16:11

textureCoordinates - Normalized texture coordinate.

For 2D textures, normalized texture coordinates are values from 0.0 to 1.0 in both x and y directions. A value of (0.0, 0.0) specifies the texel at the first byte of the texture data (the top-left corner of the image). A value of (1.0, 1.0) specifies the texel at the last byte of the texture data (the bottom-right corner of the image).

position.xy - The clip space position of the vertex.

struct RasterizerData
{
    // The [[position]] attribute qualifier of this member indicates this value is
    // the clip space position of the vertex when this structure is returned from
    // the vertex shader
    float4 position [[position]];

    // Since this member does not have a special attribute qualifier, the rasterizer
    // will interpolate its value with values of other vertices making up the triangle
    // and pass that interpolated value to the fragment shader for each fragment in
    // that triangle.
    float2 textureCoordinate;

};

What is the difference between [[textureCoordinates]] and [[position]] in Metal Shading Language

1 Answers1