Segment SDF

Based on work by Inigo Quilez: https://iquilezles.org/articles/distfunctions2d/

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The sdf function is imported, but we then need to set up a proxy function. This is because the macro system relies on inlining, and we want to create a function definition, not just inline the body at the point of use.

This shader also uses an imported 'fill' function to calculate a nice colour gradient.

Note that we can't just render the line segment, because at 0.0f the SDF is 0.0f, so we need the annular, i.e. abs(sdf) - thickness-of-border.

  inline def fragment =
    Shader[FragmentEnv] { env =>
      import ultraviolet.sdf.*
      import FillColorHelper.*

      def proxy: (vec2, vec2, vec2) => Float =
        (p, a, b) => segment(p, a, b)

      def calculateColour: (vec2, Float) => vec4 = (uv, sdf) => fill(uv, sdf)

      def fragment(color: vec4): vec4 =
        val segSDF = proxy(env.UV, vec2(0.2f), vec2(0.8f))
        calculateColour(env.UV, abs(segSDF) - 0.1f)
    }