anchor.diff:              //return V4d(moonNorm * alpha, 1.0)
               //return V4d(moonNorm, 1.0)
               //return V4d(shade, shade, shade, 1.0)
  -            return V4d(moonNorm * tex.XYZ, 1.0)
  +            return V4d(moonColor * tex.XYZ, 1.0)
           }
   
       type Vertex = {
  
positive.code:         member x.PlanetColor : V3d = x?PlanetColor
  
      let borderPixelSize = 64.0
      let sunCoronaExponent = 256.0
  
      let sunSpriteGs (v : Point<VertexSky>) =
          
  
negative.code:             let shade = max 0.0 (Vec.dot moonSurfaceNormal uniform.RealSunDirection)
              let shade = 0.0001 + shade * 0.9999 // 0.01% reflection from earth https://en.wikipedia.org/wiki/Planetshine 
              
              let moonColor = uniform.MoonColor * shade / texNorm
  
              // when rendering to half-precision: max value is 65,504 
              //  -> scale all luminance values by /1000
              //  -> as additive blending is used, clamp color to 30,000 for half-precision output support
              //let colMax = max moonColor.X (max moonColor.Y moonColor.Z)
              //let moonNorm = moonColor * 30000.0 / max 30000.0 colMax
  
              //if alpha = 1.0 then
              //    return V4d(30000.0, 0.0, 0.0, 1.0)
  