makeFrame (cam :: Mat 4 4 Float)
          (skybox :: Texture)
          (skypoints :: PrimitiveStream Point ((Vec 3 Float)))

    = imageFrame (emptyDepthImage 1, emptyColorImage (V4 0 0 0.4 1))
      `overlay`
          skypoints
        & mapPrimitives (\((p)) -> ( cam *. V4 p%x p%y p%z 1, V3 p%x (-p%y) (-p%z) ) )
        & rasterizePrimitives (PointCtx (PointSize 1.0)
                                        1.0        -- GL_POINT_FADE_THRESHOLD
                                        UpperLeft) -- texture y increases downward
                              ((Smooth))
        & mapFragments (\((d)) -> ((texture2D (Sampler PointFilter MirroredRepeat skybox) d%xy )))
        & accumulateWith (DepthOp Always True, ColorOp NoBlending (V4 True True True True))


textureCubeSlot s = Texture2DSlot s

main = renderFrame $
   makeFrame (Uniform "Cam")
             (textureCubeSlot "Skybox")
             (fetch "skypoints" ((Attribute "position")))