makeFrame (projmat :: Mat 4 4 Float)
          (prims :: PrimitiveStream Triangle (Vec 4 Float))

    = imageFrame (emptyDepthImage 1000, emptyColorImage navy)
  `overlay`
      prims
    & mapPrimitives (\x -> (scale 0.5 $ projmat *. x, x))
    & rasterizePrimitives (TriangleCtx CullNone PolygonFill NoOffset LastVertex) Smooth
    & filterFragments (\_ -> True)
    & mapFragments (\x -> x)
    & accumulateWith ({-defaultFragOp @(Depth Float) @_ -}DepthOp Less True, ColorOp NoBlending (V4 True True True True))

main = renderFrame $
   makeFrame (Uniform "MVP")
             (fetch_ "stream4" (Attribute "position4"))