Test scene 2

The source

plane{z,0

  pigment{gradient y
    pigment_map{
      [0 rgb 1]
      [0.47 rgb 1]
      [0.47 rgb 0]
      [0.53 rgb 0]
      [0.53 rgb 1]
      [1 rgb 1]
    }
  }
  finish{ambient 1}
  scale .1
  rotate y*-50
}

camera{
  location <0,0,-5>
  look_at 0
}

Without Anti-Aliasing

Perfect

Test I - limited at about 4 samples per pixel

double size and scaling - 4.00 SPP

Method 1 (old) [+AM1 +A0.1 +R3 +J0] - 3.48 SPP

Method 2 (old) [+AM2 +A0.1 +R2 +J0] - 3.11 SPP

Method 3 (like 2 but mostly using less SPP) [+AM3 +A0.1 +R2 +J0] - 3.11 SPP

Method 4 [+AM4 +A0.01 +R2 +J0] - 2.69 SPP (updated)

Method 5 (like 4 but forcing midpoint always traced) [+AM5 +A0.05 +R2 +J0] - 3.59 SPP (updated)

Test II - limited at about 9 samples per pixel

triple size and scaling - 9.00 SPP

Method 1 (old) [+AM1 +A0.1 +R5 +J0] - 9.70 SPP

Method 2 (old) [+AM2 +A0.05 +R3 +J0] - 6.28 SPP

Method 3 (like 2 but mostly using less SPP) [+AM3 +A0.05 +R3 +J0] - 6.28 SPP

Method 4 [+AM4 +A0.01 +R3 +J0] - 5.84 SPP (updated)

Method 5 (like 4 but forcing midpoint always traced) [+AM5 +A.01 +R3 +J0] - 6.59 SPP

Test III - wanting it perfect - limited at about 16 samples per pixel

Method 1 (old) [+AM1 +A0.01 +R6 +J0] - 14.58 SPP (new)

Method 2 (old) [+AM2 +A0.01 +R4 +J0] - 13.08 SPP (new)

Method 3 (like 2 but mostly using less SPP) [+AM3 +A0.01 +R4 +J0] - 13.08 SPP (new)

Method 4 [+AM4 +A0.01 +R4 +J0] - 13.47 SPP (new)

Method 5 (like 4 but forcing midpoint always traced) [+AM5 +A.01 +R4 +J0] - 14.63 SPP (new)