Step 3 edge coordinates for y=460 to 510:
y=460: edge at x=420 (v=119, r-b=92)
y=465: edge at x=420 (v=121, r-b=92)
y=470: edge at x=420 (v=124, r-b=94)
y=475: edge at x=420 (v=127, r-b=94)
y=480: edge at x=420 (v=132, r-b=93)
y=485: edge at x=420 (v=136, r-b=93)
y=490: edge at x=420 (v=132, r-b=96)
y=495: edge at x=394 (v=93, r-b=65)
y=500: edge at x=394 (v=104, r-b=68)
y=505: edge at x=394 (v=110, r-b=69)
y=510: edge at x=394 (v=118, r-b=69)