Strange rendereing

Here's a test case, a base layer all white, a second layer with a small black dot in the center. (The fractal where the problem came up was more complicated!) The black dot is a circle with a radius of .01 in a window of width 4, so at 800x800 it's 4 pixels wide and shows up clearly in the UF window, with or without anti-aliasing.

Problem: sometimes the rendered image does not show the black dot at all! Here's a bit of a table pasted from a screenshot of an Excel display:

I hope this is not just my machine (or my incompetence).
UF 6.04, Windows 10 pro, version 21h2, 64-bit

The upr for anyone to verify this strange result:

tmp {
::s5TZshn29i1ySuqNQ09TVz/Al3bbJZemUs5msJpyiUV+ASpAy26eFIKQMec+6TLh4tkndZzM
  mj6+0tO0tgmrt0CFV8Tv/WQgirEs8DqqmDBP4lq75pIUwdG/2dl5nC6TWbXORbcRLrkr6yP8
  78qgvJ6Z/c8ZM5MBRIHCo1KOVwpd86b5PZdv/mxVTUKoNKusO/QJt9HBFsaFrNoRyrVHCkN0
  Cu6ZOGCmqlW31QbBDGooi20A8NQixtc0ZUQF9WdOew8ry2quc87vNdlx6OpgXmjCuyFsaaFs
  HLr++lT9fe9QAQU7zBgj/KT0/J7XEcTgOodt5vLgL/A226cGjwACvujXyk9K9/yt/Hw7ANhV
  3yEUF/DWetcgA51rdMFtS2D7E0J0MU9NQvHQalKqRWGus7u8xdadpg1Nxj96aZTL7DO7hWVc
  A3x/XWOJJajTgOWCK2GfGQNugjRoV+8gruX3X9P6b5rcTvggXz2ArJJdJDD1QEz2h2wyL4tF
  CtQN8D1TA7gtAAcIgVejNK6GL6baYtC2VV+R8Z8EsQ+g12aqKxnPOjPWUc6MaJLaazPhwnRr
  gwaSXDRA+256FoKTHDmAKE6sZ9V+nsyAzNQbGbXeVKjOFNvy6sGWavfjruyvc08vnL7X5sS2
  oxW61AV2dYLrQ5Lv1rtTn1g7V5JTHRxzUYrltXto90GdTGOSycyNQQjU88msefKOvmsVX9ex
  0cYhshcFFrC8ImstGwwLxM1Eri/Dedp+0qDUhIYoDfM8dKaL0Fu4qt3HGxHyy0FICeFXljWg
  0Pdb0cq2o72No9Kz5h45aoJMyssPhBVonSiM1ULhDBYiBe8wgpliy1GrXisyl4F4bcJxjLp+
  dJzjLYkffwYfORehTX85U4LcynIgfhKg9JD4XoDYfCBZvQU03+BbqWkXDP5RFIh/KoNdHGNw
  WRuwl1VlLRnrJngsFlbcfVh54CzllmL195gNYavSOX/uap5iWD8Ylsefi3YLxPNE7xFzcQGq
  y1/ZEvQI7gDtXJEW/vMS9Wc7hvroWfE/R8CSMEfZonfp3heYN0upXQhtFEPvLMkGun0I/qQk
  LxMaHpROlgYPJbsLSjnPXZm349JbiHST8oAJ7S2k9km6XBSdlsp7IN1pCk5JZzcRamLFIbfy
  iReYVfsmTNAWYb+CQ7J+VdVubrw7JG7UJwEfJNxJxEXiBm4Ip91opPT2tacZfS7oVDH+C1I0
  ZSvvbDH6WNi8l0Ob4wROVjIHJtveOcsP1IefS7otDn8C1IxZSvvzDn4WNS9l0Ob+wpOVjUHJ
  tv+PcmP1IbfS7oFkgexDOQuSaCaIp1jE2LompCrofy1PtTPs58kh/FwDMZa5p+rVTTH+nw7k
  KgXYe49CHGW2MhJT/Y4a27vZnC0sUJDMTPF7pLosMMZ2W9UT0WnRrQYehcbA/GM6ch+x9m3J
  +3g00Md47vdrlWyBjMBCo7HsSjA3VJlwwW6fONKJ+CCeT2S2n54k0gCpQCvcwESmZOaDIOOJ
  JhgjsrdBeCrnFiHpB2xDzqPMg9XG9xJ7JRRzZwSUNNzhfv56YvycHfRh/QDFgX/dEqYq7yyc
  42ui3Q7+qPkg7akowkAYwni7sifkDTvHQL/efn5DT4r2Zxdzvq8x+BKCJpRhxhRJxrrn2W+M
  adWYWcCJL6/r6pjpT37iRbqQmv9hjxbq1OeJZHS8O//q6p1R3R9EEXn1T6wv3cdKsy8/DMUp
  fFH=
}

This happens because of the Guessing drawing method. If you check "Force Linear drawing method" in the Render to Disk dialog, you will see the black dot at the 1200x1200 size.

Thank you. As I never intentionally use Guessing, I don't remember to check that setting.

Is there ever a good use for Guessing? I wouldn't think that it would speed things up much.

It works well with classic Mandelbrot-type fractals. Try this both with Guessing and with One-pass Linear and compare the calculation times (order of magnitude difference for me):

Fractal1 {
fractal:
  title="Fractal1" width=1720 height=1290 layers=1
  credits="Frederik Slijkerman;6/14/2022" antialiasing=yes
layer:
  caption="Background" opacity=100
mapping:
  center=-0.5/0 magn=1
formula:
  maxiter=15000 percheck=off filename="Standard.ufm"
  entry="Mandelbrot" p_start=0/0 p_power=2/0 p_bailout=128
inside:
  transfer=none
outside:
  transfer=linear filename="Standard.ucl" entry="Smooth" p_power=2/0
  p_bailout=128.0
gradient:
  smooth=yes rotation=1 index=0 color=6555392 index=64 color=13331232
  index=168 color=16777197 index=257 color=43775 index=343 color=512
opacity:
  smooth=no index=0 opacity=255
}