Thank you for the information! The option makes it clear that I was wrong in thinking this is a typical antialiasing problem:
!! let outline = 0.05
let outline = 0
let ypixels = 2400
!! let ypixels = 4800
!! let ypixels = 9600
set data coads_climatology
set window/outline=`outline`
fill/L=1 SST; go fland
frame/ypixels=`ypixels`/file=tmp-aa.png
set window/outline=`outline`/noantialias
fill/L=1 SST; go fland
frame/ypixels=`ypixels`/file=tmp-na.png
With and without antialiasing, the land (plotted by go fland) has very clear horizontal white stripes. Using outline=0.05 ameliorates the problem but doesn't eradicate the white lines.
So, there is some error in plotting polygons for PNG output. When the northern latitude of one color-painted box coincides with the southern latitude of the adjacent box to the north, the PNG renderer that PyFerret uses somehow creates a wide gap between the two boxes (*footnote*).
Somehow, the PDF generator doesn't do that.
So, I guess the conclusions are
- For PNG output, adjust the outline value and the pixel resolution (through xpixels and ypixels) to eliminate or reduce the white lines.