Question on neato start and stop for one unfixated node position

[HermannSW]

4 nodes have fixated positions, and their label indicates their position.
Node d has start position pos=“2,2” but is not fixated.
Why does d not end up inside the triangle with neato as it does with fdp engine below?

[dot engine=neato]graph {label=“neato” size=“3,2” a[pos=“-2,-1!” label=“-2,1”]; b[pos=“-1,1!” label=“-1,1”]; c[pos=“1,0!” label=“-1,0”]; d[pos=“2,2” shape=box]; d1[pos=“2,2!” label=“2,2”];a --{b --{c --d}}} [/dot]

[dot engine=fdp verbose=true]graph {label=“fdp” labelloc=“t” size=“3,2” a[pos=“-2,-1!” label=“-2,1”]; b[pos=“-1,1!” label=“-1,1”]; c[pos=“1,0!” label=“-1,0”]; d[pos=“2,2” shape=box]; d1[pos=“2,2!” label=“2,2”];a --{b --{c --d}}} [/dot]

[steveroush]

neato and fdp use different algorithms to position nodes. It is not too surprising that the two engines position the un-pinned d node at different positions.
(neato actually has multiple algorithms - see mode and model attributes, while fdp has fewer tweaks available)

[scnorth]

Although we implemented this, it’s not known to be numerically stable. That is, we kind of noticed that it’s not always what you would expect.

neato -Gmode=KK gives different results.

neato -Gmode=sgd has an assert that fails.

[steveroush]

neato produced these four graphs from the above input.