The lopsidedness depends on the spin of each individual black hole. If you select a black hole and edit it via the planet editor (or make a new one from scratch using SE's scripting), you can set the spin to a custom value. The black hole in the link looks square because of the projection, as you mentioned. Double Negative's test render also does not distort the stars properly, as each star is represented as a 0D dot rather than a 2D distortable graphic - it is only a test render and definitely does not use the same techniques as the final render in Interstellar (in which a star directly behind Gargantua would become a ring, unlike the test black hole)

SpaceEngine's renderer is based on the actual metric. Some accuracy is lost by default because the shaders are otherwise taxing on the GPU, but you can increase the accuracy in the settings using "gravitational lensing precision":


Very interesting!  I hadn't even noticed that the lensing in the DNEG video used 0D dots until you pointed it out.  Yes, when I tried to match the look of the DNEG video in Space Engine I made sure to put the black hole at the same spin (about 0.999).  So I guess there was a difference but, in this case, Space Engine was the more accurate of the two (this was the only DNEG video that was not labeled equirectangular projection but I had feeling it still was in addition to the 0D dots).  Yes, they definitely have a different full render for the movie.  Most of the background star lensing is hidden behind the glare for the accretion disk during the movie but you can see the arcs and rings in some of the promotional material (there's that video called "building a black hole" on youtube where it can be seen better).  Thanks for the insights!  

I was surprised to join back here to see that my last comment I left over four months ago was the last comment in this topic!  Regardless, I return with another lensing related question!  I've noticed some artifacts with the lensing in space engine, specifically when I adjust the magnitude limits to see more stars.  Attached is a picture demonstrating the artifact.  You get this almost face like patch of brighter stars around the black hole.  It's more noticeable when I adjust the magnitude limit or if I increase the brightness but, as far as I can tell, it is always there for me.  I've tried playing with the settings and can't seem to get this to go away.  I recently upgraded laptops including an rtx 4060 so I know the gpu can more than handle space engine on higher settings.  I guess the question I have is if this is a bug or feature?  If a bug, how can I prevent this from happening, if possible?