Here's why the CGI for I Am Legend's zombie/vampire creatures the Darkseekers ended up looking so bad. I Am Legend is based on a classic novel by Richard Matheson and revolved around Robert Neville, who is seemingly the sole surviving human in a world overtaken by vampires. The book has been cited as an influence by Stephen King and George Romero, who admitted to basing Night Of The Living Dead on the story. The novel has also been adapted three times, starting with 1964's The Last Man On Earth starring Vincent Price, The Omega Man with Charlton Heston and Will Smith's I Am Legend from 2007.
The 2007 version had a storied development history, with filmmakers such as Ridley Scott and Michael Bay circling the project at different times. I Am Legend ended up being a unique fusion of melancholy character study with a blockbuster, as Will Smith's Neville struggled to overcome his loneliness in an abandoned New York. In a strange move, I Am Legend ditched the vampires from the books in favor of the "Darkseekers." These are former humans mutated by the Krippin Virus that wiped out most of mankind, and while they could be killed by the sun, they functioned more like zombies than vampires.
I Am Legend received praise for Smith's performance and its eerie vision of a deserted New York. However, one element that has been commonly cited as a serious flaw with the movie is the quality of the CGI used to bring the Darkseekers to life. I Am Legend features an almost unbearably intense setpiece where Neville has to navigate in the pitch dark around a Darkseeker nest, but once these creatures are actually shown in cartoonish, unnatural CG, the suspense is gone. Bringing I Am Legend's Darkseekers to life via CGI wasn't the original plan, however, with a late shift to visual effects being borne out necessity.
Director Francis Lawrence wanted to bring the Darkseekers to life with actors in prosthetics, mostly played by dancers. After some footage was shot with the performers in this makeup, it was swiftly decided it wasn't going to work, as they looked like "mime artists" instead of nocturnal monsters. Part of the Darkseekers design is that their adrenal glands are always open, giving them a restless, hyperactive body language, which was tough for actors in thick prosthetics to achieve without passing out. After it became clear human performers couldn't achieve the desired effect, Lawrence recalled to Den Of Geek in 2011 that “And one night, we shut that all down and decided we were going to go for CG."
This came with its own set of design problems, and the film's tight production schedule didn't help. The finale - most of which was reshot following mixed reactions to I Am Legend's original ending - was a particular problem as it featured dozens of Darkseekers attacking Neville's home. The quality of the CGI on the creatures is very inconsistent throughout, with some shots even appearing incomplete. Lawrence was also very disappointed with the quality of the CG zombies, later stating “It was better than doing the live versions at that time because it didn’t work, but we needed six more months on the post end to get all the visual effects right." Hopefully, when the creatures return for the upcoming I Am Legend 2, they will have undergone a major design overhaul.