The episodes of Flash Gordon season 1 were recut from an even better, older-skewing TV movie, with some striking stuff that wasn't used in the show, like an opening set in WWII during a bombing raid and a subplot that Ming the Merciless was colluding with Hitler. For the show, they cut out the more mature stuff, opened in medias res, and expanded the storyline of the movie with a series of episodic adventures in the middle, as well as recasting most of the voices and reanimating the main characters in standardized costumes rather than the changing and occasionally rather skimpy costumes of the movie (since movie sequences were rearranged and reused at times, so consistent costumes helped maintain continuity). The fact that so much of its animation was made with a movie budget is why the show looks so good compared to Filmation's usual work.
The tragedy is that, because Dino DeLaurentiis held the movie rights to Flash Gordon, Filmation was only allowed to air their movie once in 1982, three years after it was made, and it was never released on home video domestically. So it was hardly ever seen, even though it was a vastly better Flash Gordon movie than DeLaurentiis's was. Fortunately there were overseas releases, including a Japanese one that's available on YouTube (with the original English soundtrack).
Oh cool. I'll need to look that movie up on YouTube Christopher. Interesting info on that Flash Gordon animated film.