The image quality of the movie is bad, the quality varies significantly among scenes. In the newly added scenes, those with Akabane Karuma's solo used very poor algorithms and resulted in obvious artifacts. We manually picked those scenes, de-scaled to their native resolution and re-upscaled to 1080p with better algorithms to eliminate the artifacts.
Koro Sensei Quest is made from the American version. The source has low bitrates and heavy noise. We used advanced de-noising algorithms to separate the noise so that they don't interfere with line-art and texture processing. With the occasional aliasing, we applied routine anti-aliasing accordingly.
The web series (Kagaijugyou Hen) has a very bad image quality with serious aliasing and colour-banding. Since the banding in some images is so severe that it overwhelmed some textures, we at first applied highly intensive de-banding hoping to remove all the banding while maintaining the textures. But the banding was very stubborn. Even with such intensity, we still cannot completely remove large banding in some images, not to mention that we damaged a lot of textures. After numerous tests, we chose a balanced intensity to deal with the banding.
Pair 1 to 7 of the below screenshots are from the movie, 8 to 13 from the OVA and 14 to 16 from the web series.