Ace Film is an animation file format used on early RISC OS systems (file type D6A, AceFilm), allowing graphics created using Ace Computing's applications to be stored as a series of frames in a file, to be replayed layer using the Projector application. Applications used to generate Ace Film files included Tween, Mogul and Euclid.
[Ed. note: Need more sources/research. The files from Arcade BBS don't seem to match this format (title actually begins at offset 0x2).]
[Author's note: Agreed. The files I have follow the format described below, but those from Arcade BBS seem to be different. I should have paid a bit more attention to that.]
All lengths and offsets are in bytes. Words are 4 bytes.
0x0: File length (4 bytes) 0x4: Title (12 bytes, ASCII, low bytes ignored, can contain spaces) 0x10: Offset into the file of data (data_offset) 0x14: 0x18: 0x1c: Number of colours?
Each frame is described by a data length followed by the data itself, then followed by the data length again, presumably to make it easy to traverse the file in both directions:
data_offset: length of data (including this word) (data_length) ... data_offset + data_length - 4: data_length