On Monday, 22 June 2015 at 08:45:38 UTC, Andrea Fontana wrote:
They used to pack alpha informations with other pixel infos (color) just for simplicity and to have a convenient way to store info inside a file, I guess.
Separate mask has an advantage of lower memory consumption: it can have 1-bit depth. It was important in 90s when you didn't have lots of RAM and HDD and didn't need transparency deeper than 1 bit, masks were used to only outline a crop region, which worked with RLE compression well further decreasing mask size, so it made no sense to waste space on 8-bit alpha channel.
