All atoms and images have an appearance, which is an immutable object that can be shared by many atoms. Making changes to an object’s appearance generates new appearances, many of which may be temporary. For high-performance games, this can be a drawback. The /mutable_appearance type exists so that you can make multiple changes to an appearance without creating all the temporary objects, then turn it into a regular immutable appearance when finished.
A new mutable appearance is created via new/mutable_appearance, and giving it an atom, image, or appearance as a source object. Assigning it to an object’s appearance var will create a new immutable appearance.
mob/proc/GetAngry()
var/mutable_appearance/ma = new(src)
ma.color = rgb(51,255,51) // green
ma.transform = matrix(2,0,0,0,2,0) // scale x2
appearance = maReading certain vars, such as overlays, will create a temporary list object that can be modified easily. With regular appearances, making many changes to the overlays list results in a lot of churn.
The /mutable_appearance datum is technically a descendant of /image, but this is only for convenience, and should not be relied on for any other purpose as it is subject to change in future versions.