Sparrow Atlases
Sparrow atlases are the standard form of spritesheets in anything Codename, and are used in other places, such as the original Friday Night Funkin' and Psych Engine.
They usually come from Adobe Flash or any derivatives, as well as spritesheet generators in most cases.
A sparrow atlas comprises of an image containing all of the frames, and then an XML file declaring the metadata and subtextures of the spritesheet, which are the frames present in the image.
Loading
Codename should be able to load sparrow atlases automatically.
Do note, however, that some special consideration needs to be taken into when using sparrow atlases. Codename will usually check for the definition file first, parse the filename without the file extension, and then search for an image file with that parsed filename prepended. This unfortunately does not work with the image attribute in these definitions, so both files need to be named the same.
Filenames
Some special considerations need to be taken into in regards to filenames and, by extension, subtextures.
Spritesheet generators will typically use the filename of each frame as a subtexture's name by default. This is unfortunately plagued by the fact that art programs will typically name exported images using project metadata and the time of the export.
Generally, when you're naming subtextures, which includes frames in spritesheet generators, they should follow a convention:
anim0000anim0001anim0002
...where anim is replaced by the name of the animation, and the digits at
the end being the frame number of that animation. Any number of digits can be
included, but a good amount is four digits, which is present in the examples
above.
Note: Some spritesheet generators may include the file extension in the subtexture names. You may either need to configure the program to exclude the file extension, or remove the file extension manually.