The BGCHANGES line in a simfile is used to control what backgrounds are loaded by the simfile and when they appear.

The data is between the colon and the semicolon.
Each entry is separated from the next by a comma.
Each entry is composed of 1 to 11 values separated by equals.
The meanings of the values are as follows:
1. start beat 
2. file or folder name 
3. play rate
4. Backward compatible transition type. CrossFade is used if this is not 0. 
5. Backward compatible effect flag. StretchRewind is used if this is not 0. 
6. Backward compatible effect flag. StretchNoLoop is used if this is not 0. 
7. Name of the effect file to use. The BackgroundEffects folder will be searched for a match. 
8. Name of the second file.
9. Name of the transition file to use. The BackgroundTransitions folder will be searched for a match. 
10. Color string in either "1.0^0.5^0.75^0.25" or "#ff7fcf3f" form. The fourth channel is optional.
11. Second color string, same format.

The file names (values 2 and 8) and the colors (values 10 and 11) are passed to the effect file as thread variables.  Most effects do not use the second file.


The FGCHANGES line uses the same format, except only the start beat and first file are used (values 1 and 2).
