Opinion: Dynamic Looping With The BASS Audio API

[In this reprinted #altdevblogaday-opinion piece, indie developer Mike Slone examines a dynamic music system he recently built, explaining how it works and offering helpful pointers on how to leverage the BASS API for such a system.] I find it easiest to write about something I've done recently, so I'm going to share about the dynamic music system I made for Chickens 4 Cash on Monday. For the sake of keeping things simple, I'm going to explain this as though we're accessing the API directly, despite that it's actually wrapped in my own code. Fat Chance: A friend of mine, off whom I often bounce my game ideas, suggested putting together a dynamic music system for Chickens 4 Cash. Initially, I imagined it being the way it had been at Volition, with all kinds of awesome orchestral cues with smooth, dynamic transitions. With that in mind, my initial reaction was that the amount of work required was prohibitive, given the scope the game. New Inspiration: I had been working on music for a boss , and it became apparent, not only that the stages of the boss (because every good shmup boss has stages) each needed to have their own musical cue, but also that the level could potentially continue on indefinitely. So I got to work on a new plan for the system. Keeping it Simple With the notion that I needed to keep it simple, and some exploration into how my audio API worked, the Slonersoft dynamic music system became as follows…

The Loop Data:

The Set Data:

Using the API Getting the Times in Bytes: You'll need the start and end times of your loops in terms of bytes. To convert from time in seconds to time in bytes, you'll use the function:

…passing in the channel/stream handle and the time in seconds. Getting Notified: To get notified that you have reached the end of the current loop, you'll use the function BASS_ChannelSetSync function and pass it a callback of type SYNC_PROC *. To sync to a specific time, you'll call it like so:

If you want to sync to the end of the stream (which you'll want to do for the last loop in the set), you call it like so:

This function will pass back a value of type HSYNC, which you should store so that before you change it to something else, you can clear it with:

Sending it Back: Once you've reached the end of your loop (as denoted by a call of the callback), you'll either want to send it to the beginning of the current loop, or to the beginning of another loop. You'll do this like so:

Queuing Up Music Once you've got the music playing, you'll want to be able to queue up loops for the changes. For this, I made a linked list queue which contains the index of the loop to play, combined with the number of times we want it to play. In the case that I want it to play indefinitely until another loop comes along, I store -1 here. Script Hooks So with the code-side system all put together, all it needs is a set of script hooks. I'm using Lua for my scripting, so I'll share the Lua function prototypes I have for this. function music_play_set(set_name, loop_name = nil)…

function music_play_loop(loop_name, loop_times = -1, immediate_switch = false)…

Gotchas There were a couple 'gotchas' surrounding how you load the file. I load my files with

but the example seems to make it appear that you can also use…

which, in my brief attempt, I didn't get to work right. I also noted that if you pass BASS_SAMPLE_LOOP into the flags field, it will never hit the end-of-file callback you set up with BASS_SYNC_END. [This piece was reprinted from #AltDevBlogADay, a shared blog initiative started by @mike_acton devoted to giving game developers of all disciplines a place to motivate each other to write regularly about their personal game development passions.]