sciss / scalacollider-if

If-Then-Else blocks for ScalaCollider using nested, resource-efficient UGen graphs.

Version Matrix


Build Status Maven Central

A set of additional graph elements for ScalaCollider, allowing the nesting of a graph in a resource efficient manner, by pausing the inactive branches. This is facilitated by a new nested UGen graph builder that, unlike the standard builder, creates an entire tree of UGen graphs to which the conditional branches are decomposed. This project is (C)opyright 2016–2021 by Hanns Holger Rutz. It is published under the GNU Affero General Public License v3+.


The following artifact is available from Maven Central:

"de.sciss" %% "scalacollider-if" % v

The current version v is "1.7.6".


This project builds with sbt against Scala 2.13, 2.12, Dotty (JVM) and Scala 2.13 (JS). The last version to support Scala 2.11 was 0.9.3.

To compile sbt test:compile. To print the test output, sbt test:run.


Please see the file


A "side-effecting" block does not produce a dedicated return signal but is used for UGens that have side effects. For example:

// ---- Unit result ----

If (freq > 100) Then {,

If (freq > 100) Then {,
} Else {
  freq.poll(0, "freq")

A block where each case evaluates to a sub-type of GE produces a dedicated return signal that can then be further processed. For example:

// ---- GE result ----

val res: GE = If (freq > 100) Then {
} Else {
}, res)

val res: GE = If (freq > 1000) Then {
} ElseIf (freq > 100) Then {
} Else {
}, res * 0.5)

If a branch refers to UGens from the outer scope, for example the freq element in, the UGen graph builder automatically inserts link buses. Recursive nesting of if-then-else blocks is also supported.

Each branch can check its state using the ThisBranch() graph element. For a non-lagging If block, this creates a control signal trigger each time the branch is activated. Because of problems in the current SuperCollider versions with respect to initial states of triggers, this signal might not work in all circumstances the first time the branch is activated. For example will incorrectly ignore the initial trigger. There are workarounds, for example using Latch, which can be seen in the test suite.

Next to If(cond) ... there is an alternative element IfLag(cond, dur) ... which makes it possible to fade out a branch before it is deactivated. When cond becomes false, instead of pausing the branch immediately, a delay of dur happens. This also affects the newly selected branch, so there is no way of creating a cross-fade, while preserving the CPU saving property that no two branches are resumed at the same time. When using IfLag, the ThisBranch element instead of producing a single pulse trigger when the branch becomes active, it now provides a gate signal that remains high until the branch is released. Therefore, the branch can use, for example, an envelope generator that fades out no slower than dur when the ThisBranch gate signal becomes low.

val dur = 0.5  // seconds
val res: GE = IfLag (freq > 1000, dur) Then {
  val gate = ThisBranch()
  val env = = dur), gate) * env
} Else {
  val gate = ThisBranch()
  val env = = dur), gate) * env
}, res)

Classes are placed in the conventional de.sciss.synth package, matching with the other types and UGens of ScalaCollider.

For more background information and design considerations, see the sysson-experiments Project.


  • currently, ControlProxy elements do not properly propagate from outer to inner scope.
  • the number of cases per if-block is limited to 24
  • while the compound result signal from an if-block uses the maximum of the number of channels used by each individual case, the signals of the cases are not "wrap-expanded" as in regular multi-channel-expansion. The user should thus ensure that all branches produce signals of the same number of channels.