Add a convenient hook for Postprocessors to update

[eonarheim]

Context

Currently there is no lifecycle hook for postprocessors to update over time.

See discussion #2415

Proposal

Add some sort of hook that allows users to update uniforms after the shader has been bound.

Perhaps a onPreRender() or update? Not sure what makes the most sense?

[mattjennings]

I have personally been doing something like this:

class MyActor extends ex.Actor {
    effect = new SomePostProcessor()

    onInitialize(engine) {
          engine.graphicsContext.addPostProcessor(this.effect)
          const shader = this.effect.getShader()

          // set initial uniforms, otherwise shader errors
          shader.use()
          shader.setUniformFloat('u_time', 0)
    }

    onPreUpdate() {
          const shader = this.effect.getShader()
          shader.use()
          shader.setUniformFloat('u_time', someValue)
    }
}

which I honestly haven't minded to this point, but this is about as complex as my usage has gotten.

A lifecycle hook that allows me to set the uniforms once instead of both in initialize and an update loop would be nice. Possibly something that abstractsshader.use() when setting uniforms would be handy too (but I imagine there's good reason to have that called separately).

I guess another thought I have is, if u_time is a common thing to pass into shaders, should that be provided automatically as elapsed time of the shader?

[eonarheim]

@mattjennings I like what you've done here! That's a pretty convenient work around at the moment.

I think it makes sense to have a shader specific lifecycle hook for the situation you describe. Perhaps something specific like onUpdate(...) which would be called every frame and be guaranteed to have shader.use() constext setup? And maybe a onInitialize(...) which would be called once?

It's probably worth including a few well known uniforms by default (definitely u_time for sure). I use ShaderToy a good deal to develop shader code, maybe borrowing some from their list would be useful? At minimum it would help when sourcing shader code into excalibur?

Things that look useful on first glance might be iTime, iTimeDelta, iMouse, iResoultion`?

I'd want to also document a way to include additional textures into the composition as well.

[mattjennings]

That all sounds good to me!

A set of well-known uniforms from ShaderToy would be awesome. It seems to be the go-to for examples, and it would really help newcomers like myself with migrating to Excalibur.

[github-actions]

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