I ran into the same problem, but in the editor. The sliders to blend/fade Post Process Materials inside the Post Process Volume Details panel acted as Boolean (0 = off, !0 = full effect) on most of my Post Process Materials.
I read _[this part of the documentation a couple of times][2] _
The difference between both setups is the default value specified on the material (scalar or vector) parameters. The good setup has values that make the pass appear to not have any effect (e.g. multiply with white or lerp with 0).
and finally figured out how to get the Slider and general PP-Material Blending to work:
- You need a Post Process Material and it’s Instance. You drop the INSTANCE into the Array in the PP-Volume.
- In your Material, right before the output, you need a blend/lerp with the vanilla source as first Input, all your Post Process Effects as second input and the Alpha as parameter SET TO DEFAULT ZERO 0.0. Effectively turning the whole effect OFF
- In your Instance you then push this parameter back to 1.0 for full effect
- aaaand voila you can use the Post Process Volume Blend Weight and the per Material Slider in the Blendables array and (I guess) in your Blueprints.
[rant] This is a usability nightmare [/rant] While experimenting with PP Materials I noticed that some other Scalar Parameters magically hooked themselves to the blend weight. I have yet to figure out the pattern, because they are neither 0 to 1 ranges, nor default to 0.0, nor did they influence the Vanilla Input at all (And I don’t even want the #%&$ blend weight to control my texture scale parameter!) … So there are other ways to get the blend weight going correctly…