FX Boundary Response

Overview

Adds fixed-image obstacles to FxState2D. It creates deterministic, lightweight effects such as rain sliding over a roof, sparks bouncing from the floor, or smoke stopping at a wall without introducing a fluid or general-purpose physics engine.

Usage tips

• A common chain is fx_domain_2d → fx_emitter_2d → fx_force → fx_boundary_response → fx_rasterize. Put the boundary response after forces.
• Match the mask dimensions to the FX canvas. Preview debug_mask to see the exact pixels used by collision tests.
• For sparks, start with Bounce, restitution 0.6–0.8, and surface friction 0.1–0.3. For rain, start with Slide and friction 0–0.2.
• Enable Keep Inside Mask for effects contained inside a vessel or cave.
• Chain multiple boundary nodes when different obstacles need different responses, such as sliding on a roof and disappearing on the ground.

Common mistakes

• Particles pass through: per-frame speed may exceed mask thickness. Reduce speed or thicken the mask; this version uses per-frame pixel tests rather than continuous collision.
• Particles react too early at a translucent edge: raise Mask Threshold to 0.7–0.9.
• Stamps are clipped at canvas edges: enable horizontal or vertical wrapping in fx_domain_2d on the required axes.
• Particles cannot leave the mask: verify that Keep Inside Mask matches the intended use.

Related nodes

• fx_domain_2d — define canvas dimensions and periodic boundaries
• fx_emitter_2d — emit particles
• fx_force — add gravity, wind, and turbulence
• fx_rasterize — render the post-collision particle state