UE5 Pseudo Fluid Dissolve

Niagara's simulation stages let me do compute-like operations, which is pretty powerful. I use a specific kind of render target that can write to the current frame while also reading from the previous frame - that's critical for the advection to work. Without being able to compare current vs previous values, you can't predict motion.

This is what makes the "fluid" look fluid. Advection predicts where values were in the previous frame using velocity. I sample curl noise to get divergence-free motion (those swirling patterns you see in real fluids) without actually solving any fluid equations.

Curl noise velocity field setup

I get the grid UV, sample the velocity, and look up where that pixel was last frame. Then I pull that value forward. This is the core of what makes it feel like the dissolve is "flowing" instead of just fading.

Previous frame sampling for advection

Because of how I use dot product for the dissolve direction, I can actually dissolve from any angle: bottom to top, left to right, diagonal, whatever. Just change the vector values and the dissolve follows. I calculate the min/max bounds of the object in Blueprint to get the world-space extents.

Box intersection with dot product for direction control

I layer multiple noise textures with different speeds to get variation in the dissolve edge. The dissolve starts from a direction (controlled by the dot product) and eats away at the mesh over time. Smooth step controls the falloff.

Full material graph: noise tiling, dot product direction, smooth step, and opacity mask

The Two-Mesh Stencil Trick

The key to this effect is overlapping two identical meshes with different Custom Depth Stencil Values. The main mesh dissolves first with a standard material. The dummy mesh has a stencil value set and dissolves with a slight time delay.

Because of the timing offset, there's a brief window where the main mesh has already dissolved away but the dummy mesh hasn't yet. During this window, the stencil buffer shows only the dissolving edge region - that's where the fluid simulation gets applied.

In the material, I sample the stencil buffer and only apply the advected fluid texture where the stencil value matches the dummy mesh. Everywhere else gets the normal dissolve or is fully transparent. This creates that layered effect where the fluid seems to eat into the character along the dissolve edge.

Stencil visualization: the dummy mesh reveals where the fluid effect appears

Final output samples the advected texture with a depth test for sorting. The noise makes it look like fluid filling or draining, depending on which way you run the effect.

Final material with depth test output

By reversing the animation, you get a fluid appear effect - characters materialize from the flowing texture instead of dissolving into it. Same system, just played backwards.