I built a free kit to permanently fix missing and exploding motion blur in Houdini (H19.5–21)

[houdini vfx3d]

Missing motion blur on cached geometry is one of the most-asked questions in Houdini's history — and when it's not missing, it often overshoots into long streaks at every impact. Here's the complete method, plus a free kit if you want the code ready to paste.

Why per-frame caches lose blur

Per-frame caches (.bgeo.sc, per-frame Alembic, VDBs) throw away the between-frame info. To create blur the renderer needs either a velocity attribute v to extrapolate along, or two or more time samples to interpolate between. Changing topology (particle birth/death, fractures, fluid meshes) makes interpolation impossible — those cases must use velocity blur.

The decision table

• Particles / changing point count → velocity blur (v)
• Deforming mesh, constant topology → time samples / Cache LOP
• Volumes (pyro, smoke) → velocity blur from vel
• Camera / transform animation in LOPs → Cache LOP + matching camera shutter

The fix — if v got stripped, rebuild it before the cache (Trail SOP → Compute Velocity), or by hand in a Solver SOP:

 

vector prevP = point(1, "P", @ptnum);
v@v = (@P - prevP) / f@TimeInc;

Then enable velocity blur on the renderer. In Karma/Solaris it's read from v automatically on most geometry; animated transforms need a Cache LOP so the USD stage carries multiple time samples.

When blur overshoots at impacts, clamp velocity in the render branch only — never touch the raw cache:

 

float maxspeed = ch('max_speed');   // ~2x your hero speed
float spd = length(v@v);
if (spd > maxspeed) v@v *= maxspeed / spd;

Works on Houdini 19.5 / 20.0 / 20.5 / 21.0. In H21's Karma (Hydra 2) these rules are unchanged.

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That's the full method. I packaged it into a free kit that adds the clean copy-paste VEX + Python files, a third fix (neighbour-smoothing for impact spikes), a one-click batch-render tool, and a printable reference card:

Free on Gumroad (secure checkout): https://worldvfx.gumroad.com/l/motion-blur-fix-kit

It's a slice from a larger Houdini Automation Cookbook I'm building. Feedback welcome.

[tamagochy]

To fix MB on cached geometry you can use timeblend sop, or cache subframes if motion fast and curvy 

Also need to calculate velocity as central difference 

Edited July 7 by tamagochy

[houdini vfx3d]

Great additions, thanks. You're right on both:

timeblend is a clean way to interpolate when topology is constant — good call adding it alongside the time-samples approach.

And central difference is more accurate — (nextP - prevP) / 2 captures curved motion far better than the backward difference. The backward version in the post is the "inside a solver, no future frame available" fallback; when you have the full cache, central difference is the better choice. Subframe caching for fast curvy motion is the right move too.

Appreciate you sharpening the thread.