Category Archives: Animation and motion graphics

Houdini quick tip: random seed for your HDA instances

Sometimes you need each instance of an HDA to have a random seed for internal use. Sometimes you just can’t be arsed to do it yourself. This’ll do it – create a seed parameter in the UI, and then stick this in the onCreated handler to set it automatically on creation:

[code language=”python”]node = kwargs[‘node’]
seed = 6173*node.sessionId()+4139;
node.parm(‘seed’).set(seed % 10000)[/code]

From Leaf on the SideFX forums, who adds: “Explicitly setting the seed parameter on node create is the safest approach. Basing the seed on the node name can quickly cause problems when you go to clean up your scene and re-name everything (Toootally never accidentally done that before :)”

Blender / Cycles / Particles / Motion blur artefact fix

A render with lots of particles (baked), using Blender and Cycles. I was getting this peculiar streak artefact right next to the particle emitter:

After a bit of fiddling around, I’ve sussed what’s causing it. The default setting for Motion Blur in Cycles is to centre the motion blur around the frame time: it opens the “shutter” before the frame time, then closes it after the frame time.

So when it’s rendering frame 7 with a half-frame (180°) shutter, it’ll actually render the portion of time from 6.75 to 7.25. But it seems that when Cycles reads the particle data from the cache, it only reads the previous frame and the current frame. So the second half of the motion-blurred period doesn’t have the correct particle data to work from.

Easy fix then: choose either Start on Frame or End on Frame instead. Which one to choose will depend on your scene, but it should become obvious when you test it.

Streak artefacts gone! (Phew)

iKMC Promo cartoon

My first Blender project:

Written and created by Howard Matthews at Push Pictures for Fingo Interactive Ltd.
Created in Blender (, composited in After Effects
Music by Terry Devine-King / Adam Drake (
Dubbed in Logic Pro.

Since 3D Studio Max doesn’t run on Macs and I’m slowly getting rid of all the PCs in the studio (muhahaha), I needed an alternative 3D package. Had a quick look at Maya but thought I’d better give Blender a go – it’s free and open source, which is always appealing, but when I had a go with it a few years back the interface was a bit clunky and the learning curve looked daunting. Lots has happened since then, though; Blender’s really matured into an incredibly capable package.

No better way to learn than to try and produce a project for real – so this is it: my first Blender project. To keep things relatively manageable I stuck with Blender’s internal renderer, doing a separate render of each element using the “Edges” option to get the outlines, then using AE’s Roughen Edges to erode them a bit. Everything was assembled and composited in After Effects, though Blender has an increasingly powerful compositor of its own now – have to try it next time.

My first go at doing character animation, too; Blender’s approach to skeletal animation seems much simpler and more straightforward than Max’s, but the key thing is that there are tons of free tutorial videos available, courtesy of Blender’s growing army of supporters. Groovy.