Houdini 101
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This page was created when I learned Houdini, it might be old and wrong ! (looking at you, HOM expressions). I'll try to add more info with the hindsight.
Where to Start
There's a ton more videos nowadays then when I learned houdini, and instead of pointing to a thousand websites (well, I do just afterwards in the [links]), I'll simply point out that SideFX have a curated list of tutorials.
Links
Links to stuff I've saved (could have been a webring. remember those?). This was started long ago, some sites might be down, let's try to clean this up (2023 edit)
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Other Nifty links
- Discord servers: Think Procedural and Houdini&Chill (it's invite based, but worth it if you're worth it); plus it has system to publish the best (most liked) posts on the discord
- https://www.reddit.com/r/Houdini/ I'm there daily and I like the fact that answers there will be crawlable by search engines (contrary to discord servers).
- HDK tutorial on eigenvectors
- https://inria.hal.science/hal-02541299/file/DeformerElastic-rigid_paper.pdf paper to implement
Flips
Basic setup:
- better quality: more points (smaller point separatation) or decrease grid scale
- reseed particle in flipsolver: nice for droplets but /!\ particle IDs change
Expressions Vex & General Syntax
$NPT number of points
$CEX $CEY $CEZ centroid
$FF frame number
$T time
$PT point
$CR $CB $CG color
$OS use node name (in groups for ex)
$CY copy number in copy node, use for placement w/o having to stamp
`$OS`_`$OBJID` give objects nynamic names
opdigits($NAME)
opinputpath("sopnode",inputN) get the incoming connection (0, 1...)
op:`opinputpath("../",1)` ---> get full path of input 2 of parent
nprims("../sort1") ----> number of primitives
$TEMP/houdiniCache/simdata.`padzero(4,if($F>70,70,$F))`.simdata -----> read from cache, hold at a certain frame
$OS.`substr(chs("camera"),rindex(chs("camera"), "/")+1,200)` ------> get the name of the camera if it's a complicated name with lots of / / /
------------------------------------------------------------
in vex no uppercase:
v@Cd
i@id
set()
Print to console with precision
printf("Point %*.*g\n", 10, 10, @value);
Expressions dump
On a switch; switches between inputs if first input is an empty vdb (fix for Maxwell's shitty VDB implementation) - the second input being an 'empty' but existing vdb
if(prim(opinputpath(".",0),0,"file_voxel_count",0)!=0,1,0)
Shortcuts
general / viewport --------------------- ctrl-b: full window h: center d: display options p: show params space-shift-h: center on object h: center on grid/frame current selection w: wireframe left/right arrows =prev/next frame ctrl-left arrow: go to first frame node editor --------------------- u/i: up down hierachy c: set color ctrl-click on node blue bit: becomes purple = final output animation --------------------- alt on param boxes = keyframe alt on param name = key x y z chops: --------------------- d show points on curve graph editor: --------------------- shift-lmb: access graph editor up down arrows: play forward left right: prev next frame v-h: zoom vertically/horizontally j: show whole timeline k: key all g: group keys * drag & drop parameters vopsop ---------------------- r: reverseinputs
POPNET
- birthgroup: group of particles that were just borne, lasts 1 frame
DOPNET
- can inherit params
- 3 solvers, bullet = speed
- 'activate' node to do bullet time
- shelf tool always update latest created dopnet ('set always')
- clones for active object = active creation with modulo
General
- drag and drop nodes to get path names
Color Schemes
- Red: out geo
- Purple: VOP_name
- Yellow: creation/merge
- Blue: POP DOP
- Light Green: Pre split
Volumes
- Volume from inside as well as surface:
- add a 'volume from points' > ' stamp points ' before volume object
- with volume dops: use curl noise --> 4d with vector4 to do a time noise
- displace noise: use shader, dig into 'fireball' shader
Wtf
${OS}_new_pieces
pillar_? (group)
@Cd
$VALUE
vector vexcode
`interpreted text`
op:opinputpath
chramp(values)
rendering
- get motion blur on objects : geometry velocity motion blur
- plug stuff in a premade mantra surface (displace along normal, diffuse etc...)
Chops
- right click anim channels to add motion effects
- expressions: $I = ptnum, for curves
- object merge to grab channels
- multiply: order is important
- lookup: timeshift
- at Geo level, to get chop value: chop("../CHOPNET/out/ty0") or chopf
- recording mouse keyboard: keyboard - mouse -> record, hit scroll lock to disable shortcuts
l-systems
http://algorithmicbotany.org/papers/
proba: ':' rules function
nodes
point replicate : multiply points
attribute promote : transfer from point <> primitives
Render Stuff and Cache Geo and Alembic
- wedge: change node values an filenames with $WEDGENUM
- fetch: renders specific ROPs, can be daisy-chained with a merge and (node to node)
- multiple shapes in maya with alembic: in Houdini, need a primitive with strings called 'name', the names will be shape names, in the alembic exporter, make sur to choose Partition Mode: Use Combination Of Transform/Shape Node and choose previously created 'name' as Attribute (also Ogawa format - ?)
Fur/hair
- put RBD solver before Wire solver in merge otherwise = bug
Python
Howto's / Tricks
- Wedges and HQUEUE:
- stop stepping: disable integer in frame options, add timeblend
- using 'rest' to get more interesting fractures
- restpos: can be used in SHOPS to get object space
- instances: instancepoint() w/ full point instancing --> packed disk primitives = refs = light
- instances materials: "declare materials>declare all shops" to export shaders to instances (add using paramet interface window)
- triangulate a whole object cleanly using foreach primitive
- On crashes: load with 'Manual' and hit escape so nodes aren't read (if file is corrupt for ever)
- Blend between slowed down continuous mesh:
- RBD Point Object from sequence from odforce:
- add int attr on points (call it 'shape')
- write RBDs to disk
- in RBD point object override point value geometry path: /path/to/geo.*.bgeo
- 'allow editing of contents' in RBD point object, dive inside fin 'sopgeo2', add a stamp OBJID, $OBJID, disable 'Use External SOP'
- dive inside sopgeo, add a read file, and replace * from RBD point object to proper file path: `strreplace(chs("../../geopath"), "*", point("/obj/gridscatter/OUT", stamp("..", "OBJID", -1), "shape", 0))`
- Parenting: at object level or using 'Rivet' for deforming geo
- Group according to mesh connectivity: use 'Island 1' 'Island 2' etc...
- If you get 'unable to initialize UV rendering module with camera' when baking textures, add a new cam to your scene (cam1)
- If you are using maxwell and alembic (abc) files to render particles (realflow plug-in) you NEED a vector v and int id attributes otherwise it won't render
- Fluid simulation: collision reversed ? Use a vdb with a static solver, and reverse the vdb with a volume wrangle @surface *= -1
- Why is my motion blur not working ? You need 'v' and to check 'geometry velocity blur' on your object
- I still have a hard time with vex code rotations/matrices even with matt estela's wiki so VOP quicktip to rotate normals around an edge:
Specific Forum/Blog/Wiki articles
Modeling
- Remeshing with external software (quads, instant mesh) - lkruel
Vex
- Understanding rotations (2) in houdini.... (vex, quaternions) - mestela