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Particles from the emitter up No shelf tools allowed, maybe just one :)

Ari Danesh [email protected]

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Why particles? Students often tell me that particles are out of date and they do not use them because simulations (pyro, fluids, smoke) are much more accurate. So why talk about particles?

Particles are very lightweight/fast Particles are very art directable Particles are fast to render

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Why no shelf tools? Sometimes it is just as easy to setup without shelf tools Sometimes the network is just easier to read with out all the extra nodes

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Agenda

Projects

Creating Emitters

Radial space explosion

Setting up particle systems

Looping particles

Rendering

Rain sliding down an umbrella Motion blur packed particles Particle sprite rendering Particle instance objects

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Project 01 - Radial Space Blast

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Creating the Emitter Create the basic shape of the explosion with Surface Operators Calculate the velocity that the particles should emit at birth Drop down a Circle Object Rename - Explosion Emitter Dive inside Make the Circle Surface Operator polygonal

Completed network

Append a Normal Surface Operator and make it a point normal

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Making the emitter a ring As the explosion increases in distance we want to make sure the thickness of the ring stays the same Append and attribute wrangle You are going to use the fact the circle in centered around the origin to make the normals point inward Create the wrangle shown on the left Offset distance will determine thickness of ring Merge the outer and inner circles

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Making the emitter a ring (cont.) Append a Skin Surface Operator

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Creating the Emitter (cont.) Artist wants to have control of wobble and rotation in the explosion Append a Transform Surface Operator You have three choices for the wobble 1. Keyframe (personally, do not like to constantly update keyframes for changes) 2. Expressions (If you know basic math, piece of cake) 3. MotionFX (a.k.a CHOPs light) <—- Perfect place for CHOPs 4. In the X and Z rotation channels add a noise Motion FX

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Finish the network with an Output SOP Append an Output Surface Operator and name it PUT_RING Highly recommend using an Output Surface Operator over a Null for end of chains Helps when you are debugging

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You could have done this all in a particle simulation…. So why did we make this geometry? because it is so easy….

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Particle creation Go back up to the Object Level Drop down a Geometry Object, name it Explosion_Particles Dive inside and kill the File Surface Operator Drop down an object merge, set it to /obj/Explosion_Emitter Set transform to Into this Object

This

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very

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nt st ep

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Need to calculate the velocity of the expanding ring Append a Trail Surface Operator Set result to Compute Velocity

Velocity Vector Added

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Finally! Lets make particles Append a POPNet Dive Inside

basic POP network

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Setting up source_first_input Set emission type to Scatter onto Surfaces

Velocity created in trail Surface Operator

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Append a POP Force Want the particles to come into frame over a half second period Expression reads multiply the force by an ease curve from 0 to 1 as the particle’s age goes from 0 to 0.5 seconds.

@age is the current age in seconds. @life is the percentage of total lifespan Use @life when you want to access the percentage of the particle’s total lifespan. Useful for trails where you want to fade out as the particle starts to approach death.

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Add another POP Force This time want to add some curl noise We will make curl noise be dependent on velocity

swirl scale += @v;

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A couple of tips… When you are not sure of the name of an attribute use the pass-through menu item

When you are not sure of the names of global variables drop down a POP Vector Operator and dive inside to see the names

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What we have so far…

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Tame the simulation down Append a DRAG POP. Usually have to do this or your particles never loose momentum

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Life is not all Gray Drop down a POP Color Color Type - Ramp ramp = @nage;

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Final Particle Look

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Rendering the particles as volumes Pop up one level. Out from the particle network and back into the Surface Operator network Append a Attribute Wrangle to the network We need to give the particles a scale to render at @pscale = 0.02;

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Fill out the particles with a Point Replicate Append a Point Replicate to add more points around each particle emitted Add 5 points per point

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before pop replicate

after pop replicate

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Convert to a VDB This is a tricky step because you have to guess at numbers to get the VDB the right size Voxel Size - 0.02 Fog VDB - density Point Radius Scale - 2

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Add a Volume Visualization Want to see the results in the Scene View? Density Field - Density Diffuse Field Cd

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Add a shader and apply it to the object I used basic smoke

Dive into Shopnet and put down a basic smoke material Apply basic smoke to the Material tab of the object

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Project done…

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Project 02 - Looping Particles

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Goal This is especially good for games I want to create a particle effect that loops seamlessly every n frames. We will do every 60 frames

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Creating the emitter network Drop down a Geometry Object - name it Particle Emitter Dive inside Delete the File Surface Operator Drop down grid Size 0.2/0.2 <— It does not matter Rows/Cols 10/10 <— It does not matter Append a Scatter Surface Operator <— Here is where the magic will start Drop down a Output Surface Operator - name it OUT_LOOP_EMITTER

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Setting the scatter Go back and select the scatter Set the Force Total Count - I chose 10 The defining parameter is Global Seed Use an expression such as: ($F-1)%60 to repeat every 60 frames or

Relax iterations does not matter

($F-1)%120 to repeat every 120 frames

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Frame 10

Frame 20

Frame 30

Looping with ($F-1)%10

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Time to generate particles Pop back up to the Object level Drop down a Geometry Object - Name it Looping_Particles Dive inside and kill the File Surface Operator Add an Object Merge Object1 ../../Particle_Emitter Transform - Into This Object Append a POPNET Append an Output Surface Operator - Name it OUT_LOOPING_PARTICLES

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Creating the Particle network - Source Tab Dive inside the POPNet Select the POP Source Emission Type - All Points (Look at the Birth Tab and See what happened)

Overall network we will build

Geometry - Use First Context

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Birth Tab Notice fields deactivated

Jitter Birth Time - This is the most parameter to set. If Jitter is activated you can not have looping particles Life variance must be set to zero

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Particles so far….

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Adding some noise to the motion We want to add some interest to the particle stream by adding noise to the force Unfortunately we can not use a POP Force because curl noise is not periodic

We have to create our own noise POP Vector Operator to the rescue

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POP VOP Drop a POP Vector Operator down Dive inside We will use periodic noise…

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Periodic Noise

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Periodic Noise

The periodicity is equal to: Number of frames to repeat over/$FPS

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By definition periodic noise generates noise in the range of 0 to 1 Variance is usually bot positive and negative numbers We will fit 0 to 1 to a range of -1 to 1 so the particles will move in all directions

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Now all that is left to do is add the noise back into Force

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Frame 60

Frame 120

Frame 180

60/$FPS

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Project 03 - Rain and an Umbrella

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This time we will go much faster since you know what you are doing… We need a: Umbrella collider Ground collider Rain Emitter Rain Particle Generator

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Umbrella Collider Make sure to fuse the points Make sure to extrude tho give the umbrella some thickness for collision detection

Remember - You do not have to use this geometry for rendering. Just collision detection

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Ground Collider Remember to put the Output Node in. Functionally it does not add any geometry but it helps if you add nodes upstream

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Rain emitter One small grid over the umbrella to test sliding One large grid to test everything

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Now to the meat… Rain Particle Generator

Overall Network

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Object merge rain emitter

Append a Point Surface Operator to generate velocity and pscale

Object merge umbrella collider Velocity pointing downward

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Creating the POPNet

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POPNet Overview

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Source First Input

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POP Collision Behavior

Set the default behavior of collisions to Slide

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POP Collision Detect

Detect collisions with the umbrella If a collision is detected set the response to slide and change its color to red

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POP Collision Detect #2

Set the Group name to just_hit If a collision is detected set the response to dir and change its color to blue

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If a particle hit the ground… kill the particle and replicate it many time with a velocity scalar of -1 This will make it bounce of the ground and spawn many particles where only one has dropped

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Particle Replicate kill the particle and replicate it many time with a velocity scalar of -1 This will make it bounce of the ground and spawn many particles where only one has dropped

Key parameter set to -1 to reverse velocity

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Project 04 - Motion blur - packed particles

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Simplest Network Yet Emitter and Particle system in same network Just going to use a sphere as an emitter and then in the popnet add curl noise and some drag so we have something to blur

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Packing the geometry and saving to disk Append a Pack Surface Operator <— All that needs to be done to pack Append a ROP Output Driver to dave to disk

If you want save some meta data in a detail attribute Append a Attribute Wrangle pack Run Over Detail

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Use a File Cache to save to disk Write First, read back later

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Blurring the particles Go back up to the Object level In the Geometry tab goto to the Sampling tab turn on Geometry Velocity Blur Drop down a Mantra node in the Out context Go to the rendering tab and turn on - Allow Motion Blur Increase the samples to Geo Time Samples to something greater then 5

Render!

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Project 05 - Particle Sprite Rendering

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Making a film strip in Compositing Operators

-7

Drop down a COPNet Dive inside

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Drop down a File Compositing Operator Set the parameter for file - butterfly$F.pic Houdini will load all 7 butterfly images

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1 e m

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Make a Mosaic (filmstrip) Append a Mosaic Compositing Operator

Images per line 7 Check the results

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POPNet The Emitter is the same as the last project Dive into the POPNet

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Crack open the POP Sprite HDA Notice there is a shop net built right into the Houdini Digital Asset We will use its material to at the object level to render the particle system

Sprite fog material inside shopnet1

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Assign a material at the object level What material to assign? The Sprite Surface Operator and the POP Sprite have materials built in Access one of them

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Project 06 - Point Instance Rendering

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The layout

The magic happens here

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Source Particle Network Overview

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Here is a nice little trick to point instance particles First, I did use the particle shelf tool for this <— I was lazy Now, all the magic happens in the source particles object Remember the source particles takes the Dynamic Operators simulation back into geometry To do that is uses a Dynamics Operator I/O node

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Creating an instance path Dive into the Source Particles Append a Attribute Wrangle to the DOP I/O operator Run over points and set the instance path s@instancepath = “op:/obj/Pighead”;

You can do a scale for good measure

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The Instance Surface Operator Tada! That’s all there is to it…

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End of particle webinar

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