This is a neat way to render lots of particles fast and also fake volume or
light particles. It's got a number of other uses too, but let's start with the
very basics.
What is a sprite?
A sprite is simply a 2 dimensional image. The trick with sprites is that
they always face the camera, they're always flat to the viewer like cardboard
cutouts, so you can fake 3d effects with them, trees, people in scenes,
volumetrics... but because they're only 2d they render faster than the
full 3d image. You will in all likely hood have seen sprites in video
games, the classic one is usually trees in racing games. Of course the
effect doesn't have to be so obvious as that, it's really up to your texturing
prowess, but i'll get you started with simply emulating visible/volumetric
lights.
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The sprite is a 2d plane that faces the camera |
Giving the illusion of a solid object to the
camera itself |
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Why use sprites?
Simple, less geometry=faster render times. Usually a sprite is on a single
poly, this is about as little geometry as you can get! Much less at least than
the artists dummy in the above images.
So how do I do this with Thinking Particles?
To start off, simply add a library TP Emitter object Object->Object
Library->TP Emitter to your scene, this will create our particles
for us and saves us having to make the Xpresso network to emit our sprites.
Next add a Polygon Object to the scene Object->Primitive->Polygon
Object (please note this is different to the Object->Polygon
Object which is an empty polygon object which contains no polygon
geometry at all to start off with, it appears at first glance to look
like a Null Object), this is going to be our sprite surface,
sprites in 3d are produced by texturing a polygon and using an alpha to
mask out the shape of the object.
To make sure the sprite faces the camera we should change the way it
faces so with the Polygon Object selected go to the Attributes
Manager and change the Orientation to -Z.
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Make the Polygon Object a child of the Particle Geometry
object in the TP Emitter and delete the Cube Object that's generated
by default in the TP Emitter. |
| Next to make the emitter emit our Polygon Object select the TP
Emitter object in the Object Manager, then drag and
drop the Polygon Object into the Particle Shape
box in the Attributes Manager. |
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If we play our animation now the emitter should be emitting our Polygon
Object nicely, however it's not much use unless you're looking directly
at the emitter, we need to orientate the particles to face us at al times.
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Add a new Camera Object to the scene Object->Scene->Camera.
In the View port go to Cameras->Scene Cameras->Camera
(or whatever you've called your Camera Object) to look through
our new Camera in that View port.
In the Object manager Select the Camera Object
and add an Xpresso Tag to it (from the Object Manager
menu File->New Expression->Xpresso Expression).
The Xpresso workspace should open and here is where we will make the particles
face the camera. |
In the Xpresso workspace drag and drop our Camera Object from
the Object Manager, a node should appear called Camera.
Next add two more nodes that we will need to do this, firstly a PPass
node Right Click on the Xpresso Workspace New
Node->Thinking Particles->TP Initiator->PPass and a
TP Alignment node Right Click New Node->Thinking Particles->TP
Standard->TP Alignment. Now we have al the nodes we need to
make our particles always be flat face on to the Camera.
From the Camera Node outputs (the menu from red square in the
top right hand corner of the node) add a Global Position
output Coordinates->Global Position->Global Position.
On the TP Alignment node add an input Axis.
Select the TP Alignment node and in the Attributes Manager
change the Type to User Position. This
option makes particles align with and point towards a point in space defined
by the Axis setting. This is how we will make the particles
look at our Camera, we will do this simply by connecting the
Position of our Camera Object in the world (the Global
Position) to the Axis value, this way the particles
will always face our Camera Object, and wherever our camera
looks the particles will appear flat on (unless we get really close up
where the illusion can loose it's effectiveness as the particles try to
rotate around the camera).
To do this drag a connecting line between the dot on
the Global Position output of the Camera node
and dot on the Axis input on the TP
Alignment Node. Now the particles have the camera's position to point
towards, however we're not there yet, we still need to tell the TP
Alignment node which particles are going to face the Camera,
so to do that drag a line from the output of the PPass node to
the Particle input of the TP Alignment node.
Now we're all connected. The particles wont align till we press play,
So close the Xpresso Workspace and hit the Play
Button and try moving the camera around a bit, rotate it around
the TP Emitter object to see the particles face the Camera
Object at all times.
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Useful?
How do we make this useful? Well here's a simple example say we want to make
it look like we're emitting lots of visible lights into the scene. Visible lights
are slow to render, Sprites are much faster. So here's what we would do.
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| Make a new Material, and change the settings, first off switch off the
Color Channel, and switch on the Luminosity
Channel Make a nice color that we want our lights to be (HINT : if you want
to give the effect of really bright lights make the basic color as you would
with the light then bring the brightness up to over 100% till the color
is the brightest color that you want your "lights" to have at
any point). Next switch off the Specular channel and switch on the Alpha
channel. In here add either a BhodiNUT Gradient or a standard
Gradient, edit this and change the settings to 2D Circular
type and make it so that it's white in the middle and black at the edges,
this will give the look of a circular falloff from the "light" |
Now apply this material to the Polygon Object (our sprite), now it will render
out looking for all the world like a visible light.
Preparing to render
Then before we render this go to the Render Settings dialog
box and under the Options page change the Ray Depth
to 50 (the maximum) this will make sure we shouldn't get any
artifacts when rendering (though it will slow the render down a little bit).
Now seeing as the type of thing we're rendering is already rounded and soft
there's no need to have any anitaliasing on so go to the Antialiasing
page of the Render Settings dialog box and switch Antialiasing
type to None. Next allow the animation to play a little so
you have a few sprites and render. It should look like you're emitting visible
lights.
Other things you can try are putting in images in the Alpha channel
of the material you apply to the "sprite" this is the channel
that creates the cutout. Just make black and white images with white where
you want the sprite to be solid and black where you want the sprite to
be transparent or "cut out".
In this example the camera fly's around an emitter emitting sprite snowflakes
using this method. |
Click to play movie (200k Quicktime) |
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Another thing you can try is using a BhodiNUT Fusion in the
Alpha channel. Then put the gradient in the top channel and
a BhodiNUT 3D Noise in the bottom channel, set the blending
mode to Multiply and edit the BhodiNUT 3D Noise however you
want, try using World as the texture Space
to get the particles to move through a texture, different texture space options
create different effects so play around. This is a good way to emulate some
simple volumetric effects as the particles will move through a 3d texture showing
their "slice" of that texture for each and every frame, you can create
some wonderful effects this way. Check my upcoming tutorial on faking volumetrics
for more hints and tips with this sort of effect.
Download the source file for this image here
(zip) .
Here's another playing around this time with animating the parameters
on the noise and the Polygon Object to create a smokey emitter.
Download the cinema 4d file here (zip).
Or click the image or here
to play the movie (177k quicktime).
Hopefully by now you should realise the power and usefulness of sprites
in 3d. |
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