Refine the particle cloud
The ICE Tree isn’t that complex, but the velocity, direction and orientation of the particles are affected by multiple nodes, and they’re all trying to add their respective values at the same time. By default, Softimage will sample the ICE simulation one time per frame, but if you have contradicting forces or significant changes within that frame then your simulation might become inaccurate or unstable. If you experience this, simply increase the Subframe Sampling until the simulation becomes stable. You’ll find this value by clicking the Simulation Settings icon in the embedded Explorer of the ICE Tree (the pane on the right).
Video 2 00:05 Colour the particles
Using the Particle Gradient node, you can colour your particles based on any attribute defined in the ICE Tree
Select the pointcloud, and from the Render > Get > Material menu choose ICE Particle Volume. Press  to open a Render Tree. There are two options to add colour to your particles: you can either assign the colours directly in the ICE Tree or you can assign them in the Render Tree. Each has its limits and strengths, so which you choose depends on the situation. If you assign the colours in the ICE Tree, they are by definition part of the simulation, which means you have to resimulate the cloud every time you want to make a change. If you assign the colours in the Render Tree, they’ll be part of the shader and thus separate from the simulation.
Open the Preset Manager and get a Particle Gradient node. Connect it to the Global/Gradient Color input under the Per Cloud or Gradient section of the Particle Render node. Open the Particle Gradient PPG. The values in the first section of the PPG called Per Cloud: Density enable you to control the colour of the particles based on their density, where the density is highest at the centre of each particle and decreases towards the edge. As the cloud accumulates and particles overlap, their density will be added together.
The second set of values, Per Particle: Age %, sets the colour based on a particle’s age percentage. The intensity parameter then controls how much influence each of these values will have on the final shading of a particle. While your typical whirl of dust may not have that much colour variation, adding a slight colour shift to the gradient can help to bring out the overall shape of the cloud and the details in the individual particles.
In Per Cloud: Density, set Gradient Starts At to 1. Because the particles fade out over their lifetime until they’re barely visible at the end, you’ll need to compress the Age % colour change if you want the effect to be noticeable. By setting Gradient Ends At in the Per Particle: Age % section to 0.5, the particles will reach the complete colour shift after half their lifetime. Switch to the RGBA Gradient tab and click the Black/White preset to get rid of all but two of the colour markers. The marker on the right controls the colour at the rim of a particle as well as the colour it will have when it reaches the end of its life. Set this to a light brownish colour. The marker on the left controls the opposite: the birth and the centre of a particle. Set this colour to a darker brownish colour.
04:35 Shape the cloud
The Particle Shaper node works from left to right, where each of the three tabs adds details to the preceding one
With your particles looking a bit dirtier, you can move on to shaping the cloud. Open the Particle Shaper PPG and switch to the Cell Shape tab. These values will control the billowing or puffiness of the cloud. All of the values in this compound work in combination with one another and, as such, you’ll probably want to go back and forth fine-tuning the effect. You can leave Intensity at 0.75, but change Texture Scale to 18 to separate it into smaller puffs.
Switch to the Fractal Shape tab. This is where you can start adding details and really shape the cloud. If you’re going for a softer or fluffier type of cloud than in this project, you should keep the Intensity setting low and use higher values for Furrow Softness. In this example, you’re aiming for the opposite, where you’ll almost be able to detect the individual grains of sand. Increase Intensity to 1 and set Texture Scale to 25. Set Furrow Width to 0.35 and Furrow Softness to 0.1 to bring out the individual arms or valleys in the particles.
06:40 Add the details
By layering multiple textures you can create highly detailed and complex shapes, which in turn define the density of the cloud
The shape is starting to emerge, but you still can’t see the grains of sand I just talked about. To achieve this you’ll need to modify the factory compound a bit. A compound is essentially a set of nodes that are packed together. However, this doesn’t mean that you’re not allowed to change them. On the contrary, you can use them as a starting point for building your own variations. Click the e icon in the top left of the Particle Shaper node to open it. Get a Fractal Scalar node and connect it to the Input 5 > Input of the top Multi Math node. Open the Multi Math node PPG and change Operation 5 to Multiply. Close the PPG and open the PPG for the Fractal Scalar node instead. Begin by ticking the Switch Colors checkbox. Set Clamp Min to 0.15, Clamp Max to 0.5 and Bias to 0.75. Switch to the Noise tab and change Noise Type to Turbulence. Set Recursion Level to 3. Finish by switching to the Texture Support tab and changing Scale to 20 to bring out the details in the valleys. Close the compound by clicking the x in the top left corner of the Render Tree.
09:40 Control the density
The density parameters in the Particle Render node are central to controlling the density of the cloud
If your cloud is too transparent or it lacks definition, your intuition might tell you to head straight to the Rate parameter in the particle emission PPG and increase the number of particles. While this would affect the overall opacity and create a denser cloud, it isn’t the place to start. In fact, it’s more likely to be the last setting to change.
The first value to alter is Global Density in the Volume Density section of the Particle Renderer node. This value controls the overall density for the cloud. Higher values create a denser cloud. The default value is too low for your cloud, so increase it to about 60. The other value in this PPG affecting the density is Density Limit in the Particle section, which controls the density of the individual particles. The next parameters to play with are Contrast Amount and Contrast Center, which can be accessed in the Particle Volume Cloud PPG inside the Particle Render compound. If none of these parameters fulfils your needs, then you were probably right about increasing the emission rate in the first place.