Particle Repopulation In Krakatoa For CINEMA 4D¶

Introduction¶

The Krakatoa renderer provides an internal mechanism for increasing particle counts based on existing lower-resolution particle systems.
With some extreme settings Krakatoa Repopulation can produce too many particles for your system to handle.
Thus, understanding how the various controls affect the number of particles produced and the overall look is very important.
The following page discusses the basic parameters and workflows related to the Repopulation feature.

The Base Setup¶

We will start with an empty C4D scene with Units set to Centimeters.

Create a default Sphere with Radius of 100cm, 24 segments.
Create a default Target Spot Light.
Create a Krakatoa PRT Volume and pick the Sphere as the source mesh.
In the Mesh tab of the PRT Volume, enter 40cm as the Voxel Spacing.
Open the Render Settings dialog, set the renderer to Krakatoa.
Set the Output resolution to 640x480.
In the Options tab of the Krakatoa renderer, check Override Background, leave the default blue color.
Render to the Viewer.

RESULT: Only 57 particles will be generated in a grid with 40x40x40 spacing.

Basic Repopulation Principles¶

Now let’s try to use the Repopulation feature on this very low-resolution point cloud barely resembling a sphere.

How Does The Repopulation Work?¶

The Repopulation feature uses a voxel grid to encode all available channels of the source particles in the voxels.
It then seeds new particles in the areas with valid data, giving birth to new particles with properties acquired from the ones encoded on the grid.

Using A Very Small Fill Radius¶

Select the PRT Volume and add a Krakatoa Repopulation Tag.
Set Fill Radius to 10cm.
Keep Subdivs at 0 and Per Subdivision at 5.

RESULT: This is clearly too low for a distribution where the distance between particles is about 40cm, but we want to see what it looks like:

This created a voxel grid with spacing of 10cm and encoded the influence of the original 57 particles onto it.
Then we seeded new particles within the influence regions, taking the color, density etc. of the original particles.

Creating More Particles¶

Now let’s increase the number of particles:
- Change the Number Of Particles Per Subdivision to 100.
- Reduce the Final Pass Density Exponent from -1 to -3 to let more light pass through the new particles, otherwise our “blobs” will get too dark.
- Render 97,495 particles:

RESULT: We now have more particles within the same spherical regions with 10cm radius.

Increasing The Subdivisions¶

As with PRT Volume itself, it is generally recommended to increase the Subdivisions instead as it adds some better anti-aliasing.
- Increase the Subdivisions from 0 to 2.
- Render again.

RESULT: This will turn each voxel into 3x3x3 = 27 sub-divisions, each getting potentially 100 particles. But since we are filling only around the original particles, it won’t be too much, and we get 2,627,871 particles that look like this:

Using A Larger Fill Radius¶

Now let’s try increasing the Fill Radius a bit.
- Set the Fill Radius to 25cm .
  - This is still not enough to cover the 40cm between the original source particles, but enough for them to overlap a bit (at 20cm radius they would be touching exactly, at 25 you get 10cm overlap).
- In the Krakatoa Settings, increase the Final Density Exponent to -1 to give it more light-shadow contrast:

Setting The Fill Radius To The Voxel Spacing Value¶

Now we can set a much higher radius of 40cm which should help us recreate something similar to a Sphere.
But unlike CSI Miami, we cannot get more detail out of low resolution data, so our sphere will be quite rough:

We start noticing some patterns in the random distribution. It is generally better to increase the Subdivs and not the Number Of Particles.
Here are 7 Subdivisions, 100 particles per subdiv, creating 39,601,281 particles in 10 seconds, and rendering in 21 more for a total of 31 seconds:

There are some vertical lines that become obvious.
To improve the look, we could reduce the Particles Per Subdivision and further increase the Subdivisions count.

Increasing The Orignal Particles Resolution¶

Our cloud is getting smoother, but not more detailed - we need more base particles to get a better representation of the original Sphere.
In the PRT Volume Mesh settings,
- Change the Voxel Spacing to 10cm.
- Disable Repopulation
- Render to see the distribution of 3,959 particles:

If we would render with the original 40cm Fill Radius, this would produce 73,742,253 particles in 18 seconds and total render time of 56 seconds:

Obviously, the right thing to do is to reduce the Fill Radius to about 10cm which was our average particle distance in the base cloud.

REMEMBER!

The Subdivisions apply to the Fill Radius.

So a Fill Radius reduced 4 times will result in the subdivisions also becoming 4 times smaller, and each getting 100 particles.

Even reducing the Subdivs from 7 to 2 would produce 100 million particles!

Reduce the Number Of Particles Per Subdivision from 100 to 10
Reduce the Subdivisions from 7 to 2.
Setting the Radius to 10 will still show some grid artifacts because the original particles don’t really constitute a full smooth sphere, but a boxy approximation with 10cm voxel grid size, or 20x20x20 voxel grid with one particle in the center of each voxel.
Render to see the resulting 9,968,897 particles:

Comparing Repopulation To Original High-Resolution PRT Volume¶

Using a larger Fill Radius of 20.0 and a higher Subdiv of 4 will smooth out the edges, but of course our cloud will be about 10cm larger than the original sphere.
In the following screenshot, the top image is the PRT Volume with Voxel Spacing of 2cm and Subdiv 1, Jittered, 2 particles per region, total of 8,138,288 particles in 11 seconds.
The bottom image is the 4000 particles repopulated to 8,525,753 particles in 8 seconds:

Using The Density Falloff Start Value¶

So far, we left the Density Falloff Start value at its default of 0.
- In other words, there will be no Density falloff within the Fill Radius.
- This produces solid spheres around each original particle.
The Density Falloff Start value ranges between 0.0 and 1.0.
The following illustration shows how the value affects the beginning of the linear falloff relative to the original particle position and the Fill Radius:

Increasing the Falloff to its maximum of 1.0 causes a linear Density falloff for the full range from 1.0 at the location of the original particle to 0.0 at the influence range.
This of course reduces the overal size of the resulting spherical cloud compared to the 0 falloff version:

Repopulation And Normals Generation¶

A lot of CINEMA 4D particles don’t have their own Normals.
Krakatoa PRT Volume particles do have Normals taken from the surface of the Mesh’s LevelSet produced when voxelizing the source mesh.
- Increase the Sphere Segments from 24 to 64 to produce a smoother LevelSet surface - in the previous example of the PRT Volume you can see the slightly flat segments of the original polygons.
- Change the Shader from Isosurface to Phong.
- Set the PRT Volume’s Voxel Spacing to 2.0 cm and the Subdivisions to 1.

But it would make very little sense to transfer these to the new Repopulated particles the way we transfer Colors, Densities, Velocities etc.
So the Repopulation feature actually takes the LevelSet info produced when encoding the source particles onto the Repopulation grid, and produces NEW Normals from that!
Thus, if you take the example of 40cm Voxel Spacing, but with 20cm Repopulation “blobs” slightly touching, each individual blob will get its own correct surface Normals.
And Phong Shading works correctly!