This is just a small suggestion, but again this may be just me and my experience, or lack of, with Krakatoa…
But playing with settings such as the final pass density, filter size, and exponent…I can see resaults if I have low numbers…
for example…
final pass density… going from 5.0 to 10.0 can’t see any change, but going from .1 to 1.0 I can definitely see a change happening…
Is there something in the docs I may have missed? Otherwise, maybe some of those settings in the lighting and drawing pass density to lower default settings might be better perhaps?
The Final Pass Density values control the same property- a value of 5.0 Exponent -1 means 0.5. If you change the Exponent to -3 but keep the first value at 5.0, you get 0.005. So the Exponent lets you increase/reduce the density by an order of magnitude. If you need a value like 0.0000000005, the 3ds Max spinners wouldn’t even allow you to enter it, but with an Exponent, you can do this easily.
We cannot tell you what value to use though because it is highly dependent on the particles’ count, their Density channel value and the scene scale.
Here are some facts which I will try to include in the docs in the future:
Each particle carries a Density value. If it does not have an actual explicit Density channel, the value is assumed to be 1.0. This value is “Density per cubic generic spatial unit”. So if you imagine a cube with 3 sides of 1.0, if a single particle is found in that cube, the density of the cube will be 1.0. But if there are 10 particles with Density channel value of 1.0, the value found in that cube will be obviously 10.0. If the Density value of each particle were 0.1 though, the resulting density of the unit cube would still be 1.0 (10x0.1). This is also pretty much how the Voxel mode of Krakatoa works.
When rendering in Particle mode, Krakatoa draws each point in one pixel (Nearest filter mode), 2x2 or more (Bilinear filtering, depends on the FIlter Size), or 3x3 pixels (Bicubic). When it does this, it takes into account where the particle is in space and how its density would affect the volume the pixels to be drawn cover. So the result of drawing 10 particles close together with Density of 0.1 would be the same as drawing one particle with Density of 1.0, assuming the same pixel ends up representing them all.
So what happens if you scale up your particle system 100 times? The particles will suddenly represent the same Density per particle, but spread across 100x100x100 times more volume. So the total volume the same particles would cover would be a million (!) times bigger, and thus the resulting density would be respectively lower.
Note that a multitude of methods exist in Max and Krakatoa to scale the density as it flows up the stack:
*Density comes from Density channel, or is assumed 1.0
*Density can be changed or set via a Magma modifier
*Density will be scaled by the Opacity channel of the material (if any) assigned to the particles.
*Density will be scaled by the Visibility track of the particle object (in case of PRT objects like PRT Loader and PRT Volume)
*Density can be set or modified via a Global Channel Modifier (also Magma, but global for all scene particles)
*Density can be overwritten by the Global Density control via a value or map.
In order to be able to tweak the final look of your rendering without constantly fiddling with all of the above, we give you the set of Final Pass Density and Lighting Pass Density controls which SCALE the actual density values drawn into the pixels after all the previous calculations are done. This is also why you can lock your PCache and still change the Final Pass Density values to massage the final look - the density is scaled JUST before drawing the pixel, so it does not get cached and is “live”.
This is very helpful when the particle count changes. If you write 10 partitions with one million particles each and load only one, you will render 1 million particles. But if you enable all 10 partitions, you will be rendering 10x more particles and your spatial density will be 10 times higher, too. So just reducing the Final Pass Density Exponent value down by 1 will make the final density 10 times less, and the result will look just like the one partition density-wise, but much smoother because you will have 10 particles with slightly varied positions contributing 1/10th of the density value, thus spreading it across the pixels.
If you change the output image size, the pixel-to-volume mapping will be different (one pixel will represent more or less space behind it). As long as all particles cover all pixels, Krakatoa will take care of the adjustment of Density to Alpha calculations, but if you start getting gaps between the particles because there are too many pixels and too few particles, you will have to generate more particles and of course drop the Final Pass Density somewhat to adjust for the increased count.
Hope this gives you some idea about how the Density controls work and why they are there.
If you have any questions, please ask!