Sunday, October 25, 2009

Krakatoa 1.5 - Confusing Changes For The Better

Once again, while most of this is already covered in the online documentation, I feel that spelling it out for the few people reading my blog might be a Very Good Idea. I will probably have to update the FAQ or just link to this Blog or something like that.

First, the default lights handling.
When designing Krakatoa 1.0.0, we discovered that particle rendering with default scene lights never looked good. This was mainly because the default mode for default lights in Max is a "headlight" right behind the camera, which does not produce very good looking shadows. The alternative mode is two lights which works even worse with volumetric rendering, and requires one more light sorting/attenuation map generation pass...
So we made the decision back then to render particles as self-illuminated if no actual light node was detected in the scene. As result, one could just create some particles, hit render and get an idea where the particles were. On top of that, it worked great with Additive Mode where lighting was usually not desired (although Additive Mode + Lighting was somewhat supported).

When the Krakatoa version which ended up being released as 1.5.0 (it was initially developed as 1.2.0 and nearly shipped as 2.0.0 due to the amount of features added, but that's another story) added support for an Emission channel and an Absorption channel in addition to the Color (Scattering) channel, we had to revise this design decision. In short, having a per-particle Emission channel meant that it would be a Very Bad Idea to render particles as fully self-illuminated when no lights are found in the scene. At the same time, rendering Default Lighting was still as unusable as it was two years earlier - we looked into it again and finally decided to bite the bullet and render particles as not illuminated if there are no explicit lights in the scene.

What does this mean? In short, if you open a Krakatoa 1.1.x scene without scene lights in Krakatoa 1.5.x or just start a new Max scene, add some particles and hit render, you get... nothing. Or so it seems. Looking at the Alpha channel, the particles are there (and the log / progress dialog show that they are actually being loaded and processed). It is just that self-illumination is not applied implicitly, no scene lighting is applied either and you end up with black particles on black background (changing the background color to a brighter color shows that, too).

As you can imagine, this is one of the main problems new users of 1.5.x encounter and report on the forums and in support emails. The possible solutions are
  • Check >Override Emission and >Use checkbuttons in the Global Render Values rollout (or alternatively check >Use Emission and >Override Emission in the Main Controls rollout). The default Emission Override color is set to white, so your particles will render as white by default.
  • You can also add a Map to the Color Override in the Global Render Values if you want more interesting results.
  • If you want to emulate somewhat the Krakatoa 1.1.x behavior where each particle renders the Color as Self-Illumination, you could also add a Global Channels Override KCM and set it to Color Input>Emission Output, then check ">Use Emission" without enabling the >Emission Override option - this will copy the Color of the particle into its Emission channel. PRT Loaders and PRT Volumes will render in their actual color.
  • Alternatively, adding a Vertex Color Map to the Emission Override slot will render the Color channel and put it into the Emission channel, but this approach is generally slower compared to using a Global KCM.
  • You could of course also create a Light in the scene to illuminate the particles, but this will cause longer render times due to the illumination pass when rendering in Particle Mode.
While I admit that this seems like a step backwards, the ability to specify Emission per particle means more flexibility (as we will see later). Flipping two checkboxes or creating a light shouldn't be a big price to pay...

The second large change made to the main controls of Krakatoa was the replacement of the big >USE LIGHTING button with a much smaller >Ignore Scene Lights button. We wanted the lighting mode to be the default state of the UI and the underlying renderer, not a special mode one would have to activate. In light of (pun intended) the above discussion about emission and default / scene lights, this was the only logical way to go. On top of that, the >Ignore Scene Lights option does not switch the renderer into a different mode, it simply assumes any existing scene lights are actually turned off. (you could do that with the Light Lister, but it would be a PITA). At a certain point in the development, that option was nearly removed from the system, but it was deemed necessary to quickly produce Self-Illuminated particles using the Emission channel without taking scene lights into account.

And here we come to the third major change - Additve Rendering in Krakatoa v1.1.x used to be a distinct rendering state as opposed to Volumetric Rendering. But Krakatoa v1.5.0 implemented the full volumetric shading equation with Scattering (Color), Absorption and Emission terms, making it possible to control how much color is scattered into the eye, how much is absorbed from each color component as light passes through the particles and how much light is emitted by the particles. Additive rendering is simply fully Emissive rendering lacking Scattering and Absorption, which means that now we can define PER PARTICLE whether it should be rendered volumetrically or additively (or somewhere in between).
You can find more info here.

Knowing that our users will still want to be able to render particles additively without much clicking around the UI (Krakatoa has been doing additive rendering long before it even had that name, since around 2004), we decided to add a special option called  >Force Additive Mode. What this option does is this: it copies the Color channel into the Emission channel, while setting both Color and Absorption to black. Thus, particles do not absorb light, do not scatter any light into the eye, they just emit light and accumulate into the pixels as desired. To illustrate what is going on, the >Use Emission and >Use Absorption options are getting grayed out in that mode. In this case, scene lights are also ignored completely even if >Ignore Scene Lights is not checked - particles with black Color and Absorption would produce no lighting effects at all, while wasting time sorting for attenuation map drawing that would not be used anyway...

As you can see, we wanted to make Krakatoa a lot more flexible by providing per-particle channels for the main shading values, but this required some changes to be made to both the default behavior of the renderer and to the controls in its User Interface.

Understanding the processes going on inside the renderer and the logic behind these changes should help you master the new version and lose your old habits built in the less advanced environment of Krakatoa 1.1.x.

I have the feeling I will continue this post in the near future with some more notes on feature and UI changes, so stay tuned!...


  1. First, these posts are VERY useful, since understanding the logic behind interface decisions and why they were made makes the user much more insightful and knowledgeable, leading to less 'which button do i push' time. I know that at some point this blog will be syndicated into a 'things you really need to know' page in the Krakatoa docs and CGTalk :)
    Second, I really love the new shading/lighting model, I think its beautiful. Having all those override buttons looks quite native in the interface so I don't really mind that either. I also had my first render black, but there's nothing more intuitive than checking the alpha when you get a black render, so I wouldn't worry too much about that :)

  2. Ha! I just ran into this issue (Why aren't the particles rendering!?) yesterday trying to follow a tutorial from 1.1. Thanks for the explanation.