Tutorials covering opacity curve

[ben_hamburg]

I´ve been doing a lot of trial and error with opacity curves and I got different situations covered through that now, but I still don´t quite understand how the opacity curves in phoenix work, especially since this was my first smoe/fire sim tool and I had no previous experience.
Having a couple examples in the manual with explanations or a tutorial would have helped me a lot and I probably could still benefit from it.
Especially when you are starting out, there are so many parameters you gotta figure out what you´re doing and rendering is usually the last one you´ll think of if you, for example, don´t see as much smoke as you´d expect to.

[Svetlin.Nikolov]

Can you give me more info on what do you want explained about the curve/curves? Btw, you should just enable GPU preview or V-Ray GPU IPR and proceed if you don't find the curve intuitive.

Cheers!

[ben_hamburg]

Well, it might be just me, but sometimes I just expect something to work a different way logically and if it doesn´t, it needs to be explained in a lot of detail, so I get the wrong idea out of my head...
Basically I just don´t quite understand how the X and Y axs relate to the opacity along the way, when its smoke based. What does X stand for, and what does Y stand for?

In general, I think a tutorial would help all the more visual minded people like me, to understand where to place the points for specific effects.Like: "I wanna make it really dense", "I wanna reduce the opacity at the very edge of the visible area", "I wanna create more of a cigarette smoke look", "I just wanna fake some more detail into it".
But then you could also cover how to achieve different coloring effects, based on the different channels.

I usually always go for the smoke based, because even at its default setting it already looks more detailed than the simple smoke.

I also always use the GPU preview now, ever since I figured out how to disable lights before that, if I had several lights in the scene, it just didn´t look anything like the rendering, so I just didn´t use it anymore.

[Gosho.Genchev]

Hey Ben,

The Opacity diagram is basically a remapping tool. The X axis is the input, the Y axis - the result.

You know how smoke has different density in different regions of the container? In some areas it's thicker, in others - so thin it's barely visible...You can use the diagram to tweak those specific areas as opposed to the Simple Smoke mode which provides a global multiplier only.
I know where you're coming from - it can be confusing... Try creating a really simple, low-res smoke simulation and put a few minutes into moving some points around. I'm sure it will help clarify things.

I wanna make it really dense

Put a point at the very start (but not at 0 - this is the 'empty' value so if you place a point there, you're essentially remapping the empty voxels to have other values) of the diagram in the X(horizontal) axis and start moving it up.

"I wanna reduce the opacity at the very edge of the visible area",

Assuming that the smoke is thin at the edge - the left side of the diagram is where the low 'input' values are (remember, X is the input so thin smoke means low values which means the start of the diagram).

"I wanna create more of a cigarette smoke look"

That would be hard to give a general answer to. You simulation is quite important in a case like this.

"I just wanna fake some more detail into it"

You could try making an irregular shape with the curve in the Opacity Diagram instead of keeping it straight from 0,0 to 1,1

Hope that helps. I'll also try to think of something to make for people who are getting started with simulations in general, ie. what is a cell / voxel, how is smoke transferred through those cells, how are the channels stored, etc.
I think that's where the confusion is coming from in your case.

Cheers!

[Svetlin.Nikolov]

Hey, just one addition - for cigarette smoke you want to create 1 or 2 sharp, thin spikes in the diagram and leave the rest of the curve below zero. This will make for thin areas in the smoke having high opacity while the rest of the smoke would be invisible. You need to experiment with the placement of the spikes because it would vary depending on your cache.

[ben_hamburg]

Hey,

Thanks for all the input, I think I wasn´t quite clear: I know how to do all these effects by now, but it took me a long time experimenting, til I got there and I wasted a lot of time twaking simulation parameters, instead of just tweaking curves...

Its more of a suggestion for new users.

I´m still not 100% certain I really understand how the curves work though and I mostly just use the gpu preview (although it also took me quite some time to understand how to exclude lights, so it was usable) and try to tweak things visually.
If I imagine a base, linear curve from left to right would it be like this:

1. X closer to 0 means little smoke in the simulation (for example at the edges of the simulation, where the smoke dissipates).
2. Y closer to 0 means: less opacity.
3. Now if I wanna make the areas with very litle smoke appear very dense/opaque, I would add a point close to X=0 and increase Y to higher value.
4. If I would wanna make the areas with a lot of smoke (for example. close to the source) make appear less dense, I would add a point further away from X=0 and lower the Y value.

I kind of get it now, as I´m writing it down, but like I said: I´m more of a visual guy and I just think some more examples in the manual would help.

[Svetlin.Nikolov]

Roger, that was good info. Will add some pictures and more explanation on how to work with a diagram.

Cheers!