Flame Opacity

[magilla]

In render settings "Use Smoke Opacity" for fire looks decent but I want to mess with the smoke as well

First I duplicated the grid and set up the smoke to look good so now I have a separate grid for smoke and fire.

Rendering these together gives really crappy results, all grainy and messed up with the illumination from the fire grid not affecting the smoke grid no matter how high I crank the "light power on scene" attribute

So I try to transfer the smoke opacity to the fire opacity using save/load right click in the graph area and switch the fire to use own opacity - doesn't even vaguely resemble the 'use smoke opacity'

[Ivaylo Katev]

we have several tutorials about rendering smoke and fire, based on single grid rendering of the fire and smoke. you can see the results of some users, based on these tutorials, for example this one: https://vimeo.com/79728728
what exactly you didn't like or is missing in the results of our approach that pushed you to look for new rendering technique?

[Svetlin.Nikolov]

Yup, transferring the diagram won't work: The fire in "smoke opacity" mode gets its opacity by going through the fire opacity diagram using the temperature as base, and then this is corrected by the smoke opacity, which uses smoke as base. The fire in "own opacity" mode uses only the temperature as base and there is no way to get the smoke involved as well, and you can't get the same results.

Using two grids seems the valid option here. Tried to reproduce it here and didn't get any artifacts, could you share an image?
The drill is as follows (Sim2 is copied (not instanced!) from Sim1 after the end of simulation while all render settings were still at defaults)
- Sim1: Fire: Use smoke opacity, Smoke color - disabled;
- Sim2: Input: "$(same_as_output) PhoenixFD001" (makes it use the cache files from another sim in the scene, no need to specify the global path), Fire - disabled;
All the rest is left at defaults and the fire from Sim1 illuminates Sim2 with no issues.

Is you setup the same?

[magilla]

Svetlin - virtually the same, I didn't know about being able to direct the input to the same output as another grid, that is cool. But yes that is how I set up the rendering for the two. It also gives me a separate atmosphere pass for smoke and flame which is pretty valuable.

I think my step size was up too high on the smoke (I have been getting hanging buckets so I set it to 100) and it got a very grainy result which was otherwise unnoticeable on the smoke alone. I realised this later and got rid of the grain but the illumination still didn't carry from the flame grid to smoke. I don't know if this is related to the opacity - the plane fire example (which I used as the basis) has a very transparent flame, it looks great on black with ample internal detail but is difficult to comp into other colours. I also found it really difficult to get a reflection pass because of the low opacity so I have another flame grid with much more opaque flames just for the reflections in another pass.

Ivaylo - I have found Phoenix is great for explosions with thick mushroom clouds (and there are lots of tutorials explaining the same thing over and over) but delicate pink flames on a white background with very controlled amounts of thin purple smoke - damn near impossible for me at this stage! I'm not recreating reality, I'm recreating fantastical elements. The most difficult thing is trying to alter the smoke to look wispy and maintain volume without looking carved out. The smoke emitting from the source (rather than the flame - another reason why I tried to use fuel as a source) causes lots of grief, particularly when combined with unnatural buoyancy. I can't seem to knock the super thick smoke out of the curve at the point of emission without adversely affecting the rest of the smoke.

I'm sure we can get there but I'm finding it a little difficult without a large knowledge base from user input (maybe there is but they are not posting here). I really appreciate the inordinate amount of time you guys spend answering my endless questions but where are all the other users?

[magilla]

black smoke at base

edit: I'm finding much better results and control using temperature instead of actual smoke

[Ivaylo Katev]

for thin flames+thin smoke i think you can use fully opaque mode for the flame and simple smoke. the thin smoke are actually much easier for rendering, because they produce no significant shadowing, actually you need no alpha at all.

[magilla]

I have to disagree, simple smoke has no control and is virtually useless for every sim I have done so far. It's good for a quick test but smoke itself is just too difficult to control. When I say 'thin' I am not meaning transparent. I want thin opaque smoke that doesn't fight with the fire opacity. Temperature seems to be the only way to get this.

[Ivaylo Katev]

can you post an example image that shows approximately what you are trying to do?

[magilla]

I shall do on Monday - end of the day on Friday here - have a good weekend Ivaylo

[Ivaylo Katev]

ha ha, i see

[magilla]

This isn't the greatest example, I just grabbed this from the gpu. The left being the sim with a default simple smoke look, the right being a custom curve with the desired look.

So basically the difference is that with temperature I can separate it from the flame and using the curve (which I am used to doing from using fume) I can punch out the early thick stuff and control the falloff with the thinner smoke and therefore avoid it visibly clipping at the edge of the grid.

The simple smoke option seems to just make the whole thing less or more opaque.

[Ivaylo Katev]

well, i'm sure you can achieve perfect results using the temperature to control the opacity. actually this approach was the recommended one in the early ages of the product, but i should say that this our decision was a disaster. we preferred it because there is no difference between the smoke diffusion and the temperature diffusion (if you have no cooling specified). from technical point of view, the smoke channel seems to be pure waste of memory, that's why we recommended the temperature to be used. however from convenience point of view the smoke channel it's not a waste. the result from our pro-temperature campaign was a wave of terrible looking videos and negative comments, because our default opacity curve was not good chosen, and almost all the users was using exactly the default curve. after this bad experience we decided to accept the idea to waste 10% of the memory and to recommend the smoke based opacity. i'm glad that you successfully decided to use temperature based opacity, the idea is not bad, if you have good understanding how the opacity curve works.

[magilla]

haha - the best laid plans of mice and men. I did find it easier to manipulate the temperature with cooling as opposed to the smoke with diffusion, which was helped a lot by the viewport preview. But of course this works best with fire and smoke, I had just done some low hanging fog which really could only be done with cold smoke (afaik) and the simple smoke looked pretty good on that - a real, solid voluminous look - but I still found it gave me the desired look using the smoke curve. Having the feedback of the viewport preview, the gpu and the blue area in the curve graph makes it pretty intuitive to manipulate.

[magilla]

I'm still having no luck getting the flame of one grid to light the 'smoke' of another. My guess is because it isn't actually smoke, it is temperature with the fire opacity set to zero.

[Ivaylo Katev]

this may be a bug, by default the smoke is lit internally, otherwise there are too many shadow rays to be traced. i suppose this mechanism stops the lighting of one simulator by another, but let svetlin say more, it is the expert here.