Volumetrics: Combustion

Introduction

Combustion is a volumetric plugin for 3dsmax, Essentially what it does is create firey explosions although it can be used for many gas like real world elements such as smoke, steam, clouds and so forth. As well as tendril fire.

Combustion uses volumetrics rather than polygons, volumetrics are nothing like polygons, instead of using faces and textures they are volumetric pixels filled into a primitive shape. The way they work is the volumetric form will use a shape such as a sphere, box or cylinder and fill itself within it taking that form.
Volumetrics are much slower than faces since rather than being hollow inside they have actual content meaning you could go right inside of the shape and see what's inside rather than see the otherside of the polygons. This means there is much more to render but the effect is much better in quality.

Creating the Gizmo

To the left is the gizmo and next to it is the rendered combustion that used the gizmo to get it's shape. What combustion has done is used the gizmo to work out how to fill itself within the gizmo, much like water fitting into a cup. Of course it has it's firey look to it but the actual shape does still resemble the gizmo's original roundness.



The gizmo is an important step telling combustion the length, width and height and position in the world. To create a gizmo it is located in atmospheric apparatus in the helpers menu.

To get to this click on create (1) and then click on the helpers menu, then in the drop down box select atmospheric apparatus, choose spheregizmo.

Now a small drop down box apears, these are the basic settings that control the gizmo but do not effect combustion too much. First is radius, this is the actual radius of the gizmo, this is a major setting, next is hemisphere checkbox, this controls wether it is a sphere or just a hemisphere, the difference is one is a sphere and one is half a sphere.

This can be useful when making explosions that are sitting on the ground rather than floating in mid space. Seed controls the random fractal algorythm, which essentially just gives it a random look, new seed randomly generates a number to go in there.

As said before the gizmo only controls the shape, more the transform values of the explosion, such as length, width and height, as well position. Apart from that it is just a helper to combustion. The real settings and controls that really define the look and actions of combustion are in the combustion enviroment menu.

Getting to the actual combustion settings

Once you've created the gizmo, you've specified where in space the explosion
is going to take place but you still need to create the combustion and adjust the settings. Combustion is located in the enviroment window, go to the render menu and go down to enviroment.

Once you do that a window will pop up, click on the add button to the right and select combustion. This is what combustion looks like, all of the values that control the look and the way it acts are within this menu, so understanding what they all mean and do is very important.

To the right is what the combustion menu looks like. Showing all the default settings, now I am going to explain what each and every one of them means in english.

First up under Gizmo is Pick Gizmo this is where you select what gizmo to use for the shape. By clicking on the button and then clicking on the yellow gizmo it will allow you to choose that gizmo for your combustion.





Also once you have selected a gizmo, the remove gizmo button becomes active, so if you wnat you can remove that gizmo from combustion. It also specifies it's name within the drop down box next to remove gizmo. If you click on the down arrow next to the gizmo's name it drops down the menu with all of the gizmo's that are in combustion if you have more than one.

* Note* You can choose more than one gizmo for combustion if you want multiple combustions in the scene.

Once you have chosen your gizmo you can adjust the colour of the gizmo, this essentially is the look of it. There are three coloured boxes under colors that specify how combustion looks according to it's colour.

Colours





Inner colour by default is a kind of yellow colour, this is what the colour in the inside of the combustion will look like.

Outer colour is a red colour by default, changing this changes the edges and exterior of the explosion.

Smoke colour is the colour of the smoke, once combustion fases out at the end it turns into smoke and then fades out, the smoke colour controls what the fased out colour will be.





Under shape we have flame type, here you can choose Tendril or Fire ball. Tendril is a more natural fire for candles and bush fires, and fireball is more of an explosion type of fire.
Use them in which cases work best for you, naturally for torches, bushfires candles and most types of standard fire tendril would be the best choice, but for giant explosions and fire balls, firball makes the most sense.

Below that we have stretch, which essentially stretches the fire, the default value is 1.0. Although the way it streches it is more vertical, there is no real use for this but in some cases it can make a nice abstract effect.

Regularity by default has a value of .2 but adjusting it higher makes it fit more into the gizmo's shape, and lowering the value forces it to be more sharp and crisp on the corners and essentially smaller. If you look below, a value of 0 makes the fire very sharp but small, as you adjust the value it becomes more bloated and looks more like a round circle rather than fire.



Characteristics


Here you have flame size, this controls believe it or not, the size of the flames. Essentially the smaller the flames the more there are, but if they get too small they becoem like confetti and look very ugly.
The bigger they are they become very soft and it also looks ugly. A safe value is anywhere between 10.0 and 35.0. The default value is 35.0.

Density controls how thick the fire is in a way, how transparent or dense the fire is. If the density is too low the explosion will lack colour and if there are other objects in the scene (eg. a non black background) the explosion will be very hard to see because it is so transparent.
But if it is too dense there will be colour bleeding because the inner colour will have higher majority over the outer colour. If you look below at the density values demonstrated. between 15 and 30 looks exceptable, and maybe 100 to demonstrate extreme heat in the explosion, but anything other than that looks out of place. The default value is 15.0.


Flame detail is similar to flame size, but is more designed for the edges, the values are limited from 0 to 10, and with a value of 0 it looks very round and blurry, with a value of 10 the edges are very detailed and crisp. The default value is 3.0.

Samples also controls the detail, samples is more the volumetric detail, the smaller the value the quicker it will render, but you will need a balue between 15-30 for the combustion to look exceptable. Anything above 30 isn't worth it since the detail will not be noticable and will render a lot slower. The default is 15. 20 is a good value to use.

Motion


Phase is a timeline which controls the birth and death of the combustion. 0 is default, but as it cycles through it dies out at later values.
Whilst it phases the pattern changes, the faster it's animated the more furious the pattern cycles. Essentially phase is like a seed, a mathematical algorythm. You need to animate this value adjusting, or use the explosion feature metioned just a bit further down the page.

Drift conrols how quickly or slowly the fire animats in the Z axis. Meaning it will rise quickly or slowly upward.



Motion (cont.)

Explosion, enablng explosion adjusts the phase values for you allowing it to apear and explode like a proper explosion. This option is necessary to make a propper explosion rather than the fire being there ever continuously going. Enable this for 99% of your explosions unless you want it to be there rather than grow out and explode.

Smoke is the third colour in the colour boxes, if enabled when the explosion begins to die out it fades into black and then fades out completely.


If you click on set up explosion it will pop up a box which has start and end times for combstion, allowing you to specify when the explosion starts and finishes. If you want to enable this though you will need to click the up and then down arrow on the end time, just adjusting the values slightly to tell it to work, it's something that needs to be done for it to work.

Fury controls how quickly or slowly the explosion cycles from start to beginning through the colours, wether it starts off fast and fades out slowly or starts off slowly and fades out quickly ect. It essentially accelerates or decelerates the speed of phase, so it churns quicker or slower. Higher values make it animate faster, and lower values make it slower.

This covers all of the settings within combustion. When rendering remember to render in the perspective viewpoint or a camera, orthographical viewpoints will not render combustion due to there being no FOV.


If you are using multiple combustion gizmo's and want them to look slightly different instead of synchronized then adjust the gizmo's seed in the modify panel. This will give it the differentiation needed.

I hope this covers everything you need to know on combustion. There is a lot more to combustion, but the best way to learning it is to play with it, but knowing what does what can always help.

source:----------------------------------------------
http://www.3dluvr.com/content/article/63

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