--- title: "Smoke Opacity" canonical: "https://documentation.chaos.com/space/PHX4MAYA/125637963/Smoke%20Opacity" format: markdown --- This page provides general information about the Smoke Opacity sub-section of the Rendering rollout of Chaos Phoenix. ## **Overview** --- This dialog controls the Opacity component of the Volumetric Shader. In the volume shader, there are two types of content — **diffuse** and** emissive**. For simplicity's sake, we call the diffuse content **Smoke** and the emissive content **Fire**. Diffuse (Smoke) needs external light in order to become visible, and also casts shadows. On the other hand, emissive (Fire) is visible even without being lit by lights. It also ignores their illumination, and does not cast shadows on itself. > Macro (ui-text-box) > > Phoenix has different tools that help you to control how the emissive fire illuminates the smoke and the scene - see the [Fire Lights](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/126060907) sub-rollout for more details. Fire and smoke also have their own Color and Alpha. Alpha is a synonym for opacity, and transparency is the opposite of opacity. The **Smoke Opacity**, **Smoke Color**, as well as the **Fire's Color** and **Opacity**, can be mapped to a physical **Grid Channel** from the simulation, using the [Based on](https://docs-chaos.atlassian.net/wiki/spaces/PHX3MAYA4EDIT/pages/117295374) parameter. The **Based on** parameter specifies the source channel that will be rendered, and is set independently for the Fire, Smoke Color, and Smoke Opacity respectively. This makes shading very flexible, so that you can even render a simulation's fire as smoke, and smoke as fire, depending on the channel you set the Based on parameter to for each. > Macro (ui-text-box) > > Fire, Smoke Color, and Smoke Opacity, can also be** mapped to a texture**, or to a Grid Channel multiplied by a texture, to achieve a wide variety of different results. > > Textures have infinite resolution, and so they can increase the detail above the resolution of the grid, meaning that even with a low resolution simulation, you can get a detailed result when using a texture. > > By default, textures are static in space. However, if you export the **Grid Texture UVW Channel** and use the [TexUVW](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/125832240) feature, textures can move together with the fluid to create even more possibilities, such as melting textured icing on a cake. > Macro (ui-text-box) > > The **Smoke Opacity** settings below will affect the rendering of **Fire** if the [Fire Opacity Mode](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/125996126/Fire#FireOpacity) in the **Fire Rollout** is set to **Use Smoke Opacity**. > Macro (ui-text-box) > > UI Path: ||Select [PhoenixFDSim](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/125895496)|| **> Attribute Editor > Rendering rollout > Smoke Opacity rollout** ## **Parameters** --- > Macro (ui-text-box) > > When there is both **Smoke **and **Fire **(diffuse and emissive) in the same voxel, you can use the [Fire Opacity Mode](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/125996126/Fire#FireOpacity) option in the **Fire sub-rollout**, to determine the resulting color and alpha. > > If the Fire Opacity Mode is set to **Use Smoke Opacity**, then the **Smoke Opacity **settings below will affect the rendering of Fire. > > Otherwise, you can separate their opacities for additional control, using either the **Fully Visible **or **Use Own Opacity** modes in the Fire sub-rollout. > Macro (ui-text-box) > > Using the **Based on** parameter below, you can achieve a wide variety of results when shading** Fire **& **Smoke**. > > For example, the [Fire](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/125996126/Fire#BasedOn) (emissive content) is by default based on the **Temperature Grid Channel**. If instead it were based on **Smoke Grid Channel **data, then voxels containing Smoke values would be shaded as emissive (Fire). In other words, the Smoke would be rendered as Fire. > > On the other hand, if the [Smoke Color](https://docs-chaos.atlassian.net/wiki/spaces/PHX4MAYA/pages/126060985/Smoke+Color#BasedOn) (diffuse) & **Smoke Opacity** were based on Temperature, then voxels with Temperature values would be shaded as diffuse volumetrics (Smoke Color & Opacity). In other words, the Fire would be rendered as Smoke. > Macro (ui-text-box) > > You can use the following controls in the Opacity Curve: > > **Double click** – Creates a new point or changes an existing one.** ** > **Left button drag over a point** – Moves the point. If several points were selected beforehand, they will move the same amount. > **Left button drag over several points** – Selects several points. > **Middle button drag over the background** – Drags the visible area. If the Shift key is pressed, scales the curve in the corresponding direction.** ** > **Mouse wheel** – Zooms in/out. > **Mouse wheel near the borders of the control** – Zooms in/out only along X or only along Y. > **Right-click** – Displays a drop-down menu where you can add a point, edit or delete a selected point, and fit the entire curve into the view. If multiple points are selected, they can be edited simultaneously. Multiple selected points can also be scaled and flipped with the **Scale Selection** option as seen below. > > ![image](media://3be41d50-67b4-43c5-8585-640f84c9ed41)   > Macro (anchor) #### Example: Absorption Constant Color --- > Macro (ui-text-box) > > To make the **Absorption Constant Color **more pronounced when combined with a grey** Smoke Color**, you’ll generally need to introduce saturation for the absorption, and increase its color HSV brightness value a bit. > Macro (ui-text-box) > > The **Absorption color** can affect the opacity of the smoke as well, depending on how bright or dark its color is. Brighter colors make the volume more transparent, while darker colors make it more opaque (denser). > > Using bright and highly saturated absorption can produce very prominent colors. Introducing color into the **Smoke Color **can create even more striking results. > > Here are some examples showing different Absorption Constant Color and Smoke Color combinations. ![image](media://c53f47a3-6454-44d2-a9a7-ba3d9904dc69) **Example #6** ![image](media://763a7106-5a04-4b78-a018-9da998d3ccbb) **Example #7**