A Compositing Pass is an object belonging to a compositing Element. Passes are individual steps that are executed when rendering the Element, and are run in sequential order.
There are three compositing pass types:
-
Inputs
-
Transforms
-
Outputs
Most Passes are responsible for rendering an image. For Inputs and Transforms, those images are available to subsequent passes and can be referenced in rendering materials.
Blueprintable
Passes are Blueprintable, so you can easily create your own Pass and use it with the compositing system. Just create a Blueprint and inherit from CompositingElementInput, CompositingElementTransform, or CompositingElementOutput.
Shared Settings
Each Pass has its own set of properties, but every Pass shares the below properties:
| Property | Description |
|---|---|
| Enabled | Like Elements, individual Passes can be disabled. When turned off, the Element behaves as if the Pass is not there. |
| Pass Name | Passes have names so that they can be referenced by other subsequent Passes. A Pass must be uniquely named if you want to reference it in a render material. |
| Intermediate | For each render Pass, a render target is allocated. By default, it assumes that you only need that result through the next Pass. After that, to save on render resources, it releases its render target so it can be used by another pass. If you need the Pass's result to be accessible longer, uncheck this box. |
Inputs
Media Texture Compositing Input
This Pass is responsible for routing video into the compositing system. It requires a media texture to reference. Its Media Source is processed through the Pass's Material.
Without a Media Source, the media Pass will be blank. However, you can set up a default still image in your game config file: [/Script/Composure.ComposureGameSettings] StaticVideoPlateDebugImage="/Game/Path/To/My/TextureAsset"
Texture Input
This Pass provides you with an easy way to source static textures into the compositing system.
Transforms
Transforms are responsible for taking input image(s) and spitting out a processed image. Traditionally, this is where compositing is done – a Material Pass that references sub-Elements, combining them.
Compositing Element Material Pass
This Pass lets the user author a custom material, in which they can reference other Elements/Passes. This is the bread-and-butter of the compositing system.
Compositing Post Process Pass
This Pass applies a set of post processing effects on the previous Pass (inoperable if it's the first pass).
Only certain effects are available (Bloom and Tonemapping). Users can create more effects by creating a sub-class of ComposurePostProcessPassPolicy.
Compositing Tonemap Pass
This Pass applies finishing post-process tonemapping on the previous Pass.
This is helpful for converting images back from linear color space. It is useful as a preview transform, or on an output Pass (intermediate material operations expect linear colors).
Multi Pass Chroma Keyer
This Pass is used by the Media Plate Element to key an image. The Key Colors property specifies the color(s) to turn transparent.
If needed, you can switch out the material for your own chroma keyer using the parameters listed in the Required Material Parameters field.
This Pass runs multiple times, once for each key color.
Multi Pass Despill
This Pass removes chroma bounce from the image ("spill" from the green screen).
Much like the Chroma Keyer Pass, you specify Key Colors that you want removed. Also like the Chroma Keyer Pass, this Pass runs multiple times – once for each key. You can switch out the material for your own despill process (it just requires the parameters listed in the "Required Material Parameters" field).
You can find information on chroma keying and despill in UE4 in this blog post.
OpenColorIO Pass
This Pass is used to apply a color conversion (color space transform) onto the input image pixel colors.
Outputs
Output passes define a destination for the Element's fully processed image to be broadcast to. Some outputs perform a transform of their own before routing the image.
Compositing Media Capture Output
This Pass routes the Element's result to a media capture target. The Capture Output field expects a media output asset, which is a configuration file detailing where to route the image to (capture card, port ID, pixel format etc.).
This Pass has a Color Conversion transform associated with it, that runs prior to outputting the image.
EXRFile Compositing Output
This Pass will save a .EXR image file to your hard drive for each frame the Element renders.
This Pass will start writing out images (one for each frame) as soon as it has an Output Directory. For more control, disable the Pass first.
Player Viewport Compositing Output
Use this Pass to take over the player's viewport in game, and replace the display with the Element's result.
This Pass has a Color Conversion transform associated with it that it runs before displaying the image. Play in Editor to see it in action.
Render Target Compositing Output
This Pass will write the Element's result to a render target.
Use the Render Target field to specify which asset.