What’s the difference between a Bitmap and Passthrough composite in rigging?

When creating a rig, there are two main types of composites that are used. This article will outline their differences.

Passthrough Composite Overview

• Allows Z-depth information (and other information such as transparency) to ‘pass through’ the composite layer by layer.
• Heavier than a Bitmap composite due to its multi-layered output.
• Shows as a lighter blue trapezoid in the Node view:

Bitmap Composite Overview

• Flattens Z-depth information like a bitmap image would.
• Lighter than a Passthrough composite as it outputs a single image.
• Shows as a deep blue rectangle in the Node view:

When to use a Passthrough composite in rigging

When rigging, we need to consider what will be needed for animation. For most cut-out animation, different layers and parts will be interacting with each other and may need to move in front, or behind, of another part/layer. 

This could be something simple, like a cut-out character bringing their arm forward from behind their back. Or it could be more complex, like two cut-out characters interacting.

In these cases we don’t want to lose the ability of Z-depth, so Passthrough Composites tend to be the best option for rigs. However, Passthrough composites can also be heavier because of how they filter every layer. 

Passthrough Composite	Bitmap Composite

When to use a Bitmap composite in rigging

Bitmap composites can be used in rigs, but only in areas where flattening will not directly affect the parts that need to move in Z-depth.

Some places where a Bitmap composite can be used inside of a rig:

• When it is being used to collect more than one layer to use as a matte in a matte port (like a Cutter node matte).
• When flattening sections of artwork that are visually part of a single piece, so they do not need to move in Z-depth amongst themselves.
• When the parts connected to the composite are the same colour in line and fill, so the Z-depth movement would go unnoticed. (E.g: Separate eyelashes that are completely black in colour).

Bitmap composites are also extremely helpful for adding effects to rigs because adding an effect directly to a rig’s Passthrough composite will result in partially transparent, overlapping artwork as the effect is being applied to the individual parts.

A Bitmap composite is ideal in this case, as it will flatten the rig’s parts without compromising the setting of the Passthrough composites above. 

In this case, the Bitmap composite should be placed underneath the rig’s main Passthrough composite, but above the effect nodes.

Passthrough Composite	Bitmap Composite