Projector View Blending

Projector View Blending is a per-surface option on a projection surface that makes the surface semi-transparent in the projector view, so a projector can “see through” it to whatever is positioned behind. It is set in the screen’s output properties, and the surface’s blend must be set to Alpha for it to take effect.

It is the basis for techniques that deliberately project past a front surface, such as casting light onto scenery behind it, or fading a downstage curtain away to reveal the scenery upstage.

A recent update in r32.0.3 changed how a surface with Projector View Blending enabled is treated, making these transparent-surface techniques possible. This page explains how the feature works, what it is used for, and how it differs from the legacy behaviour.

How it works

When a projector renders its view, it works out which surfaces and objects fall within its line of sight and in what depth order. Normally, surfaces in front occlude whatever sits behind them, just like a real projector.

With Projector View Blending enabled, the surface no longer occludes the objects behind it. The projector treats it as transparent and “sees through” it to the scenery, performers or other surfaces upstage. The surface’s own content still blends into the output according to its alpha: opaque pixels appear as content or light, while transparent areas reveal whatever is behind.

When to use it

Use Projector View Blending wherever you want to project past a transparent front surface onto what is behind it:

• Light Casting: Position a transparent surface in 3D space and use it as a virtual gobo. Its opaque pixels cast light, patterns and colour washes onto the scenery behind it, reusing your projectors as flexible virtual lighting fixtures.

• Downstage Curtains: Keep upstage scenery projection-mapped continuously while a front curtain fades in and out. Because the projector sees through the curtain to the scenery behind, you can cross-fade the curtain away with no visible “snap” as it is raised.

Legacy and new behaviour

Before this change, a Projector View Blending surface acted as a solid occluder: it blocked the projector’s view of anything positioned behind it. Scenery, performers and Mask Objects upstage of the surface were hidden from the projector, so you could not project through the surface onto them, and they could not appear on the surface. The new behaviour treats the surface as transparent instead, which is what makes the techniques above possible.

	Legacy behaviour	New behaviour (default)
Projector View Blending surface in the projector’s view	Acts as a solid occluder	Treated as transparent (see-through)
Scenery / performers behind the surface	Hidden from the projector	Visible; can be projected onto
Mask Object upstage of the surface	Occluded; does not appear on the front surface	Visible; can appear on the front surface
Casting light/content through the surface	Not possible	Supported (See: Light Casting)
Downstage curtain cross-fade	Not possible	Supported (See: Downstage Curtains)

Caution

Because the projector now sees past a Projector View Blending surface, a Mask Object placed upstage of the surface is no longer occluded by it and can be drawn onto the front surface. This can look like an unexpected shadow of an upstage object falling on a front surface. For example, the silhouette of an upstage set piece showing up on a front cloth. If your show relies on a front surface hiding the objects behind it, use the legacy behaviour below.

Switching to the legacy behaviour

The new behaviour is the default. If your show depends on the legacy behaviour, you can restore it:

1. In d3 Manager, open Advanced Machine Settings.
2. Enable the useLegacyProjectViewBlending option switch.
3. Restart d3 for the change to take effect.

This restores the previous behaviour for every Projector View Blending surface in the project.

Note

The useLegacyProjectViewBlending option is available from r32.4.10 and r33.2.2 onward.