3.3 Projection surfaces — Throwcast Manual

+ Surface asks the same way. A surface is a screen: its width is measured along the surface rather than across it, and it can be made rear-projection.

The Add a surface menu — Figure 3.3a The surface shapes. **Flat** is a plane; **Curved** is an open arc; **Cylinder** is that arc closed into a 360° tube; **Dome** is that same arc swept round instead, zenith up.

A test card projected onto a curved surface, its grid bending around the curve — Figure 3.3b The demo project’s curved screen: 12 m of screen on a 5.7 m radius, lit by two machines from across the room. **Content preview is switched on here** (§6.3) — an unlit surface photographs as a black rectangle, whereas the test card’s grid bends around the curve and the trapezoid shows the tilt’s keystone. Metrics fit a tangent plane at the point the beam actually hits.

The shape can be changed at any time from the inspector, and doing so keeps the surface’s position, name and any annotations pinned to it.

The rule

Switching shape conserves width — how much screen you have never changes. Only the radius moves, because a closed tube holding a given width of screen has exactly one radius (width / 2π). A cylinder therefore shows you a radius field rather than a width field that would snap back.

Figure 3.3c The surface inspector. The shape buttons sit above the dimensions; the fields a shape does not use are hidden rather than disabled, and their values are kept.

Domes

A Dome is the curved screen swept round rather than extruded: a planetarium. It is described by two numbers — its radius and its dome angle, which is the angle the trade quotes. 180° is a hemisphere; more than that is an overshoot dome, wrapping below the spring line to a narrower rim; 360° closes it into a sphere, at exactly the width that closes a cylinder. Under the two fields the inspector reads back the rim diameter, how deep the bowl is, and how much screen that comes to.

A test card projected onto a dome by three machines, its grid bending in two directions — Figure 3.3d A 12 m dome — R 6 m at 180° — with three machines at the cove throwing across it. **Content preview is on** (§6.3), and the test card is what makes the shape read: its grid bends in *two* directions at once, where a curved screen bends it in one. The dark band above the rim is simply where no image lands. The screen is set to rear projection so the interior reads from outside — the rig is inside a shell the camera is not.

A dome is placed by its centre, not its skin — as a cylinder is placed by its axis. The object’s origin is the centre of curvature, with the zenith straight up from it, which is where the audience sits and where a survey works from. So a tilted dome is just the object rotated, tilting about the point it really tilts about, and a dome of radius r sits exactly on a cylinder of radius r if you want a silo. Height is not a dome’s to set — a surface of revolution has none — so that field is hidden, and the value comes back if you make the surface flat or curved again.

The inspector for a dome — Figure 3.3e The same inspector, on the dome in Figure 3.3d. Width and Height are gone; **Radius** and **Dome angle** stand in their place, and the derived row reads back the rim diameter, the depth and the screen area that pair comes to. Compare Figure 3.3c: one panel, four shapes, and only the fields a shape really has.

Everything else is unchanged: a doorway is a cutter pushed through it, the heatmap and content preview land on it, the probe reads it, and metrics fit a tangent plane at the point the beam hits.

Rear projection

A surface is opaque by default: it catches light on the face turned toward each projector, and you see that light only from the same side. Turning that off makes it a rear-projection screen — light passes through, and any projector lights the face you are looking at. Occlusion does not change either way; a screen blocks light from both sides regardless of which face shows the image.