Projection mapping on a Mac, from an empty room

Most introductions to projection mapping start with content — here is a nice loop, here is how to warp it onto a wall. That order is backwards, and it is why first jobs take three times as long as they should.

Start with the surface. Everything else is downstream of a decision you make with a tape measure.

Step one: measure the surface, in the surface’s own units

Before you open any software, get the real dimensions of what you are projecting onto. Not the room — the surface. Width, height, and the position and size of anything you must avoid or land on precisely: a doorway, a window, a company logo, the join between two flats.

Write it down in millimetres. You will convert to pixels later, and you want one authoritative set of real-world numbers to convert from.

While you are there, note two things that are easy to forget and expensive to discover:

• Is the surface flat? A wall that looks flat and bows by 40mm in the middle is a mesh warp job, not a corner pin job. Sight along it.
• What colour and texture is it? Projection onto grey brick is a different design brief from projection onto white scrim. Dark and textured surfaces eat contrast, and no amount of software fixes that.

Step two: place the projector before you build anything

The projector’s position determines the shape of the correction you will have to apply, and the amount of resolution you throw away applying it. Every degree off-axis costs you pixels: the software has to squeeze part of the image to make the geometry right, and the squeezed part is softer than the rest.

So the priority order is physical, then digital:

1. Get the projector as square to the surface as the venue allows. On-axis, centred, at the right distance.
2. Use lens shift before you use keystone. Lens shift moves the image optically with no loss. Keystone is a digital resample. If the projector has shift, spend it first.
3. Use the projector’s own geometry tools next, if it has decent ones. Warping in the projector leaves your canvas clean.
4. Warp in software last, for what is left, and for anything the projector cannot express.

Rigging is not a formality here. Twenty minutes spent getting the projector square saves an hour of warping and gives a visibly sharper result.

Step three: build a canvas that matches the surface

Now go to software, and build a canvas whose aspect ratio matches the surface you measured. Not the projector’s native resolution — the surface.

This is the step that separates jobs that go smoothly from jobs that fight you. If the surface is 6000mm × 2400mm, that is 2.5:1, and your canvas should be something like 2500 × 1000 or 3000 × 1200. Design content to that canvas and it lands correctly without anyone having to think about it. Design to 1920 × 1080 because that is what the projector does, and every piece of content needs a mental transformation applied before you can judge it.

Pick the actual pixel dimensions by working out how many projector pixels land on the surface. If a 1920 × 1200 projector covers the whole 6000mm width, that is roughly 0.32 pixels per millimetre, so a 2500-pixel-wide canvas is slightly over-sampled and safe. Going much higher wastes GPU for no visible gain; going much lower means you are upscaling.

In SpectraMap the canvas is the first thing you set up and the thing everything else hangs off — outputs, masks and warps are all defined as regions of it. That is deliberate: the canvas is the shared coordinate system between the designer making content and the tech pointing projectors at a wall.

Step four: mask before you warp

Masking is deciding which parts of the canvas produce light at all. Warping is deciding where that light lands. Do them in that order, because masks are much easier to draw on an un-warped rectangle.

What to mask:

• Anything that should stay black. Gaps between flats, a doorway people walk through, the floor below the surface.
• Spill onto things that are not the surface. Audience faces, a lectern, the ceiling. Spill is the single most common reason a map looks amateur.
• Objects in front of the surface. A truss, a speaker cluster, a plant.

Do this with the projector live and the house lights off, adjusting against what you can actually see, not against a plan.

Step five: warp, and stop early

Now correct the geometry. On a flat surface with a reasonably square projector, a four-corner pin will do it and you should not reach for anything more. On a curve, a segmented surface, or a wall with a bow, you need a mesh — a grid you can push locally.

The rule that saves the most time: get the edges right and accept the middle. Audiences read the boundary of a projection against the physical object. They do not notice a 5mm error in the centre of a flat field. Chasing centre-field perfection with a mesh warp usually introduces waviness that is more visible than the error it fixed.

Use a grid or a set of alignment marks as your test pattern rather than content. Content hides geometry errors, which sounds good and means you find them during the show.

The choice between corner pin and mesh has more to it than “flat or curved” — we went through it properly here.

Step six: only now, content

With a canvas that matches the surface, masks that define the shape, and geometry that lands, content becomes a normal design job. Build to the canvas dimensions, keep anything important away from the edges, and check it on the surface early rather than at the end.

Two practical notes:

• Test with the worst content, not the best. A slow dark loop hides everything. A hard-edged white grid moving quickly shows you every geometry error, every seam and every frame drop.
• Check it from where the audience will be. Mapping looks correct from the projector’s position by definition. It is the audience’s angle that reveals whether the surface was really flat.

Where the pixels come from

Mapping software has to get content from somewhere. On a Mac that is usually one of three routes: media files played by the mapper itself, a live source arriving over NDI from another machine, or a Syphon hand-off from another application on the same Mac — a media server, a generative patch, or a captured window via SpectraBridge. Which one to reach for is covered in NDI or Syphon?

If the content is generative rather than a file, TouchDesigner is the usual answer on a Mac and it publishes Syphon in both directions — sharing frames with TouchDesigner covers the hand-off.

What matters at this stage is that the source’s resolution should meet or exceed the region of canvas it lands on. Upscaling a 1280-wide source across a 2500-pixel canvas is the most common reason a map that is geometrically perfect still looks soft.

The short version

1. Measure the surface in millimetres.
2. Rig the projector square; spend lens shift before keystone.
3. Build a canvas matching the surface’s aspect ratio.
4. Mask what should stay dark.
5. Warp the edges; leave the middle alone.
6. Then make content, and test it with a hard grid.

Do them in that order on your first paid job and it will look like it was not your first.