prism docs · Designing · Masks

Masks

A mask is a shape that erases instead of drawing. Put one over a surface to cut a hole in it, such as a window in a wall or the gap round a door handle, or turn it inside out to keep only what is inside. A mask never shows anything itself.

WhereDesign ▸ Add ▸ Mask, or K

Cutsthe surface or group you had selected, or everything below it

Invertkeeps only what is inside the mask

ModeSimple Draw a mask (and K)
Advanced mask quad, circle, triangle, polygon, Pen mask

1 · Add a mask 2 · What a mask cuts 3 · Invert: keep only the inside 4 · Soft edges 5 · Draw and reshape masks 6 · Masks on the canvas and in the list 7 · When its surface is gone 8 · Masks on a mesh-warped surface 9 · Surface to mask, and back 10 · Hints and tips 11 · When something looks wrong 12 · Mouse and keyboard

1Add a mask

Select the surface (or group) you want to cut, on the canvas or in the list.
Press Add ▸ Mask and choose its shape: Mask quad (K), Mask circle, Mask triangle, Mask polygon, Draw a mask (freehand) or Pen mask (points and curves). Right-click the canvas ▸ Add mask has the same list.
The mask appears, already cutting the surface you selected. Drag it into place and pull its corners like any shape.

The Add menu with the Mask submenu open — Add ▸ Mask: four shapes, then Draw a mask and Pen mask. In Simple mode the submenu has Draw a mask only; `K` still adds a mask quad.

2What a mask cuts

Select a mask and the inspector shows its Mask settings. Cuts says what it erases.

The Mask settings: Cuts, Invert and Soft edge — Cuts, Invert and Soft edge.

A round hole cut in an aurora wall, the door beside it untouched — A mask circle that cuts **Wall**: a round hole in the aurora. The Door is not cut, even where it's near.

A surface or group (the usual case): it erases only that surface, or every surface in that group.
Everything below it: it erases everything drawn below it in the list. At the top of the list that leaves black; inside a group, what is below the group shows through. A mask added with nothing selected starts this way.

The mask selected: its dashed outline on the canvas, its row under the Wall, its settings on the right — The mask selected. On the canvas: a dashed coral outline and its name. In the list: its row sits under the Wall it cuts. On the right: its shape and **Mask** settings; a mask has no look.

3Invert: keep only the inside

Turn on Invert and the mask does the opposite: it keeps only what is inside its shape and erases the rest of the surface it cuts.

The mask erases inside: a hole — Invert off: a hole.

The mask inverted: only a round window of the wall remains — Invert on: only the round window of the Wall remains. The Door is not cut, so it still shows.

4Soft edges

Soft edge (0 to 10 px) fades the mask's edge instead of cutting it hard. A soft round mask, inverted, makes a spotlight or vignette.

5Draw and reshape masks

Draw a mask works like Draw: draw the outline in one stroke and it becomes a clean shape. Pen mask works like the Pen.

A hand-drawn mask cutting a curved shape out of the door — A mask drawn freehand with the Door selected: it cuts the Door, so the aurora on the Wall behind shows through the hole. The mask's smooth points have handles.

Masks are shapes, so everything from Shapes and curves works: drag corners and points, double-click an edge to add a point, nudge with the arrow keys, curve an edge.

6Masks on the canvas and in the list

On the canvas a mask is a dashed coral outline with its name in coral.
The Masks button appears in the canvas tool bar when this projector has masks. Turn it off to hide their outlines while you judge the picture; hidden masks can't be clicked, except the selected one.
In the list a mask that cuts a surface sits right under it, indented, with a dashed coral border. Its second line says what it does: “Mask · Quad · cuts Wall”, “masks everything below”, “inverted”.
An attached mask moves with its surface in the list; it has no place of its own. To make it cut something else, change Cuts.

7When its surface is gone

If you delete the surface a mask cut, or ungroup the group it cut, the mask hides itself so it can't erase the wrong thing. Its row reads needs a target with a warning sign.

A mask whose surface was deleted: the warning in the list and the inspector — The Door was deleted, so the mask that cut it says “Its surface or group is gone”. Pick what it cuts, or press Keep it unattached. (The note at the bottom offers to undo the delete.)

Select the mask.
Choose a surface or group in Cuts, or press Keep it unattached to let it mask everything below it. It shows again.

Deleting a group deletes the masks that cut it too. Deleting a single surface keeps its masks, flagged like this.

8Masks on a mesh-warped surface

When the surface a mask cuts has a mesh warp, the mask bends with the picture: each of its points keeps its place on the picture as you bend the grid. Turn off Bends with the mesh to keep the mask where it is on the projector instead.

A round mask bent along with the wall's mesh — Bends with the mesh on (the default): two grid points were dragged and the hole bent with the picture.

The same bend with the mask staying round — Off: the same bend, but the hole stays round, where it was on the projector.

The Bends with the mesh switch in the Mask settings — The switch appears only when the mask cuts a surface with a mesh warp.

9Surface to mask, and back

Any shape can become a mask: Make it a mask at the top of its shape settings, or right-click ▸ Make it a mask. Its layers go, and it masks everything below it until you choose what it cuts.
A mask's Make it a surface turns it back into a surface. It starts dark: pick a look.
Surfaces from older pixel scans can't become masks.

10Hints and tips

Cut, don't build round. A wall with a window is one wall and one mask, not four strips.
Name your masks (F2 in the list): “Window” and “Handle” beat “Mask 3”.
Mask a group to cut every surface in it with one shape, for example the faces of a box.
Spotlight: a soft circle mask, inverted, over a busy wall.
Hide the outlines with Masks when you judge the picture on the projector.

11When something looks wrong

You see	Why	Fix
Nothing is cut	The mask cuts another surface, or it is hidden.	Check Cuts, and the eye in its row.
Everything under the mask went black	It is unattached: it erases everything below it.	Set Cuts to the surface you meant.
A mask vanished and its row has a warning	The surface or group it cut was deleted or ungrouped.	Choose Cuts, or Keep it unattached.
A mask can't be clicked	Masks is off in the tool bar.	Turn it on, or select the mask in the list.
A mask won't take a look	Masks only erase; they have no layers.	Make it a surface.
The hole doesn't follow the mesh	Bends with the mesh is off, or it cuts a 3D object's face (not supported yet).	Turn the switch on.

12Mouse and keyboard

Action	How
Add a mask quad (cuts the selection)	`K`
Other masks	Add ▸ Mask · right-click the canvas ▸ Add mask
Reshape	Drag corners and points · double-click an edge · arrows to nudge
Delete	`Delete`
Rename	`F2` in the list