Masks
A mask decides where something happens rather than what happens. SlopMotion has one general masking system - the Mask composite on a group - and a handful of layers that carry a mask shape of their own.
The Mask composite
Set a group's Output blend mode to Mask and the group stops being an image. Its rendered buffer becomes a luma stencil: bright areas of the buffer are the hole, dark areas are solid.
That means anything you can build inside a group becomes a mask. A Shape Layer becomes a moving window. A Pattern becomes a torn-up stencil. A Text Layer becomes type that the video shows through.
Mask needs a real buffer to work from, so the group's Start must be Black, Clip, or the other group. A group started on Chain runs inline and has no Output section at all.
Controls
Picking Mask reveals five extra controls next to the group's Amount:
| Control | What it does |
|---|---|
| Reveal | What the stencil lets through - Below (an earlier state of the chain) or Black. |
| Reach | With Reveal set to Below, how far back up the chain that earlier state is sampled from: All goes to the picture before any effects ran, Prev takes the step immediately before, and 2 – 6 count further back. |
| Threshold | The luma cut-off in the group buffer. Defaults to 0.35 - lower it to open the hole up, raise it to keep only the brightest areas. |
| Softness | Edge falloff either side of the threshold, 0.01 – 0.5. Small values give a hard cut; large values give a feathered vignette. |
| Invert | Swaps which side of the threshold is the hole. |
Amount still fades the whole thing in. At 0 the mask does nothing and the picture passes through untouched, which makes it a good target for a band or a trigger - the frame tears open on the kick and seals up again.
The two Reveal modes
Black is the simple one: the bright parts of the buffer keep the current picture, everything else is cut to black. Use it for hard graphic shapes on a dark stage.
Below is the interesting one: the bright parts of the buffer punch an earlier state of the chain back over the current one. Set Reach to All and the mask reveals the untreated clip through the middle of a heavily processed frame. Set it to Prev and you reveal the picture as it was one step ago, which is a very cheap way to isolate a single effect to one shape.
A worked example
- Add a second group and set its Start to Black.
- Drop a Shape Layer inside it, set to a filled circle.
- Set the group's Output blend mode to Mask, with Reveal on Below and Reach on All.
- Stack whatever you like on the main chain above the group.
The circle is now a porthole showing the clean clip through the effects. Map the group's Amount to the kick and it blinks; map the Shape Layer's size to the bass and it breathes.
Layers that mask on their own
Not every mask needs a group. Several layers carry their own:
- Mask - procedural shapes (lines, circle, square, noise, perspective) applied directly to the chain, with its own four-mode blend.
- Video Map - uses a second clip as the stencil. In Mask mode the second video's bright regions punch through, with the same Threshold / Softness / Invert idea.
- Video Window - a picture-in-picture cut-out showing another moment from the same clip.
- Luma Lock - not a mask exactly, but it finds the bright regions a luma mask would key on, and draws them.
- Vignette - the simplest radial mask there is.
Where to go next
- FX groups - how to get a buffer worth masking with.
- Blend modes - the other six ways two images can meet.