feature: edgeDepth, smoothing and snapAbove added to PictureEffect

[Gohnnyman]

What's new

Extends PictureEffect's area sampling with the border-strip part of #138.

• edgeDepth — the outermost lights reach this far into the picture instead of their own sliver at the edge
• smoothing — each light eases toward its new colour over several pictures, which settles a noisy source
• snapAbove — a move bigger than this lands whole, so a cut stays a cut
• 5 tests on top of the existing 6: the depth, edgeDepth 0 as a no-op, exact convergence, a snapped cut, and no smoothing following tick-for-tick

Note

smoothing and snapAbove are almost no-op at this point as these settings could only be observed on dynamic changes.
Right now PictureService doesn't implement Video/USB MJPEG source, but when It'll be added, these setting will take effect

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[ewowi]

Hi @Gohnnyman, thanks, this is exactly the shape we hoped for: one topic, two files, on main, and five tests that read as what the controls do. The edge depth on the existing area ranges, and the 8.8 state that keeps fractions of a step between pictures, are both nicely done.

Your branch allows maintainer edits, so we'll add one commit for the remaining points and merge:

1. Smoothing by time, not by tick. While a light is still easing, the effect redraws every render tick, so the easing advances per tick rather than per picture. A 30 fps capture on a 100 fps render then settles three times faster than on a 30 fps render. We'll make it the textbook exponential smoothing: a half-life in milliseconds, scaled by the time since the last frame, so it settles at the same speed on every source and every board.
2. edgeDepth's help says a deeper edge "costs reads in proportion", but the area average samples each light at a stride (about 16 reads), so it costs about the same. We'll correct the line.
3. American spelling: "colour" becomes "color" in the help and the test comment.
4. Credit: "Hyperion samples ~8%" moves into the header as prior art.

One thing for later, on our side: the smoothing state is sized per grid cell, so a backlight frame keeps state for the empty middle too. Sizing it by the lit lights needs a list of a layer's lit cells, which is core work of ours.

As you noted, smoothing and snapAbove only show their worth on a moving source, so the natural next step is the USB capture from #139: a fresh PR on main that adds the grabber as a PictureService source, with the points from our earlier comment (Teensy stubs, no 3 s freeze without a grabber, the USB tasks on core 1, the UVC header length from its flags, the flash cost measured, mm::formatTo). Then #139 can close.

Thanks again!