Dividing the centering shift by top_scale (previous commit) avoided the
strong-zoom black border, but made the shift a nonlinear function of the
scale value being interpolated -- mid-animation, the pan no longer
tracked the zoom at a constant rate, which read as jerky/skippy motion.
Reverted shift to the original linear formula, (1-s)*(ART_SIZE/2), which
moves smoothly in lockstep with Scale's own eased interpolation. Instead,
capped ZOOM_LEVELS["strong"] at 1.45 (down from 1.7) -- the highest value
where this plain linear formula stays safely inside the art's bounds,
verified with the calibration tool (no black border across the full
animation cycle, at both 1.45x and against a live station).
The centering Translate formula, (1-s)*(ART_SIZE/2), was derived assuming
it's applied in final output pixels. That held up at a moderate zoom
(1.4x) but pushed past the image's actual edge at a strong one (1.7x) --
a real, on-device black border in the bottom-right corner, confirmed with
the calibration tool (true black RGB(0,0,0) pixels, not just dark
content).
Dividing the shift by the scale factor keeps the crop inside the image's
bounds at every zoom level the app offers, verified via the calibration
tool at 1.0/1.15/1.4/1.7x and against a live station -- at the cost of
the most extreme (1.7x) setting being very slightly off-center rather
than perfectly centered, which is a trade worth making since going
outside the art's bounds is worse. Moderate and subtle remain visibly
centered.
dev/calibration.png is a synthetic grid (concentric color rings, corner
markers, a crosshair) and dev/calibration_test.star runs it through the
same zoom logic as the real _art_widget, so cropping can be checked
visually without depending on what a live station happens to be playing.
This is what was used to find and verify the centered-zoom fix.
poprhythm