// The brush's own physics, shared by FX's two tools: HEAL copies a patch of the
// photo over what it paints, MOSAIC hides what it paints, and both lay circular
// spots from the same gesture — a circle that rides the cursor, a stroke laid
// down as the pointer travels, and a wheel that sizes it. What a spot MEANS is
// each tool's own business (heal.ts, mosaic.ts); how big it is and how far apart
// a stroke lays them is this file's.
//
// The bounds are fractions of the photo's width. Past BRUSH_MAX_R a spot is a
// quarter of the frame — a repair would be borrowing a patch from off the
// photo, and a mosaic block would be enormous — and below BRUSH_MIN_R the circle
// is finer than the pixels it is drawn on at preview scale, which is a size the
// user cannot see well enough to aim.
export const BRUSH_MIN_R = 0.003;
export const BRUSH_MAX_R = 0.25;
// How far apart a drag lays its spots, in radii: any closer and the run is the
// same spot written twice, which is work for nothing.
export const BRUSH_SPACING = 0.6;

// The brush radius after a wheel notch. Multiplicative, so the brush grows by
// the same proportion wherever it starts, and bounded as above — a trackpad's
// small deltas and a mouse's 100px notch are then the same gesture at two
// speeds.
export function wheelBrushR(r: number, deltaY: number): number {
  const next = r * Math.exp(-deltaY * 0.0015);
  return next < BRUSH_MIN_R ? BRUSH_MIN_R : next > BRUSH_MAX_R ? BRUSH_MAX_R : next;
}