rectAt method
Sprite sheet дотор тухайн thumbnail-ийн байршил.
sheetWidth, sheetHeight — бодит зургийн хэмжээ (pixels).
Implementation
Rect rectAt(Duration position, {double? sheetWidth, double? sheetHeight}) {
final tw = sheetWidth != null ? sheetWidth / columns : (_thumbWidth ?? 160);
final th = sheetHeight != null ? sheetHeight / rows : (_thumbHeight ?? 90);
final g = gridAt(position);
return Rect.fromLTWH(g.col * tw, g.row * th, tw, th);
}