coverFitScaleOffset function

({double offsetX, double offsetY, double scale}) coverFitScaleOffset(
  1. int sourceW,
  2. int sourceH,
  3. double viewW,
  4. double viewH,
)

Cover-fit scale + offset for rendering a source region of size (sourceW, sourceH) into a viewport of size (viewW, viewH).

Preserves aspect ratio and centers; the source is scaled to cover the viewport (by the larger of the width/height ratios), so the axis that overflows is centered with a zero or negative offset and the other axis offset is zero. The record (scale, offsetX, offsetY) is what a CustomPainter typically needs to transform source coordinates to viewport coordinates: x_view = x_source * scale + offsetX.

Implementation

({double scale, double offsetX, double offsetY}) coverFitScaleOffset(
  int sourceW,
  int sourceH,
  double viewW,
  double viewH,
) {
  final double sourceAspect = sourceW / sourceH;
  final double viewAspect = viewW / viewH;
  if (sourceAspect > viewAspect) {
    final double s = viewH / sourceH;
    return (scale: s, offsetX: (viewW - sourceW * s) / 2, offsetY: 0.0);
  }
  final double s = viewW / sourceW;
  return (scale: s, offsetX: 0.0, offsetY: (viewH - sourceH * s) / 2);
}