calculateScanWindowRelativeToTextureInPercentage static method

Rect calculateScanWindowRelativeToTextureInPercentage(BoxFit fit, Rect scanWindow, { required Size textureSize, required Size widgetSize, })

Calculate the scan window rectangle relative to the texture size.

The scanWindow rectangle will be relative and scaled to widgetSize, not textureSize. Depending on the given fit, the scanWindow can partially overlap the textureSize, or not at all.

Due to using BoxFit the content will always be centered on its parent, which enables converting the rectangle to be relative to the texture.

Because the size of the actual texture and the size of the texture in widget-space can be different, calculate the size of the scan window in percentages, rather than pixels.

Returns a Rect that represents the position and size of the scan window in the texture.

Implementation

static Rect calculateScanWindowRelativeToTextureInPercentage(
  BoxFit fit,
  Rect scanWindow, {
  required Size textureSize,
  required Size widgetSize,
}) {
  // Convert the texture size to a size in widget-space, with the box fit
  // applied.
  final fittedTextureSize = applyBoxFit(fit, textureSize, widgetSize);

  // Get the correct scaling values depending on the given BoxFit mode
  var sx = fittedTextureSize.destination.width / textureSize.width;
  var sy = fittedTextureSize.destination.height / textureSize.height;

  switch (fit) {
    case BoxFit.fill:
      // No-op, just use sx and sy.
      break;
    case BoxFit.contain:
      final double s = min(sx, sy);
      sx = s;
      sy = s;
    case BoxFit.cover:
      final double s = max(sx, sy);
      sx = s;
      sy = s;
    case BoxFit.fitWidth:
      sy = sx;
    case BoxFit.fitHeight:
      sx = sy;
    case BoxFit.none:
      sx = 1.0;
      sy = 1.0;
    case BoxFit.scaleDown:
      final double s = min(sx, sy);
      sx = s;
      sy = s;
  }

  // Fit the texture size to the widget rectangle given by the scaling values
  // above.
  final textureWindow = Alignment.center.inscribe(
    Size(textureSize.width * sx, textureSize.height * sy),
    Rect.fromLTWH(0, 0, widgetSize.width, widgetSize.height),
  );

  // Transform the scan window from widget coordinates to texture coordinates.
  final scanWindowInTexSpace = Rect.fromLTRB(
    (1 / sx) * (scanWindow.left - textureWindow.left),
    (1 / sy) * (scanWindow.top - textureWindow.top),
    (1 / sx) * (scanWindow.right - textureWindow.left),
    (1 / sy) * (scanWindow.bottom - textureWindow.top),
  );

  // Clip the scan window in texture coordinates with the texture bounds.
  // This prevents percentages outside the range [0; 1].
  final clippedScanWndInTexSpace = scanWindowInTexSpace.intersect(
    Rect.fromLTWH(0, 0, textureSize.width, textureSize.height),
  );

  // Compute relative rectangle coordinates,
  // with respect to the texture size, i.e. scan image.
  final percentageLeft = clippedScanWndInTexSpace.left / textureSize.width;
  final percentageTop = clippedScanWndInTexSpace.top / textureSize.height;
  final percentageRight = clippedScanWndInTexSpace.right / textureSize.width;
  final percentageBottom =
      clippedScanWndInTexSpace.bottom / textureSize.height;

  // This rectangle can be used to cut out a rectangle of the scan image.
  return Rect.fromLTRB(
    percentageLeft,
    percentageTop,
    percentageRight,
    percentageBottom,
  );
}