RenderTextureQuad.slice(RenderTextureQuad renderTextureQuad, Rectangle<int> sourceRectangle, Rectangle<int> offsetRectangle, [ int rotation = 0 ])

Source 

factory RenderTextureQuad.slice(
    RenderTextureQuad renderTextureQuad,
    Rectangle<int> sourceRectangle,
    Rectangle<int> offsetRectangle,
    [int rotation = 0]) {

  var renderTexture = renderTextureQuad.renderTexture;
  var pixelRatio = renderTextureQuad.pixelRatio;

  var oldRotation = renderTextureQuad.rotation;
  var oldSourceL = renderTextureQuad.sourceRectangle.left;
  var oldSourceT = renderTextureQuad.sourceRectangle.top;
  var oldSourceR = renderTextureQuad.sourceRectangle.right;
  var oldSourceB = renderTextureQuad.sourceRectangle.bottom;
  var oldOffsetL = renderTextureQuad.offsetRectangle.left;
  var oldOffsetT = renderTextureQuad.offsetRectangle.top;

  var newRotation = (renderTextureQuad.rotation + rotation) % 4;
  var newSourceL = sourceRectangle.left;
  var newSourceT = sourceRectangle.top;
  var newSourceR = sourceRectangle.right;
  var newSourceB = sourceRectangle.bottom;
  var newOffsetL = offsetRectangle.left;
  var newOffsetT = offsetRectangle.top;
  var newOffsetW = offsetRectangle.width;
  var newOffsetH = offsetRectangle.height;

  var tmpSourceL = 0;
  var tmpSourceT = 0;
  var tmpSourceR = 0;
  var tmpSourceB = 0;

  if (oldRotation == 0) {
    tmpSourceL = oldSourceL + oldOffsetL + newSourceL;
    tmpSourceT = oldSourceT + oldOffsetT + newSourceT;
    tmpSourceR = oldSourceL + oldOffsetL + newSourceR;
    tmpSourceB = oldSourceT + oldOffsetT + newSourceB;
  } else if (oldRotation == 1) {
    tmpSourceL = oldSourceR - oldOffsetT - newSourceB;
    tmpSourceT = oldSourceT + oldOffsetL + newSourceL;
    tmpSourceR = oldSourceR - oldOffsetT - newSourceT;
    tmpSourceB = oldSourceT + oldOffsetL + newSourceR;
  } else if (oldRotation == 2) {
    tmpSourceL = oldSourceR - oldOffsetL - newSourceR;
    tmpSourceT = oldSourceB - oldOffsetT - newSourceB;
    tmpSourceR = oldSourceR - oldOffsetL - newSourceL;
    tmpSourceB = oldSourceB - oldOffsetT - newSourceT;
  } else if (oldRotation == 3) {
    tmpSourceL = oldSourceL + oldOffsetT + newSourceT;
    tmpSourceT = oldSourceB - oldOffsetL - newSourceR;
    tmpSourceR = oldSourceL + oldOffsetT + newSourceB;
    tmpSourceB = oldSourceB - oldOffsetL - newSourceL;
  }

  newSourceL = clampInt(tmpSourceL, oldSourceL, oldSourceR);
  newSourceT = clampInt(tmpSourceT, oldSourceT, oldSourceB);
  newSourceR = clampInt(tmpSourceR, oldSourceL, oldSourceR);
  newSourceB = clampInt(tmpSourceB, oldSourceT, oldSourceB);

  if (newRotation == 0) {
    newOffsetL += tmpSourceL - newSourceL;
    newOffsetT += tmpSourceT - newSourceT;
  } else if (newRotation == 1) {
    newOffsetL += tmpSourceT - newSourceT;
    newOffsetT += newSourceR - tmpSourceR;
  } else if (newRotation == 2) {
    newOffsetL += newSourceR - tmpSourceR;
    newOffsetT += tmpSourceB - newSourceB;
  } else if (newRotation == 3) {
    newOffsetL += newSourceB - tmpSourceB;
    newOffsetT += newSourceL - tmpSourceL;
  }

  var newSourceW = newSourceR - newSourceL;
  var newSourceH = newSourceB - newSourceT;

  return new RenderTextureQuad(renderTexture,
      new Rectangle<int>(newSourceL, newSourceT, newSourceW, newSourceH),
      new Rectangle<int>(newOffsetL, newOffsetT, newOffsetW, newOffsetH),
      newRotation, pixelRatio);
}