private void drawGuidelines(Canvas canvas) {
    if (mGuidelinePaint != null) {
      float sw = mBorderPaint != null ? mBorderPaint.getStrokeWidth() : 0;
      RectF rect = mCropWindowHandler.getRect();
      rect.inset(sw, sw);

      float oneThirdCropWidth = rect.width() / 3;
      float oneThirdCropHeight = rect.height() / 3;

      if (mCropShape == CropImageView.CropShape.OVAL) {

        float w = rect.width() / 2 - sw;
        float h = rect.height() / 2 - sw;

        // Draw vertical guidelines.
        float x1 = rect.left + oneThirdCropWidth;
        float x2 = rect.right - oneThirdCropWidth;
        float yv = (float) (h * Math.sin(Math.acos((w - oneThirdCropWidth) / w)));
        canvas.drawLine(x1, rect.top + h - yv, x1, rect.bottom - h + yv, mGuidelinePaint);
        canvas.drawLine(x2, rect.top + h - yv, x2, rect.bottom - h + yv, mGuidelinePaint);

        // Draw horizontal guidelines.
        float y1 = rect.top + oneThirdCropHeight;
        float y2 = rect.bottom - oneThirdCropHeight;
        float xv = (float) (w * Math.cos(Math.asin((h - oneThirdCropHeight) / h)));
        canvas.drawLine(rect.left + w - xv, y1, rect.right - w + xv, y1, mGuidelinePaint);
        canvas.drawLine(rect.left + w - xv, y2, rect.right - w + xv, y2, mGuidelinePaint);
      } else {

        // Draw vertical guidelines.
        float x1 = rect.left + oneThirdCropWidth;
        float x2 = rect.right - oneThirdCropWidth;
        canvas.drawLine(x1, rect.top, x1, rect.bottom, mGuidelinePaint);
        canvas.drawLine(x2, rect.top, x2, rect.bottom, mGuidelinePaint);

        // Draw horizontal guidelines.
        float y1 = rect.top + oneThirdCropHeight;
        float y2 = rect.bottom - oneThirdCropHeight;
        canvas.drawLine(rect.left, y1, rect.right, y1, mGuidelinePaint);
        canvas.drawLine(rect.left, y2, rect.right, y2, mGuidelinePaint);
      }
    }
  }
Draw 2 veritcal and 2 horizontal guidelines inside the cropping area to split it into 9 equal
parts.
core/src/main/java/com/theartofdev/edmodo/cropper/CropOverlayView.java