title: How can I get a transparent background in ImageJ/Fiji
body: <p>I have a hexagonal image, stitched together from several single images, which is saved as a rectangle 8bit TIFF with black corners. I want to set the black corners to transparent. Because I want to stitch several hexagons together. I tried already the threshold function and set the black areas to NaN but after reopening the image the corners were still black. I also tried making a selection and clear outside which resulted in a white background. 
  I need to save it as a TIFF file again for further processing.
  Does somebody has an idea how to solve that issue?</p>