Impressive shows annoying white stripes in gradients. How can I get rid of them?
This is a common problem with a whole bunch of PDF viewers and results from the way gradients are stored in PDF files: They aren’t. In PDF versions prior to 1.5 (I guess), gradients have to be »simulated« using a series of rectangular »strips« whose colors are interpolated from the gradient start color to the gradient end color. Almost every application produces PDFs that way. The problem arises if the PDF display application tries to apply fancy anti-aliasing to smooth the edges of text and lines. Usually, this is done by an algorithm called »multisampling« that smooths the edges of each object individually. So, if one of the gradient strips ends in the middle of a pixel, the antialiasing would blend that pixel towards white (the page background color). But then, the next strip is rendered, and it (of course) starts in the middle of the very same pixel. So, the renderer blends the strip’s color with the color that is already in the buffer — the one that has already been blended with w
