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The default compression method for the image's pixel data is the value specified for /display/openexr/compression in rendermn.ini, which may take the value of none, rle, zip, zips, pixarpiz, pxr24, b44, b44a, dwaa or pizdwab; if not specified therein it defaults to zips. A lossy compression (b44a, dwaa, and dwab) will confuse denoising, to the point that you will have regions in your denoised image that appear to not have any denoising on them at all - the problem gets worse as you increase samples in the region - and can introduce very slightly negative values (eg -0.00001).

You can select a different compression method by adding a "compression" argument to the Display command. For example:

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