#87 noise in black areas

Kevin Egan

from .rib file:

# This attempts to show that there is strange noise in areas that should be
# black, either from no objects (edges of image), or from lack of
# illumination (under side of sphere). I am not sure if the noise is just
# more visible with OpenEXR output, or if exr output is part of the problem.
# If you turn off all lights there is still noise in the image.

Thanks for the quick fix on the last bug!



  • Kevin Egan

    Kevin Egan - 2008-04-07

    rib scene file

  • Okan Arikan

    Okan Arikan - 2008-04-07

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    The problem is in the various display driver's handling of the value quantization (this is not a renderer issue).

    RenderMan interface includes a quantization function that introduces a random noise to the output values for dithering. It seems like OpenEXR and libTIFF are handling this noise (in the color and alpha channels) completely differently. What's worse is a generated tiff file, when opened by various image processing programs, look completely different. Pixie adds the noise to color and alpha channels. Perhaps it only needs to be added to the color. Still trying to test this with PrMan.

    Meanwhile, when generating openEXR images, you can turn off quantization by
    Quantize "rgba" 0 0 0 0
    which will generate a high dynamic range image or
    Quantize "rgba" 255 0 255 0
    which will generate a 8 bit per channel image with no additional noise (the last number is the noise amount).

    Will keep this bug report open until I figure out what's going on in OpenEXR/libTIFF.

    Any help is appreciated.


  • Okan Arikan

    Okan Arikan - 2008-04-07
    • status: open --> open-accepted
  • Kevin Egan

    Kevin Egan - 2008-04-07

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    Originator: YES

    OK, that makes sense, and turning off quantization made the noise go away. Thanks for checking that out.

    This web page from RenderDotC seems to agree with what you have (in terms of quantization being on by default, and quantizing the alpha values as well). RenderDotC adds an extension so that you can set the rgb quantization levels differently than the alpha level.


    It would be nice to double check that prman has the same behavior. How are OpenEXR and the tiff library doing things differently? Let me know what kind of help would be useful.



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