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Thread: Downsampling filtering

  1. #1

    Default Downsampling filtering

    Hello,

    Irfanview is really great piece of software for viewing! I however noticed, that standard settings on common monitors (no 4k etc) are a little bit suboptimal when viewing higher megapixel photos.

    So I have experimented a bit and found great combination for viewing

    1) enable resampling, resampling when zoom and sharpen after resampling in the windowed view mode
    2) with sharpening, set the smallest (1) magnitude of this filter for viewing
    3) use Hermit filter, disable use fast filter for downsampling.

    If anyone wants to have higher sharpness he or she can use 5,7 or 10 for sharpen magnitude, but for me the smallest value on my two monitors (desktop and notebook) works well.
    Also Lanczos filter can be tried but for downsampling it is probably unnecessarily advanced and does little aliasing and ringing effects.

    It is very good that Irfanview can do Hermit filtering!

    With these settings the results are better than standard settings.
    Last edited by jumpingjackflash5; 13.04.2020 at 03:04 PM.

  2. #2

    Default

    Just an addendum after some time, it seems that on different displays different sharpening levels look good. So feel free to experiment a little with Sharpening effect from 1 to 10 to get subjectively best results on your device :-)

  3. #3

    Default

    And one more addendum, I also found out that using Triangle, Mitchell or even Lanczos filter without any post-sharpening works nicely for downsampling/viewing images. Sharpening can be disabled with unchecking "Apply sharpen after resample" in View menu.
    Of course, the display results are also a matter of personal preference (the degree of sharpness, the accuracy versus appearance of photos etc), therefore any user can select a variant that he likes the most, also based on his monitor device. I am just mentioning the possibilities that work reasonably for me. Actually I realized that downsampling is a complex issue with many opinions on filtering results, also discussed on forums of different image software, sometimes deeply.

    Still, the options in Irfanview are awesome, the user can change the display according to his/her preference.
    Last edited by jumpingjackflash5; 13.04.2020 at 06:30 PM.

  4. #4

    Default

    I have found new nice downsampling filter combination - Mitchell filter and Sharpening level 5. I tried it for downsampling from 20 megapixel Canon camera photos to 1920x1080 monitor.

  5. #5
    Power User j7n's Avatar
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    Default

    I find that the algorithm choice is mostly dictated by the image content and available CPU speed, not the monitor device. In general, photographs look good with Lanczos or Cubic, which have good anti-aliasing properties but slow speed. The oversharpening halos are usually not noticeable amongst small details and texture, or do not occur if the photo is large with only low frequencies. Simple, artificial images look better with weighted average or triangle (linear) or mitchell, which are all positive kernels, trading aliasing versus sharpness. With real-time zooming, the resampling should be simple with little lag on scrolling at the expense of quality, unless one has a the latest technology computer. IrfanViews on-the-fly resampling isn't very efficient.

  6. #6

    Default

    Thank you for reaction.
    Yes I agree that image content and CPU speed is primary. I just showed the examples when I am using it and under which conditions the results seem good to me. I now use Mitchell/sharpness 5 on all my devices, except one where I use sharpness 7 (I usually view some documents there).
    Yes Lanczos is recommended on several websites as optimal for photos. But I like other algorithms more.
    Cubic produces a little bit blurry results, so for my experience the sharpening afterwards would have to be much higher to achieve sharp results.
    On majority of todays computers the zoom is fast enough even with Mitchell algorithm and zooming. My main concern is how the photos will look like when browsing through the folder with a series of photos and for that purpose speed is very good.

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