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Thread: Downscaling images - Bicubic-like filter needed

  1. #1

    Default Downscaling images - Bicubic-like filter needed

    Dear IrfanView Community!

    I would like to ask if there is any Bicubic resample filter plugin for IrfanView, because as I noticed IrfanView's (4.25) Lanczos filter is worse when downsizing images (other IV filters [Bell, B-Spline etc.] or simple resize even worse). I've attached two images as comparison about a picture showing block of flats. blocks_bicubic.png (on the left) was downscaled by Photoshop, blocks_lanczos.png (on the right) with IV's Lanczos filter. I'll talk about the third (blocks_imagemagick.png) attachment later. There is no difference at first sight, but try to look more carefully. Lanczos filter causes some sharpness loss at edges. The best way to see the difference is to place them into Photoshop (or GIMP or any software that supports layers) over each other, then start turning the upper layer on/off focusing on the sharp edges. You'll see the difference.


    This third image was created using ImageMagick (IMDisplay under Windows). ImageMagick produces the same image as Photoshop, so it must have Bicubic method of resizing (as mentioned in the manual):


    The original image was 461x352 (was downsized to 320x244).

    Thanks in advance
    stringZ
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	blocks_bicubic.png 
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ID:	2067   Click image for larger version. 

Name:	blocks_lanczos.png 
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ID:	2068   Click image for larger version. 

Name:	blocks_imagemagick.png 
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Size:	140.7 KB 
ID:	2069  
    Last edited by stringZ; 15.01.2010 at 12:25 AM.

  2. #2
    Moderator Sam_Zen's Avatar
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    Bicubic is indeed a good method when decreasing dimensions
    In IrfanView I compensate this often by applying a Sharpen before the resampling.
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  3. #3
    Professional User Skippybox's Avatar
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    They definitely are different, but it is not Lanczos.

    See this post about the special filter IrfanView uses exclusively for downsampling.

  4. #4

    Default

    Bicubic is indeed a good method when decreasing dimensions
    In IrfanView I compensate this often by applying a Sharpen before the resampling.
    Well, I tried this with sharpen values 2 to 14. With value 3 and 4 it's almost the same as Bicubic/Photoshop, with higher values it produces even sharper image than PS but there is some luminance loss at homogene areas, but this is a good idea, I'd use this method until a Bicubic method isn't available.

    They definitely are different, but it is not Lanczos.
    See this post about the special filter IrfanView uses exclusively for downsampling.
    I've read the post you linked and now I see it clearly, thanks.

    I did the same test with upscaling a monoscope image from 640x480 to 1024x768 with Photoshop and all IV filters. Photoshop resized it pretty good, at IV's side Triangle filter (fastest) was the best compared to it. All filters produced poorer quality, this can be seen if you do a little gamma correction/levels filter and get down the gamma. I'm thinking of starting a feature request thread for Bicubic filter as an upscaling/enlarging method.

  5. #5
    IV Beginner Silversleeves's Avatar
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    You want Bicubic? Try My View Pad. Personally I think Lanczos is better on all counts. Spline, Bell and Mitchell are pretty good too.
    I often wish someone would write a Photoshop plug-in to insinuate Lanczos into its image resize, but I'm given to think the filters there are too well-enmeshed in the application to even reach for a tweak or addition. A sharpen-and-export filter might work, though.

    BZT

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