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Author Topic: Downsampling High Resolution Images  (Read 6899 times)
Ron AKA
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« on: July 18, 2014, 03:57:36 AM »

I will try and make a long story short. I have been puzzled for some time as to why high resolution images that have been downsampled by Internet Explorer at times appear to be oversharpened. It showed up most recently in a thread at the DPReview forum where I pushed an image with extra, but to my eye acceptable sharpening. I did the basic development in Capture One Pro 7, and then some pixel editing in Photoshop Elements Editor. In both applications the image looked very good to my eye. Sharp but not too sharp. However when the high resolution image was displayed in Internet explorer with the View Original feature at DPReview, it looked brutally bad - way oversharpened. That prompted me to get to the bottom of it, and I made a post on the issue there.

http://www.dpreview.com/forums/post/54017256

Again, to make a long story short, while I had some trouble accepting it, the answer seems to be that Microsoft is using Nearest Neighbor or something very close to it, for downsampling, and it causes brutal artifacts which look like sharpening (on certain images more than others). To convince myself of the issue I used Nearest Neighbor, Bilinear, Bicubic, Bicubic Sharper, and Bicubic Smoother to make a downsampled image that could be displayed on the screen at the same size without any resampling. Nearest neighbor was almost identical and as brutally bad as the image downsampled by Internet Explorer. There was a small variation in the rest of the methods with Bicubic Sharper being the sharpest ( but quite acceptable) to Bicubic Smoother as the smoothest and still quite acceptable. What I saw in Capture One Pro and Elements would likely have been about in the middle of those -- probably close to just Bicubic.

So now my question about Qimage Ultimate. What method is being used to interpolate high resolution images to display them in the editor? And the reason I raise the issue is that in the process of this investigation I decide to look at this problematic high resolution image in Qimage Ultimate. I have to be blunt. It looked oversharpened. Perhaps not quite as bad as Nearest Neighbor in IE11 or the high res image downsampled by IE11, but frankly not acceptable. It simply did not match what I saw in Capture One Pro or Photoshop Explorer. I did print the image in Qimage using the default DFS at level 5, and it printed fine -- just as I saw it in Capture One Pro and Elements, but not as I saw it in Qimage Editor. I also opened the images I had downrezed using the methods I listed above, so they could be displayed in Qimage Editor without resampling. They all displayed fine with the minor but acceptable variations in sharpness.

I also think back to my attempt to use Qimage Ultimate for image editing a few weeks back, and becoming frustrated with the sharpening and noise reduction. I now think what I was seeing was not necessarily real noise and sharpening, but potentially artifacts caused by the method used to dowrez the image in the editor.

If you want to look at the images, you can find a couple of them in the post at the link below. The second image is my original high resolution variation. The last one is a downsampled Bicubic version. Night and day difference if you view them in Internet Explorer, and Qimage as well...

http://www.dpreview.com/forums/post/54042200
« Last Edit: July 18, 2014, 02:45:20 PM by Ron AKA » Logged
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« Reply #1 on: August 04, 2014, 03:17:25 PM »

Since we've been talking in the other thread, I figured I'd switch to this one to talk about your specific images.  I looked at the DPreview examples and I'm not sure I know what I'm looking at.  I see one image that looks reasonable and another that has been ruined by ridiculous oversharpening.  The one you labeled "post processing" has so much sharpening added that it has created incredible amounts of noise.  That noise can "clump" when images are downsized using a fast downsampling routine, making the downsampled version look pitted.  Are we talking about the post processed image or the original (which looks reasonable and not so much noise)?

I also noticed that you probably sharpened in a program other than Qimage Ultimate because if you sharpen at that level in QU using DFS, you won't get all those bright specular spots that are so bright it looks like one of the petals actually has white spots on it.  The noise may still be enhanced with any sharpening method but at least DFS won't create blown out (white) specular dots like USM.

Just want to make sure what we're dealing with here.  Super-noisy images can always look a bit odd and speckly when downsampled for screen resolutions.

Mike
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