Started Nov 7, 2012 | Questions thread
Detail Man
Detail Man Forum Pro • Posts: 16,688
Re: Sharpness ?!

Ulric wrote:

Detail Man wrote:

Ulric wrote:

There's enough material here for an entire conference.

After a bit of experimenting, I've found that the forum software does this: The uploaded or linked image, it doesn't matter which, is resized and stored in several versions. All of these are unsharp, no matter what the original looks like. It doesn't look like regular jpeg artifacts, they are simply unsharp.

When the picture is viewed inline (in the forum post) one of these unsharp copies is displayed and then again resized by the browser, further reducing sharpness.


My tests indicate that it is likely that the in-post displayed images are down-sampled directly from the stored original uploaded JPG image. The re-sampling algorithm may well be Lanczos-3, 2x2 (vertical as well as horizontal) Chroma Sub-sampling is performed, and the Quality Factor was around 89 early on with the new system, but not long after became around 90, instead (which represent CSS and QF values that are identical to the DPR Image Gallery "Large", Medium", and "Small" viewable, and download-able, images).

The evidence that the in-post displayed JPGs are not down-sampled twice is that there is no measurable color-shift distortion that occurs in them (relative to the uploaded original) - whereas the "Large", Medium" and "Small" versions all show varying amounts of RGB color-shift artifacts (which would appear in the tested in-post displayed JPGs, as well). However, that is not the case. See the results of the RGB color-shift artifacts tests here:

I would provide a direct link to the post, but the DPR system is largely down for maintenance.

What (I think) it is that people are seeing is that while the "Large" size DPR Image Gallery JPGs are down-sampled so that the largest (horizontal or vertical) pixel-dimension is 1200 pixels, the in-post displayed JPGs are down-sampled so that the largest (horizontal or vertical) pixel-dimension is a smaller 940 pixels (representing a significantly larger amount of down-sampling).

Since (many) LCD flat-screen monitors are only around 100 pixels/inch physical resolution, the displayed physical size is smaller, and additional pixelize-ation effects take place when viewing 940 pixel maximum sized images that add further distortion arising out of the display medium.

When viewing images in the previous system, clicking twice on the images used to display them at the same 1200 pixel (maximum) dimension as the "Large" size DPR Image Gallery JPGs.

The new system restricts the displayed pixel-size to 940 pixels maximum, instead.

Believe it or not, surveys as recent as Spring of 2012 (of the monitor screen-sizes in use on systems as reported by internet browsers to web servers) indicate that a clear majority of screens are 1024x768 pixels (or even smaller). This is very likely due to the preponderance of laptops and other compact display devices which are used to view images on the internet.

While a case could be made that restricting the maximum displayed pixel-size of images to 940 pixels (or even less than that) would result in the least amount of (browser/viewer) down-sampling required to occur when such images are viewed on laptop and compact devices, a choice to restrict displayed images to these small pixel-sizes seems quite adverse to viewers using larger sized (typically 1200 pixel-height, and in some cases higher) flat-screen monitors in a non-portable, non-mobile application (such as my own HP ZR24w monitor).


Hi DM and welcome back.
The url to the in-post images look a bit like this:
Obviously a program that fetches the file and resizes it. The resized image is cached; to not do so would be terribly slow. This is the reason why removing the original image does not remove it from the post.

Yes, it seems that the in-post displayed images are distinct from the uploaded original JPG images.

However, despite the resized image being at most 940x940 pixels, it is displayed by the browser at a larger size. I took a screenshot of the image and it was 1129x637 pixels. This final resizing (upsampling :-O) is done by the browser using an algorithm and at a quality over which Dpreview have no control whatsoever. Of course, the image that was already unsharp from the first resizing can only be even less sharp after this final upsampling.

I pasted the URL that you cite above into Firefox 3.6.28, and made a screen-shot of the browser's display. I manually cropped the image from the screen-shot, and it has dimensions of 940x531 pixels. The pixel-size of the image is rather small (at least, on my 1920x1200 screen-size monitor), but Firefox 3.6.28 does not up-sample the displayed image (and appears to have no settings for doing such things under direct user control). What browser were you using that did that ? That does not sound like a very desirable operation. Perhaps that browser has some user-settings that might allow you to disable such things ?

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