What Causes Loss of Resolution

[BobH]

When uploading pic(.jpg, .png, etc) files, I've noticed that uploaded images lose resolution. In the sample image attached, the blurring was not present in the file itself. If it matters the image was exported from SketchUp and cropped using FastStone Image Viewer.

Tower Fan Wiring.png

[HansV]

Moved here from Windows General.

[HansV]

What happens if you download the picture again (right-click it and select Save Image As...), and then view the downloaded picture?
Is that lower quality than the picture that you uploaded?

[stuck]

When uploading pic...

Uploading to where? This forum or somewhere else?

Ken

[ChrisGreaves]

When uploading pic(.jpg, .png, etc) files,

Hi Bob. Uploading (or downloading) a file cannot cause loss of resolution. The file, with a checksum, is transcribed faithfully, in your case all 114,128 bits of it.
Any loss of resolution must come about by a Process applied to your file data.

Such a process might be a browser, opening the file and making it ("processing it so that it is ...") suitable for display on a screen.
Obvious examples of loss of resolution arise when an image editor or viewer(!) loads the data to a screen view and then saves it back to disk. Viewer? I am hedging my bets here because I have seen Windows Photo Viewer save a rotated image. Maybe Windows Photo Viewer just has a 2-bit field to signal rotation, but maybe it saves the file after remapping all the bits.

Most processing of image data should have a form of entropy where things can only degrade to a worse state. Except for FBI image processing to get the registration plate of the getaway vehicle, but although Shannon's information content might go up, the image data is still changed on the file which is the point here.

The folks over at Audacity have several posts on audio-file degradation. Audacity processes audio files in 32-bit floating-point, so converting (say) and MP3 file to 32-bit fpt, and then simply saving it as MP3 under a new name will cause rounding errors. They advise, of course, maintaining the original track in FLAC format until one is ready for the final saveAs MP3.

Your resolution issues must be caused by your image editors, browsers, or any other program that processes your file data after it has been uploaded.

Cheers, Chris

[stuck]

...Uploading (or downloading) a file cannot cause loss of resolution...

I disagree, some websites will adjust an image to be in keeping with the site's requirement, e.g. Facebook. That is why I asked Bob about where he is uploading to.

Ken

[BobH]

To answer Hans' question: The image I posted was exported from SketchUp and saved as a .png file. I was that .png file that I uploaded here. I became less sharp overall. I can view it in FastStone and other apps without the apparent fuzziness I see here using Firefox 113.0.2.

To answer Ken's question: I've seen this on at least 3 different webpages where I've posted images.

Although I can't say definitively, I think that this usually happens when I've saved an image and then posted that saved image or in this case created an image by exporting then posting the exported image.

I'll go to the test area later and try several different sorts of images to see if I can pin anything down.

[ChrisGreaves]

...Uploading (or downloading) a file cannot cause loss of resolution...

I disagree, some websites will adjust an image to be in keeping with the site's requirement, e.g. Facebook. That is why I asked Bob about where he is uploading to.

Hi Ken. I understand this point (about Facebook modifying the image to suite Facebook's purposes), but I was trying to make a distinction between the upload of a file and the processing of a file.
In the Facebook example, I suspect that that is akin to Audacity opening a file - say an MP3 track - and then converting it (processing it) into 32-bit floating-point format. Here it is an Audacity Process, but is not a file Upload.

Can we still be friends?

I am being somewhat pedantic in this, but my eye was caught by Bob's "Uploading" rather than "Opening" I think of "Uploading" as rather like an FTP transfer - it moves a file as a string of bits, without regard to the file's contents.

I daresay someone can come up with an application that uploads-and-processes as a result of a single command, but then I would say that this was a Process (that incorporated an upload) rather than an Upload.

Cheers, Chris

[HansV]

When you upload a picture to Eileen's Lounge, the file is stored on the server "as is". But the image displayed in the post may be downsized (you can then click on the image to see a larger version).

Messages apps such as WhatsApp and Signal severely compress images and videos attached to messages.

[ChrisGreaves]

Messages apps such as WhatsApp and Signal severely compress images and videos attached to messages.

I have no experience with WhatsApp and Signal , but this sounds to me like "process then upload", which is a Process.

On sending a message with an attached video, first the video is compressed, and only then is it transmitted ("uploaded")
Cheers, Chris

[BobH]

When you upload a picture to Eileen's Lounge, the file is stored on the server "as is". But the image displayed in the post may be downsized (you can then click on the image to see a larger version).

Messages apps such as WhatsApp and Signal severely compress images and videos attached to messages.

Seems I'm too easily confused these days. Do you mean to say that I can upload an image of any size and that the Lounge software will accept and store it but will resize it for display? I know that other forums do this, but I thought there was a 256k limit on images uploaded to the Lounge.

[HansV]

There *is* a 256KB limit on the file size, but if you upload a picture of, say, 1920 by 667 pixels, the post will display it much smaller (at least on Windows and Mac, it may not work on phones).
See A green path. The image displayed in the post is only 600 pixels wide, but if you click on it, it will be enlarged (the exact size will depend on the size of the browser window).

[stuck]

...exported from SketchUp and saved as a .png file. I was that .png file that I uploaded here. I became less sharp overall. I can view it in FastStone and other apps without the apparent fuzziness I see here using Firefox 113.0.2...

If I download view the file you uploaded here and open it in IrfanView it looks as fuzzy as it does when I view it within your post using Firefox but given that the file is a mere 14 KB I wouldn't expect a razor sharp image.

When you say you can view the image in Fastsone and other apps are you sure you are opening the tiny 14 KB file you uploaded here or are you actually viewing a 'master copy' / the original file exported from SketchUp before you resized it for upload?

...Although I can't say definitively, I think that this usually happens when I've saved an image and then posted that saved image or in this case created an image by exporting then posting the exported image...

Again that's what I'd expect unless you are careful about the save / export settings you selected. The greater the compression required to get an image down to a particular file size, e.g. The Lounge's 256 KB limit, the greater the number of bits that have to be chucked away and that means image quality will suffer.

Ken

[BobH]

Ken, the file that I view in FastStone is the same file that I upload to the Lounge. It was cropped after exporting it as a .png file of 14 kb.

Just now, I exported the image from SketchUp again, this time as a .jpg file of approximately 1950 x 950 pixels. I then used FastStone IV to crop it to a 527 x ?? image retaining .jpg and it comes to a file of 77kb. Here is the uploaded result:

tower fan redux cropped.jpg

It is a better image than the first but some sharpness has been lost.

[HansV]

For images like that, consisting of text and lines and only a few colors, I use .png format. It results in a better quality/size ratio than .jpg.
For photographs, on the other hand, .jpg is much better than .png.