when exporting .png images to .jpg I get all black background - never before using latest OS 15.4.1

ribbo39 wrote:

When exporting screenshots in .png format, the images, using Preview app, to .jpg all of a sudden I'm getting a "Black" background.

Yes. Unlike PNG, JPEG images do not support transparency. By default, a screenshot of a window will include the outer shadow as transparent.

The is always an "undefined" problem. What is the colour of a transparent area? The alpha channel certainly has a value, but what about the red, green, and blue channels? In many cases, they are just set to zero. That will show up as black if the alpha channel is removed.

Sometimes, clever apps will try to guess what the transparent colour should be. Apple Preview is a special case. Apple rewrites the entire thing every year and breaks tons of stuff every time. By the time they finally get everything fixed, they do it all over again. So if you use some other app like Pixelmator Pro, you won't have this problem.

Also, PNG files are already quite small. You shouldn't need to save them as JPEG. In addition to losing the transparency, you lose a lot of quality. If you don't need to share these JPEG files with any PC users, you could save them as HEIC instead. You'll still lose quality as HEIC is lossy, but it does support transparency.

Also when opening the screenshots they open in full screen width

I'm not sure what you're asking here.

ribbo39 wrote:

When doing Genealogy research on the net, I select an image  and d/l to my desktop in the png format. I then try to clean up and annotate the image giving it a title and then export from the d/top to the jpg format.

JPG is a lower-quality format. Why are you doing genealogy research in the first place? Why don't you just go to the central store of information where all data was perfectly recorded and passed down to those who needed it?

That's a trick question, of course. You have to do the research because at every point in the past, someone copied down the same information and, every time, introduced an error, or didn't copy all of the information. Maybe they felt it wasn't important. No way to tell. Maybe they didn't bother to print clearly. Maybe they were rushed. Who knows?

But my point here is that, as someone dealing with an information domain plagued by poor data quality, it is your responsibility to Break The Cycle. Stop reproducing data from a high-quality format to a lower-quality format.

When doing so I'm I get a permissions problem that I do not  own the file or have the permission to write to but to duplicate. (images attached)

Maybe you should've led with that.

No problems in the past but have now got a new mac mini (3 days ago) and many problems using  preview.

Now I'm confused. Your original post indicated that this was a habitual practice and that macOS 15.4.1 introduced changes that broke that practice. Now you are suggesting that this a a brand new computer with all new problems? And you throw in the major configuration problem with permissions as an aside? Are you converting to JPEG because you can't change the original PNG?

When I now open d/l documents they usually open almost full screen size.

Let's just ignore that so that we can focus on the more critical problems.

Tried Apple techs but didn't really get any conclusive answer only it must be something to do with resolutions. If so how to alter them, not individually but all?

I don't know who "Apple techs" are. But anyone who saw those screenshots showing a major permissions probelm and then thought that default image window sizes was something they should be spending time on is not someone you want to be consulting with.

Could write more but don't wish to burden anyone.

This is a user-to-user tech support forum for users of Apple products. Probably the single worst thing you can do is omit information.

Consider how we got here. You asked a question regarding minute details about how different image file formats handle the alpha channel when saving to a format that doesn't support transparency. And you were concerned about window sizes or something.

Absolutely none of that matters. Don't ever save PNG files to JPG. Window sizes don't matter. Permissions matter. Please provide more details about how you are downloading these PNG files. What web browser are you using? You said you are downloading to the desktop. That seems to work. But then you are exporting from the desktop. Exporting to where?

Can you show us an example where we can see the before and after?

Where do you get black? You might see black if the png had transparency - because jpeg does not support transparency; but for a screenshot there is no transparency involved.

ribbo39 wrote:

New mac mini 15.4.1.

When exporting screenshots in .png format, the images, using Preview app, to .jpg all of a sudden I'm getting a "Black" background.

Anyone know the answer.

Also when opening the screenshots they open in full screen width

"Exporting" from where? Is there another application involve— certainly not clear

I would look at your settings...

Preview.app>Settings>General... make a change(?)