lessrahkmin
Member
Hello. After i have used STF and then Histogram Transformation, how do i save the file to make it the best image visible to the final image that i can send to others (.jpeg, .png, etc) they all seem to change the original. Thanks
After i change to png or jpeg, gets grainy
- Thread starter lessrahkmin
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fredvanner
Well-known member
After you have stretched your image, you have a high resolution uncompressed 32 bit floating point .xisf image. If you save as a lower resolution, compressed format (e.g. jpeg) the image quality will be reduced. Just how reduced depends on the settings you choose for the target format.
sharpie1178
Member
personally I save to 16 bit TIFF for display purposes or use in other software. if you save as 32 bit TIFF you may find it doesn't display at all.
jpeg or png would display ok on a phone for example or as an online posting but as Fred says, image quality would be lower.
TIFF is a common format that should be ok for sharing to other PC's.
Defo save it as full res .XISF first though. You always want an original final image saved in PI's native format.
jpeg or png would display ok on a phone for example or as an online posting but as Fred says, image quality would be lower.
TIFF is a common format that should be ok for sharing to other PC's.
Defo save it as full res .XISF first though. You always want an original final image saved in PI's native format.
cloudbait
Well-known member
A properly saved JPEG image should look virtually identical to a 32-bit processed image. It is the appropriate format for display and sharing purposes, not generally TIFF.
sharpie1178
Member
.....but will only be 8 bit quality....not great for further processing in other software or in PI. 16 bit TIFF gives that option and can be displayed fine on most devices. it's only an option of course. As you say and in my own experience...jpeg should look virtually identical and is the usual way to share or display an image due to it's smaller file size for uploading.
...suspect Tom may have sorted out the OP's question and this is all academic though
cloudbait
Well-known member
Absolutely. You never want to do any further processing on a JPEG! But the issue here is one of visual appearance, and there's no reason a JPEG should look any different than an uncompressed, high bit depth image on a screen. As you said... the master should be saved as an XISF file, and a display version created from that... almost always in JPEG. 16-bit TIFF format is usually the appropriate file type for moving an image into another app for processing steps that you aren't going to do (or can't do) in PI. PNG is rarely an appropriate display format for photographic imagery......but will only be 8 bit quality....not great for further processing in other software or in PI. 16 bit TIFF gives that option and can be displayed fine on most devices. it's only an option of course. As you say and in my own experience...jpeg should look virtually identical and is the usual way to share or display an image due to it's smaller file size for uploading.
...suspect Tom may have sorted out the OP's question and this is all academic though
fredvanner
Well-known member
Perhaps a bit more detail will help your decisions:
- The XISF image will (unless you mess about with it) have 32 bit floating point data for each colour channel. This is equivalent to 24 bit integer precision. Since you are very unlikely to have a display with more than 8 bits per channel, this is much more than you need for display, but the precision is available for future processing, which can make it visible.
- TIFF can be saved at 16 bit per channel. This allows you to export some of your "excess precision", for example for processing in other apps outside PI. The extra bits will not provide any improvement simply for display.
- PNG provides lossless storage of 8 bit per channel data, which is fine for display purposes (and is my preferred export format unless file size is a constraint).
- JPG provides lossy compressed storage of 8 bit per channel data (it can store lossless, but has no advantages over PNG in that mode). Because the storage is lossy, the image is degraded. You can select the degree of compression to balance image quality against file size.
cloudbait
Well-known member
JPEG also allows for lossless images, and I find that lossless JPEG images are usually smaller than PNGs (and PNGs can be lossy, as well).
About jpg as a final image format, a test I like to do is to save a 32bit xisf to jpg (at 100% quality) then reopen the jpg in Pix and compare it to the original file. It's very difficult (almost impossible) to find any differences, even at a very small scale.
And if you subtract one image from another in Pixelmath, the differences are really marginal
And if you subtract one image from another in Pixelmath, the differences are really marginal
sharpie1178
Member
d'ya know, I've never even saved anything as a PNG apart from maybe a downloaded image. Never even thought to try it to be fair. I've only ever used JPEG's for upload, TIFF's for processing in a certain other piece of software and native .XISF's in PI.
...mostly for the reasons Fred shared above.
...mostly for the reasons Fred shared above.
sharpie1178
Member
Is that the same with PNG do you know ?
cloudbait
Well-known member
Indeed, PNGs are usually a poor choice for images. The format is basically a replacement for GIFs, where you want graphic elements like lines and fonts to be crisp and clear. For photographic imagery (like astroimages) you'll not get any visual difference, but the files will usually be much larger. JPEG is really the appropriate format most of the time for sharing astroimages for viewing purposes.
sharpie1178
Member
I probably read that somewhere about 5 years ago and that's why I don't bother with them. Guess they'd be more appropriate for design than our uses ?
Do we even start bothering to talk about DPI/PPI and screen resolution or is that a long one for another day ?
fredvanner
Well-known member
This is a slightly grey area. The PNG format is strictly lossless (it is usually compressed, but with lossless compression). However, some PNG encoders may support lossy pre-processing (mainly colour reduction). I never use lossy PNG pre-processing (though I may quite separately reduce colour depth), and I have usually found lossless file sizes to be smaller than lossless JPG - but that may depend on other encoder settings.
I don't. And I can't really test it because I encounter this limitation:
future fix - Unable to load file, "Error reading bitmap" — large PNG, JWST public image
Citing the same configurations as this post I am unable to load a JWST public release PNG. The image is STScI-01GA6KKWG229B16K4Q38CH3BXS.png (124.71MB) which can be obtained from https://webbtelescope.org/contents/media/images/2022/031/01G77PKB8NKR7S8Z6HBXMYATG On both Mac and Linux platforms...
pixinsight.com
sharpie1178
Member
that's a limitation I could give no thoughts on. it's well out of my remit. another reason for the OP to not bother with PNG though.