Exported jpg file size

[gcellison]

I’ve searched for a previous thread on the topic, but none appear with an answer using my keywords (jpg size) back through 2016. So, I would like to ask if anyone might shed some light.

I am experimenting a bit with photo editing in P&GD. The photos will be for a website. I am familiar with saving jpegs at lower resolution to reduce the file size. I am familiar with the optimization process. Etc.

My objective is to set a benchmark of sorts by comparing different levels of export quality with the corresponding file sizes. So, I opened a photo (4592 x 2576; 8.93 MB (9,371,648 bytes)) in the photo editor (not a Xara project file) at its original size. (Mind you, I have no intention of using images at that size… I am just trying to establish a benchmark free of modifications outside of quality reduction.) I made absolutely no modifications to the image, and then I exported it at the default 96dpi. (Correct me if I’m wrong, but I assume that the software opens it at the default resolution as well, meaning that there should be no modification in that regard.) Note that saving the image rather than exporting it gives the exact same result.

As expected, exporting at 85% quality renders a smaller file: 4.29 MB (4,506,047 bytes), and at 75%, smaller still: 3.22 MB (3,382,589 bytes). What puzzles me is that exporting at 100% quality increases the file size substantially: 14.9 MB (15,712,983 bytes)… almost 70% bigger.

Theoretically, one would expect that the image should remain identical and therefore have the same size. But this is obviously not the case. Does anyone know why? Is this typical of all photo-editing software? Or am I missing something?

Thanks for any advice.

[handrawn]

a jpeg is compressed - the compression is not [necessarily] linear - most of that compression is between 85- 100% quality

[gcellison]

a jpeg is compressed - the compression is not [necessarily] linear - most of that compression is between 85- 100% quality

Thanks, handrawn.

If I understand well, that means that the Xara software uncompresses the original image file when it is loaded, but (by default) it does not re-compress it on save / export (at least in the same way). Including a quality reduction in the export would be required for that. I suppose the obvious ramification is that to achieve the original file size inherently means loss of quality. Thanks for the insight.

[handrawn]

xara certainly uncompesses PNG when they are loaded, and to the best of my knowledge [I hardly ever use them in xara] it does for JPG, it would not really make sense otherwise

xara is set to export JPG at 85% by default - you can change this on the options tab of the export dialog

85% compression is generally considered the 'sweet spot' when compressing JPG to reduce file size because, as you have noticed, there is a significant reduction in file size, but not too much loss of quality - below that you start to risk meaningful issue with artifacts

check in the bitmap gallery - you should see the file size of the loaded JPG - does it equate to 100% [no compression]

[Egg Bramhill]

My reading of it is that you import a jpg that has a predefined compression when & wherever it was exported from. Lets say it was exported at 80% and you load it into Xara.

You can either set the compression to be greater (60%) and Xara exports a smaller file size.

However you can also set the compression to a lesser value (100%). This then creates a very large filesized image. However it's pointless doing so as the original image quality remains unchanged. You can't add detail to the original image.

[gcellison]

Thanks, handrawn and Egg.

The default quality reduction is something I’m aware of and always use, as well as optimization when it’s time for the final export for the website. As I explained, my objective was to make a comparison of difference levels of controlled export quality reduction. I suppose I naively assumed that loading an image and exporting it at 100% quality would render the identical image.
The photos, by the way, came directly from a camera; so, any compression is part of the camera’s internal algorithm. Admittedly, I do not use the RAW format, as it would result in much larger files… but perhaps it would be a better indicator of how Xara processes the images.
In any case, I think I have a better understanding now of how the photo editing processor works, so I can make my decisions on that basis.

Many thanks again.

[handrawn]

jpg is a lossy format - you cannot put back what has been removed by compression - unlike a png which you can uncompress without loss - if what egg says is true, and he will know better than me, then every time you resave a jpg [unless at 100% ?] you degrade it

one of the reasons I never use them, except for a backgraound maybe that is static within the animation

[Egg Bramhill]

Hi again Gary. I think you'd be better off restarting the exercise with a raw file. Save it as a jpg without compression (100%) and check the files size. Repeat again with the original raw file and save as 95%, 90%, 85%, 80% & 75% etc.

HD, resaving a jpg many many times has no reduction of image quality. It will only degrade if you alter the photo either by adjusting hue, contrast etc or you save at a compression different from the original at each change.

[gcellison]

Hi again Gary. I think you'd be better off restarting the exercise with a raw file. Save it as a jpg without compression (100%) and check the files size. Repeat again with the original raw file and save as 95%, 90%, 85%, 80% & 75% etc.

Thanks Egg. In fact, if I could go back in time, perhaps I would shoot at least some of the photos in RAW format precisely the have a definitive, lossless, uncompromised image. Alas, hindsight is 20/20. They were taken several years ago before I knew anything of the added potential of using RAW.

[handrawn]

@EGG

well yes, resave it after changes - why would you need to resave it if no changes had been made ?

https://www.lifewire.com/jpeg-myths-and-facts-1701548