Need help understanding file size difference between GIMP & Photoshop

iggy · **Posted:** Fri May 17, 2019 8:30 pm **(#1)**

GIMP Version: 2.8.14
Operating System: Linux
GIMP Experience: Intermediate Level

Hey GIMPers. I made a file in Photoshop Elements 19. I saved it as a .jpg at PE's "maximum" setting (which is at 10 with the top being 12), which gave me a file of 1.3MB. I opened the file in GIMP (2.8) and cropped it very slightly. (1838X2775-->1800X2700) I automatically went to save it at 100%. The resulting file was 4.3MB. Then I tried a do-over and I noticed that the % in GIMP (for the .jpg save) was set at 93% and when I went with that it produced a file size of 1.2MB. (As if GIMP was tweaking things to give me a similar file size--at least that's the way it seemed.) This cover is for an ebook cover so I want really good without the file being ridiculously large (the 1.2 or 1.3MB range is fine.) But I'm wondering is GIMP's 100% roughly the same as Photoshop's level 12? (I just tested saving the .psd file at the 12 rate and the result was 3.2MB.) Thanks.

P.S. Sad about Tim Conway. Nice of you guys to remember him.

ofnuts · **Joined:** Oct 25, 2010 **Posts:** 4812

There are two things in a JPEG that make the file smaller, but you have to understand how a JPEG is made first.

The JPEG doesn't work in RGB, but, like color TV of yore, with a Luminance channel (which is basically the B&W signal) and two "chroma" channels that carry the color information.

Now, the human eye is much more sensitive to "luma" than to "chroma" so in a JPEG file the chroma images are scaled down (along one dimension (half-size) or two (quarter-size) before being stored. This alone accounts for a large part in the space saving. With quartered chroma, before compression, insead of saving 1+1+1=3 full-size image you save the equivalent of 1.+.25+.25=1.5 image, so before doing any compression, you have reduced the size by two... Of course during decompression the chroma is scaled up. This is the "chroma subsampling" in Gimp's advanced settings.

New you have the compression, which is done with a given precision, a lower precision removing detail from the image.

Then depending on application "Quality" is just the precision, and the chroma subsampling is another setting (as in Gimp) or "quality" is a general setting that combines both, and where it is expected that high quality uses full chroma (but in some apps it is fixed, usually to half-size). An indication is that there is no separate setting for the chroma subsampling.

iggy · **Posted:** Sat May 18, 2019 5:06 pm **(#3)**

ofnuts wrote:

There are two things in a JPEG that make the file smaller, but you have to understand how a JPEG is made first.

The JPEG doesn't work in RGB, but, like color TV of yore, with a Luminance channel (which is basically the B&W signal) and two "chroma" channels that carry the color information.

Now, the human eye is much more sensitive to "luma" than to "chroma" so in a JPEG file the chroma images are scaled down (along one dimension (half-size) or two (quarter-size) before being stored. This alone accounts for a large part in the space saving. With quartered chroma, before compression, insead of saving 1+1+1=3 full-size image you save the equivalent of 1.+.25+.25=1.5 image, so before doing any compression, you have reduced the size by two... Of course during decompression the chroma is scaled up. This is the "chroma subsampling" in Gimp's advanced settings.

New you have the compression, which is done with a given precision, a lower precision removing detail from the image.

Then depending on application "Quality" is just the precision, and the chroma subsampling is another setting (as in Gimp) or "quality" is a general setting that combines both, and where it is expected that high quality uses full chroma (but in some apps it is fixed, usually to half-size). An indication is that there is no separate setting for the chroma subsampling.

Thanks, ofnuts, for the explanation. It's a tad technical for me, but it was very helpful.