[Gimp-developer] jpeg quality factor.
I'm noticing big differences between jpeg files from gimp and photoshop. The same image exported as jpeg with the same quality factor (let's take 75% as an example) gives very different results in both programs. In my tests, Photoshop image has better image quality, but its size is 75 KB. Gimp image shows lots of artifacts and its quality is lower, but its size is 35 KB. Doing more tests I found that the factor 75% of Photoshop is equivalent to the 93% in gimp. Conclusion: Both programs calculate the compression ratio differently. I'd like to know how the compression factor is set (if gimp or the jpeg library manages that). Most people consider Photoshop as a industry standard (which is not, but is a de facto standard) so I'd like to know which program isn't working as it should (I mean, if the qualiy factor is 80%, and the compression algorythm is the same, it sounds ilogical to get different results). I'm worried about that, because one who cames from photoshop to gimp may thing that Gimp jpg files have less quality than photoshop ones. In my personal experience, I find the default compression quality of the jpeg in Gimp to be very destructive. And I don't know if it happens because gimp uses the compression factor from the file and recompresses to its own equivalent. Trying to be more specific: If I open an image from my digital camera with gimp, adjusts its levels or curves, and re-save it, the saved image is very deteriorated. If I do the same with Photoshop that doesn't happen. I think it's problem, but let me know if I'm wrong. At least I know that I must re-save images from other sources using a higher quality factor. Regards, Gez. ___ Gimp-developer mailing list Gimp-developer@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
Re: [Gimp-developer] jpeg quality factor.
Trying to be more specific: If I open an image from my digital camera with gimp, adjusts its levels or curves, and re-save it, the saved image is very deteriorated. If I do the same with Photoshop that doesn't happen. I think it's problem, but let me know if I'm wrong. At least I know that I must re-save images from other sources using a higher quality factor. Interesting, what platform are you using? Here if I can do say 10 re-saves at 85% quality, it produces no discernible changes in picture quality. In fact I have tried to prove that recompressing jpg pictures reduces the picture quality and got bored doing it at 85% (which btw is the Gimp default) However I could degrade quality by repeatedly saving at some lower quality, like 50% Opinion. Y You should never work on a jpeg, take it in off your camera, save it as an xcf and when finished, recreate it as a jpeg if you want. Owen ___ Gimp-developer mailing list Gimp-developer@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
Re: [Gimp-developer] jpeg quality factor.
Mark. Thank you for your reply. I'd like to clarify some of my comments. You obviously have to compare qualities at similar filesizes. Everything else is irreelvant. I don't think the way the quality is expressed (I know, it's not quality but compression ratio) is irrelevant. If you came from a program where you saved at 70% and it gave you a high quality image, when you save at 70% and you get an image with heavy compression artifacts, it matters. Don't get me wrong, I'm not asking do it like photoshop, but for the user it's confusing. The first thing that crossed my mind saving a jpeg with gimp was it sucks at 70%... Had to look at the filesize to figure that the 70% of Gimp was not the 70% of Photoshop. You'll say it doesn't matter, Gimp is not Photoshop, and that's ok. But I'm a designer, worked more than 10 years with Photoshop and switched to Gimp a couple of months ago. I'm sure that I'm not the only one following that path. Maybe it's a good idea to document this difference and/or display a warning in the exporter. It is free software, you can look at how it does things any time you want. I know and I'd love to. But I'm not a coder, just a user. Uneducated and jumpy people might jump to all sorts of conclusions - that generally is their problem. I'm not an uneducated or jumpy person, but I swear that the first thing I thought was something is wrong with the jpeg exporter. Higher than? JEPG images do not store a quality factor, and the very notion of using the same quality is simply not achievable with a lossy format such as JPEG. In Gimp the compression factor is expressed as quality factor. So 100% is the best and 0% is the worst. If it would be labeled as compression ratio, more compression should be the worst. When I mention quality I'm meaning the % selected during the export, not the perceptible image quality. I know it's lossy compression and I know it's impossible to get the same quality. In the terms that the percentage is expressed in the export dialog, the user will think that, in a scale between the best and the worst quality achievable with jpeg compression, a 70% is a 70%. Well, 70% isn't the same in Gimp and in Photoshop. And it doesn't sound very logical. That's what I'm talking about. Regards, Gez. ___ Gimp-developer mailing list Gimp-developer@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
Re: [Gimp-developer] jpeg quality factor.
Owen: Interesting, what platform are you using? Ubuntu Linux (7.04) and Gimp 2.3.18 Here if I can do say 10 re-saves at 85% quality, it produces no discernible changes in picture quality. In fact I have tried to prove that recompressing jpg pictures reduces the picture quality and got bored doing it at 85% (which btw is the Gimp default) I can't say the same. Today my wife uploaded a couple of photos to her flickr, and she noticed the same. http://www.flickr.com/photos/superdd/738669627/ Se only opened the image from the camera, adjusted the curves, and scaled it down (BTW, the downscale code should do oversamplig by default. It always breaks a little the edges). Until she saved, the image quality was good. Then she saved with CTRL+S, without changing the quality factor, and the picture turned out like that. Heavily compressed. Opinion. Y You should never work on a jpeg, take it in off your camera, save it as an xcf and when finished, recreate it as a jpeg if you want. Of course. I always do that. I use XCF (or PNG if the image is a single layer) for work. But usually I take the pictures from my digital camera and make a quick levels and color adjustment, and that's when the problem pops up. If you just want to adjust a bunch of pictures from your camera, it's not very handy to save the pictures as XCF. It takes more space and it's not a very popular format for viewers of other platforms. Gez. ___ Gimp-developer mailing list Gimp-developer@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
