On Monday 24 Feb 2003 1:22 pm, Jan Wilson wrote: > * Anne Wilson <[EMAIL PROTECTED]> [030224 07:00]: > > I thought that png was a lossless compression - am I wrong? > > No, you are right. > > > If I start from a jpg file from my camera, 397 KB, why does saving it as > > a png come out at 2.4MB? > > Because .png IS a lossless compression. Your camera is using a lossy > format, .jpg, to reduce your images to 397 KB. If you then save it as > a .png, you are saving the already lossy image in a lossless format, > resulting in a much larger file. The .png you make does not contain > any additional information than what was in the original .jpg, so it > is questionable how useful that is. > > So why use .png at all? Well, I use it a lot for screenshots for > training. One thing .jpg does NOT do well is represent typical > application program screenshots. Areas that should appear all the > same color often have "artifacts" (distortions) when saved in .jpg > format. Actually, photos do too, but usually they are less obvious. > > Another good use for a lossless compression like .png is when you will > be using a photo editor to edit the file. If you edit a .jpg, you > take an already lossy image, edit it ... all photo editors I know use > a lossless compression technique internally, at least while they are > editing the image ... and then save it. If you save it as a .jpg, the > image is compressed in a lossy manner, and these losses can > accumulate. The GIMP's native format, .xcf, is lossless, and can get > HUGE, but that's the expense of using lossless but better quality > compression.
Though I suppose that if I want to make progressive saves while working on a picture, then it would be well worth it, but then make the final save in jpg format again? Anne -- Registered Linux User No.293302
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