In the last Episode I looked under the hood of JP(E)G and PNG. This time it gets a bit more practical – which is better for what?
I tackle two examples from the GIMP Magazine web site and test, if they would be better saved as JPG or PNG. The Plugin “Save for Web” is really usefull for this task.(The image for this blog entry is a PNG by the way, showing JPG compression artifacts. As a JPG it would be five times the size. )
I “developed” a method for comparing two layers – just set the top layer mode to “difference”, make a new layer from visible and check that with the threshold tool for pixels, that are not completely black. After locating the problematic zones in an image with this tool, one can decide what settings are “good enough”.
Conclusion: It depends. It depends on the file, your use case, your level of “good enough” and your compassion for people on a mobile device in EDGE-Hell.
The show starts with a little extension of the last show, Pascal mentioned some options for saving a JPG file that I had overlooked.
00:00:00 Start of video
00:01:00 Progressive mode in JPEG
00:04:09 Progressive mode is not fully supported by browsers
00:04:23 Optimized mode
00:06:17 The quality setting
00:07:09 GIMPMagazine and MTG header image – PNG or JPG?
00:09:23 Checking for quality loss in JPG
00:10:03 Comparing two layers with difference mode
00:10:48 Using the histogram for analysis of the amount of difference
00:11:25 Locating the differences
00:13:50 Trying 85, 75 and 90 as quality settings
00:16:13 When in doubt, compare different settings
00:16:36 Save your work as XCF.GZ
00:17:12 Second example – a drawing
00:23:19 Stay at 4:4:4 for subsampling with photos
00:25:16 Final words of wisdom
00:26:07 End of video
Meet the GIMP Video Podcast by Rolf Steinort is licensed under a Creative Commons Attribution 3.0 Unported License.
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