The last episode was for absolute beginners, this one is for Geeks. I try to explain (and understand on the way) how images are stored in PNG and JPEG files. PNG (pronounced “PING”) does this lossless, the image can be retrieved in the same quality as the original. PNG works wonders with graphics with a lot of lines and clear colour areas, comics and logos for example, but it creates monster files out of photos and similar images. JPEG looses details, aquires artefacts and generally mangles the image. But it has so beautifully small files and the losses are in most cases invisible – except in the area where PNG is good. So both have their niche to live in.
How is this done? I try to explain this without the math, using analogies, plaing with GIMP to reenact some stages and reducing the complexity a lot. If you want to know the exact facts, read up in Wikipedia, which was also my source of information, or look for other sources. I hope that I never crossed the border between simplification and telling wrong stuff – but I am really not sure. The math is really over my head, last time I had to tackle such a level a Pentax ME Super was still a new camera model. I am happy about any comments that improve my understanding – and all other comments too.
00:02:22 Basics about digital images – pixel, RGB
00:05:25 Storing the colour values in the computer
00:06:24 Palettes for reducing the image size
00:06:50 Run length encoding as simple compression method
00:07:20 PNG, JPG and a WARNING
00:08:44 The PNG format
00:10:12 HexDump and structure of a PNG image
00:13:32 Compression in PNG
00:15:15 PNG is suitable for …
00:16:26 JPG analysed – file structure
00:18:13 First stage: Change the colour model to YCbCr
00:18:54 Decomposing an image to YCbCr in GIMP
00:20:32 Reduce the colours
00:22:20 Testing the concept with GIMP
00:26:17 Splicing into blocks and DCT
00:31:20 Testing the compression
00:35:19 Final words
00:36:10 End of video
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