Jim Snyder wrote: [chop] you can stand a little bit of image quality loss, use ZIP [chop] Hmmmm - this email list needs an FAQ - or some pointers to certain image FAQs on the web now and again. Image compression is a rather complex mathematical process that usually requires some 'dumping' of image data to gain good compression ratios - thus these compression schemes are 'lossy'. Non-lossy compression schemes use LZW type compressors which are good when there is a lot of replicated data in a file - but not so good for images that have a large variation of data components. The problem with most people is the mixup of file formats with compression schemes. For example, TIF can be compressed or uncompressed - it uses LZW to compress - but two TIF files are still called XXX.TIF and YYY.TIF even though one is raw data and one is compressed data. There is no such thing as an 'LZW' extension - only file formats that use it. Ive attached a small HTML doc with some specs. Not exact, but a guide - if anyone wants to add formats then do so. bert Filmscanners archive at: http://phi.res.cse.dmu.ac.uk/Filmscan/Title: Compression
File Extension | Developed for? | Compression Scheme | Effect of compression | % Saving for images | Useable for? |
TIF | image storage | LZW or none | lossless | 15% | archive copy |
ZIP | general file store | fancy LZW | lossless | 18% | archive copy |
JPG
(Joint Pictures expert Group) |
image storage | JPEG | lossy | 80% | non-archive (web!) |
GIF
(good! interchange format) |
image storage | LZW | lossless | 15% | nasty 256 colour only |
PNG
(portable network grahpics) |
image storage | fancy LZW | lossless | 22% | archive copy |
WIF
(wavelet image format) |
image storage | waveform mathematics | lossy | 95% | proprietary |
FIF
(fractal image format) |
image storage | fractal mathematics | lossy | 90% | proprietary |
PCD
(Kodak PhotoCD) |
image storage | fancy JPEG | lossy | 70% | archive (but not perfect copy) |
FPX
(Kodak Flashpix) |
image storage | sortof JPEG / PCD mix | lossy | 80% | non-archive web apps |