> Correct me if I am wrong, but JPEG is all about > "Lossy" compression - the lower the compression > quality, the higher the loss of information in the > picture (leading to artifacts, etc.). Now at 100%, the > loss may be minimal but there should still be some > compression
Sorry Harmanm, going to have to correct you here. JPEG is all about lossy, but 100% quality does retain all the detail in the image. I don't know how much people here know about Fourier transforms but basically a JPEG stores your image as a bunch of sine and cosine waves. The fewer waves the better the compression but the more approximate the stored image (hence the loss). At 100% the number of sine waves used to store the image is however many it takes to reproduce the original exactly. Typically this is ~4000 waves (90% however will need only ~30 waves and produce something almost indistiguishable). Even at 100% though, this alternative form of storage can prove to be more efficient (for photographs anyway), and so the file still will shrink, even though no information is lost. JPEG falls down on subjects that don't lend themselves to storage as waveforms, such a diagrams with lots of hard edges (high frequency waves). If you were to produce a JPEG at 100% quality for (say) a checkerboard you might find that the image ends up very large (although still probably not as large as the original BMP). Why do BMPs almost always turn out large? Because all the images we tend to care about are (to a certain extent) predictable. Compression algorithms work because they use less space on your hard disk to store the 'predictable bits', since they are 'easy'. BMP uses the same amount of space regardless of whether an image is predictable or not. For example, an all white image is really predictable (I've had 60000 white pixels so far, there's a good chance the next is too), but BMP would still just give the next pixel 8,16 or 24 bits as appropriate for the file. However, for JPEG, an all white image is a flat wave (ie a constant, very low frequency) so only one waveform is required to store it, and it compresses very well. Hope this helps David PS To try and lend credibility to this, I am a software engineer with a background in compression algorithms and data storage. * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
