Guillermo Espertino writes: > I didn't know that PS compression scale doesn't follow the jpeg > specification. There is no such specification for a compression scale or quality factor. Inside an JPEG image, what actually defines the lossiness of the compression are a set of so-called "quantization tables". A quantization table is a table of 64 8- or 16-bit integers. (Typically 8-bit values are used.) Typically, one such table is used for the "Y" channel (luminance, i.e. "B&W" information) and another for the Cb and Cr channels (colour information). These tables are actually stored inside each JPEG image. (There are not some standard one(s) that would implicitly be referenced to by some single index that would say "use standard table 1" or something like that. In retrospect, that might have been a good idea I think.) I.e. what actually determines the lossiness and "loss style" of the compression, what information is thrown away, are 128 (or just 64, or even 192) numbers (!). Not a single quality value. (It is in fact even possible to use different quantization tables for different parts of an image, I think. I have no idea how common such JPEG images are, and if any software in common use produces such.) The single quality value 1..100 that GIMP uses is passed to libjpeg's jpeg_set_quality() function. This function is used to scale two sample quantization tables given in the JPEG specification. But nothing forces some application to use linear scalings of these sample quantization tables. I don't know if the sample tables are even normative in the standard, or just informative. One might imagine some application even doing a clever analysis of an individual image to come up with image-specific quantization tables. The website http://www.impulseadventure.com/photo/jpeg-quantization.html seems to have a good collection and comparison of quantization tables used by different firmware and software implementations of JPEG compression. http://markcox.googlecode.com/svn/trunk/jpegdump/jpegdump/ has sources to a nice program one can dump the contents of a JPEG file, if one wants to have a look at the quantization tables used. There is another program with the same name at http://www.programmersheaven.com/download/17054/download.aspx that is less useful in general, but has one nice feature: it attempts a guess at the libjpeg-style quality factor used for the quantization tables. --tml _______________________________________________ Gimp-developer mailing list Gimp-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
