On Sat, Feb 25, 2006 at 02:46:33PM -0700, Joseph Tainter wrote: > "> Why not? The DS and DL are able to compress RAW files." > > > "They don't compress. They pack two 12-bit quantities into > three 8-bit bytes (unlike the D, which pads out to 16 bits), > but there's no true compression...." > > Okay. I partly understand this -- the words anyway. Could I > trouble you to elaborate a bit more on how this happens and what > it means?
All the current Pentax cameras use the same sensor, which produces an array of 12-bit values. But the file storage on a memory card uses a byte-organised architecture. This means that the 12-bit values from the sensor have to be stored in 8-bit bytes on the card. The *ist-D took the simple approach of padding each 12-bit value with 4 extra zero bits, and storing the resulting 16-bit value in two 8-bit bytes. For the DS, and later cameras, Pentax implemented a slightly more complicated scheme, where two 12-bit quantities (which require 24 bits of storage) were packed into three 8-bit bytes, with no pad. > Should we expect the D2 to use the method of the D or the method > of the DS? If Pentax stay with PEF format, I expect the method of the DS et al. There's (almost*) no benefit (other than simplicity) to the method used in the D, and it wastes 25% of the file space. And now that Pentax have created the firmware routines to deal with the packed format, there's no reason not to use them in any new camera. Implementing true compression would shrink the files even further. Rather than see Pentax invest in developing their own compression techniques, though, I'd prefer to see them move to adopting DNG, which already has compression support built in. There's very little difference between DNG and PEF, when you look under the covers - both are built on the TIFF-E architecture (as are the raw file formats from several other manufacturers). * I say "almost no benefit", because technically the extension to the TIFF-E format Pentax use for their raw files violates the letter of the TIFF extension specifications, by overloading the meaning of one of the Tag values. So if you're trying to read Pentax raw files using a standard TIFF library you'll be able to handle files from the "D" with no difficulty, but won't be able to deal with the files from the later cameras. I doubt if anyone cares about this, but to avoid nitpicking ...
