On Sat, Nov 18, 2006 at 01:11:05PM -0700, Joseph Tainter wrote: > How does one know that a compressed RAW file is actually lossless? > > Over at dpreview, Steve (apparently the first kid on the block with a > K10D) has posted a DNG file taken at ISO 1600 here: > > http://www.sendspace.com/file/zsdg52 > > It is a 6 mb file. When you open it in CS2, then resave it as a TIFF, it > becomes a 29 mb file. So clearly the DNG was compressed.
That doesn't prove it - when I open a (13MB) RAW file from my *ist-D, or a 10MB RAW file from a *ist-DS, and save as TIFF, it can take 36MB. Even an 8-bit TIFF needs 18MB, which is larger than either RAW file. What *does* prove it is compressed, though, is that a 10MP file of 12-bit raw values is going to need at least 15MB of uncompressed space. If the DNG file only occupies 6MB then it's obviously being compressed. > > I don't want to use RAW compression unless I cam be completely certain > that it is lossless. How does one know? Because the DNG specs say that compressed DNGs use lossless JPEG compression, and the JPEG consortium assure us it really is lossless. Beyond that, you'd actually need to understand the algorithms used. But (unless you're a conspiracy nutcase) you can be certain that a compressed DNG isn't throwing away any of your precious data. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
