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June 2006
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2006.06.13 16:02 "Re: Editing a tiff image using libtiff", by Joris Van Damme
Rob, Tillaart, Rob van den wrote: > If space waste is an issue one could introduce a micro-memory manager > (MMM). > > Suppopse one wants to update a strip/tile, the traditional scenario > is: 1) the strip is loaded and decompressed in memory. > 2) the strip is edited > 3) the strip is compressed again > 4) the strip is stored at the end of the TIFF file > 5) and tags are updated > > An MMM would interact just before step 4. > It looks at the size of the new strip and compares it to the previous > size > If (strip fits) reuse() > Else append() > > On average there is a 50% chance (depends on type of editing) the new > strip will fit, so on average the filesize will grow less fast. That is of course a good idea. > An additional way to handle space is the introduction of an > 'alloc-tag'. > This tag indicates the size of the 'allocated' block in the file in > contrast to the size tag which indicates the count of the bytes used. > The MMM described above can make a better decission when to reuse the > existing strip space if these 'alloc-tags' would be in the file. Note > that if these 'alloc-tags' would be removed TIFF would work just as we > know. Drawback of the 'alloc-tags' is of course the space they cost :) That is an attempt at inserting memory-management data at the wrong level. There's not just compressed image data blocks to manage, but also IFD blocks, tag value blocks (which can be quite large), etc. If ever a TIFF with memory-management data were to be designed, the data better accompanies each data block. Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html