[mkgmap-dev] [Patch v1] improve LinePreparer to reduce img sizeFrom Gerd Petermann gpetermann_muenchen at hotmail.com on Sun Jul 17 15:48:05 BST 2016
Hi Nick, nwillink wrote > As a matter of interest how long does a delta need to be for it to be > treated as a special case? This depends on the number of bits which are used. The typical case with a coastline polygon is that we have a lot of very small delta values and one or two large values for the straight lines. The unpatched algo uses these large values to calculate the needed number of bits (e.g. 8), the patch implements an iterative search which decreases the number of bits and tries if the resulting bit stream is shorter. Each value that cannot be represented with e.g. 7 bits needs the trick. If only one value is stored with the trick, and we have to store 50 values, we have 50 * 8 = 400 bits with the old and 49 * 7 + 1 * (7+7) = 357 bits for the trick. So, the iteration would continue with 6 bits, maybe 5 bits and so on until too many values are encoded with the trick (or the the large values are encoeded with too many bits) and the bit stream length increases again. I hope this makes it a bit clearer? Gerd -- View this message in context: http://gis.19327.n5.nabble.com/Patch-v1-improve-LinePreparer-to-reduce-img-size-tp5878545p5878568.html Sent from the Mkgmap Development mailing list archive at Nabble.com.
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