Hi. One of the things I have been always looking for is the method for enhancing PNG sizes made by mapnik. Right now, each map tile (250x250px) is by average 90Kbyte (PNG-24bit) in city centers..(where we have POIs, roads and other colorful features.).
This file size is not suitable for web mapping. I checked google maps and in those crowded areas, the size of PNGs are maximm 12Kbyte !! One way is to use PNG256 in mapnik. But since we use Tilecache for generating the tiles, Metatile does not support PNG256 and if we turn metatile off, then the we loose the texts on edges and even speed for making tiles is much less. our maps are full screen, so 7x4=28 tiles per view. I even made PNG256 with mapnik alone and still the size is around 25 to 30Kbyte ...! comparing to gooogle, it is double or more.. I used Photoshop "save for web" feature and chose Web Snap option in save menue...the result PNG is like 9Kbyte !!! I used Convert function in ImageMagick and tried to set it for lowest possible quality and even ImageMagick can't make 9Kbyte PNG...it could make it like 22Kbyte... I don't know what Photoshop does..is it a hidden algorithm? So I am still looking for possible Batch optimization of 600,000 PNGs which are created by mapnik/tilecache. We usually update our maps every week and we need to find a solution for that. Anyone has experience on methods and tips for enhancing the sizes? it is a matter of giga bytes of transfer per day..... thanks. Alireza
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