Disclaimer : I just read the 2 first paragraphs. Indeed, if you want to be able to using bilinear resampling, cubic, etc..., you can't do that on a paletted dataset. You must convert it before to 24 bit. So the solution is to pct2rgb.py your input files (or use the new -expand rgb|rgba option in gdal_translate in GDAL development version) and then warp them into epsg 3395. Then you should get correct quality.
Le Tuesday 26 August 2008 15:41:04 Wendell Turner, vous avez écrit : > Hello! > > I am trying to do something I probably shouldn't ... > > I have about 30 geotiffs that tile the continental US (aviation sectional > charts). Each of them uses a slightly different projection -- LCC with > different standard parallels and origins for each chart. > > Using gdalwarp, I converted everything to a single projection -- epsg 3395 > -- but the quality suffered (I was never able to get any resampling method > other than "near" to work properly; the colormaps kept getting screwed up. > There doesn't seem to be any way to tell gdalwarp "stop trying to make a > new colormap, just use the old one!") > > The result is OK for display on a screen (I have a nifty OpenLayers > interface to it), but I also need to print good-quality versions. Sometimes > the prints will need to cross the boundaries between the geotiffs. These > prints also need to be at a (nearly) constant scale, which is inconsistent > with using 3395 anyway, since it is a Mercator projection. > > I could warp everything to some other projection (LCC with a single set of > standard parallels, say) but I am willing to accept a small amount of > mis-registration at the boundaries between charts if it would save me the > loss of quality in the warp. (In fact, people do this with the paper > charts all the time -- at many airports you'll see multiple charts glued to > the wall, abutting. The misregistration is typically less than a few > millimeters across a four-foot chart.) > > [Of course, the misregistration is probably cumulative in some sense; with > the paper charts one can simply "eyeball" the error and slide the chart so > as to minimize it. I'm not sure how to express this mathematically ... ] > > So, what I am thinking of doing is to rewrite the geotiff metadata so as to > simply lie about the projection. gdal_translate has options called -a_srs > and -a_ullr that appear to do at least part of what I want. > > Is there a "null projection" that I could give to gdal_translate so as to > not to munge any of the actual pixels? > > Is there some better way of re-writing the geotiff metadata than by passing > it through gdal_translate? > > Is this a silly idea that I should receive the cartographic equivalent of > being disbarred for even suggesting? _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
