Hi, You have an interesting theoretical challenge but I would still make it in another way. First, it would be easier to use single band 16-bit png instead of 3-band 8-bit png. And second, compressed tiff is not much bigger than png. Use the deflate method which is the same method that png is using. For DEMs I would recommend to use the option "predictor=2".
Here are numbers from a quick test with 16-bit DEM file. gdal_translate -of GTiff -co compress=deflate -co tiled=yes -co predictor=2 dem.png dem.tif png: 1326381 bytes tiff: 1394597 bytes Tiled tiff with overviews added with the gdaladdo utility is also much better (=faster) format for MapServer that png files. Overviews add 25% to the file size but it is worth it. -Jukka Rahkonen- Lähettäjä: mapserver-users [mailto:[email protected]] Puolesta Tellez-Arenas Agnes Lähetetty: 4. tammikuuta 2018 16:40 Vastaanottaja: [email protected] Aihe: [mapserver-users] Classification of RGB png (or tiff) using [red] [green] [blue] Dear all, I am trying to classify a PNG (could be a tiff) which has the 3 bands (red, green, blue), each from 0..255. I need to classify (not just display it as it is), because I have used those 3 bands to "code" elevation values (it's a hightmap). I am using a PNG to encode the elevation value on those r,g,b bands, rather than one tiff containing directly the elevation on the pixel, because the size of the PNG is smaller than tiff (and I need to store a big amount of images...). The formula to "encode" the elevation is #min=-50, max=1000 ((r * 256 + g + b / 256) * (Zmax-Zmin)/(65536)) + Zmin* Which I translated as CLASS #min=-50, max=1000 ((r * 256 + g + b / 256) * (Zmax-Zmin)/(65536)) + Zmin* EXPRESSION ( ((( (red]*256) +[green] +([blue]/256)) * (1050/65536)) - 50) <= 0) STYLE COLOR 100 195 215 OPACITY 35 END END But, [red]=[green]=[blue]= the value of the first band (red). Always. In documentation I can see (http://mapserver.org/mapfile/expressions.html) For Raster Data<http://mapserver.org/input/raster.html#raster> layers special attributes have been defined that can be used for classification, for example: * [PIXEL] ... will become the pixel value as number * [RED], [GREEN], [BLUE] ... will become the color value for the red, green and blue component in the pixel value, respectively. And (http://mapserver.org/el/input/raster.html) There is no need to specify a CLASSITEM. The raw pixel value itself ("[pixel]") and, for paletted images, the red, green and blue color associated with that pixel value ("[red]", "[green]" and "[blue]") are available for use in classifications. Hope someone can help me on that! Thanks! Here is my MapFile. MAP NAME "test" STATUS OFF SIZE 256 256 MAXSIZE 4096 RESOLUTION 96 EXTENT 253999.500 6884000.500 254999.500 6885000.500 UNITS DD IMAGETYPE png IMAGECOLOR 255 255 255 PROJECTION "init=epsg:2154" END WEB TEMPLATE "../etc/html_template/getcapabilities_wms_130.html" METADATA (...) END END LAYER NAME "test" TYPE RASTER DATA "/mnt/nemesis_resultats/flood/v3/v4_1m/RGEALTI_FXX_0224_6753_MNT_LAMB93_IGN69_png.png" #PROCESSING "BANDS=1,2,3" #this line crashes with a "drawGDAL(): Unable to access file. Attempt to classify 24bit image, this is unsupported." STATUS ON PROJECTION "init=epsg:2154" END METADATA (...) END CLASS #min=-50, max=1000 ((r * 256 + g + b / 256) * (Zmax-Zmin)/(65536)) + Zmin* EXPRESSION ( ((( (red]*256) +[green] +([blue]/256)) * (1050/65536)) - 50) <= 0) STYLE COLOR 100 195 215 OPACITY 35 END END END #LAYER END # End of MapFile
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