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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