On Monday 23 June 2008 23:00, Eric Firing wrote:
> Delbert Franz wrote:
> > I have been working to display an image of a USGS 7.5 minute quad sheet.
> > These are provided at various locations about the Web. Since the
> > range of colors on these maps is limited, the *.tif files appear to
> > use an indexed color map wherein each pixel has a value 0 to 255 and
> > the color is found from a table with 256 entries having triplets of
> > the RGB in the range of 0-255. I have not been able to sort out
> > how to get the gdal package to give me the color map from within python, so
> > I dumped it
> > from the image file using gdalinfo and then cut and pasted to get
> > the following script:
> >
> > ---------------------------------------------------------------------------------
> > import numpy as np
> > import matplotlib.pyplot as plt
> > import matplotlib.cm as cm
> > from matplotlib.colors import ListedColormap
>
> from matplotlib.colors import NoNorm
>
> >
> > import osgeo.gdal as gdal
> > from osgeo.gdalconst import *
> >
> > gd = gdal.Open('o37122d1.tif')
> >
> > #Setup to compute the colormap from the RGB triplets from the geotif file
> > div = np.zeros( (256,3), np.float32)
> >
> > ax = fig.add_subplot(111)
> > ax.imshow(suba, cmap=usgscm, origin='upper')
>
> Instead, try:
>
> ax.imshow(suba, cmap=usgscm, norm=NoNorm(), origin='upper')
>
>
> >
> > plt.show()
> > ---------------------------------------------------------------------------------
> > This script does give me an image--but in badly wrong colors:( The script
> > does
> > properly display gray-scaled Digital Ortho-quadrangles using cm.gray as the
> > color
> > map in imshow. Consequently something is not quite correct with respect to
> > the
> > definition or the use of the color map. It appears that each map, and
> > there
> > are about 56,000 of them available on one site, could have its own color
> > map.
> > Thus my application must be able to compute a color map unique to each of
> > the
> > topographic maps.
> >
> > Questions:
> >
> > 1. What am I missing to get imshow to pick out the correct colors from the
> > color map?
>
> The default norm will scale your inputs; specifying the norm as a NoNorm
> instance will pass the integers through directly, so they will be used
> as indices into the colormap.
>
>
> >
> > 2. Should I be using the name, usgs, given in the ListedColormap instance
> > someplace?
> > Not clear to me what role that name plays.
> >
>
> None, really. I can imagine ways in which it could be useful, but
> unless you know you need it, consider it optional.
>
>
> > 3. Is there a way to use ctab directly in the ListedColormap instance?
> > Class Colormap
> > has a bytes argument, False by default, but I am not yet sure if it has any
> > bearing
> > on my problem.
>
> No, sorry, but the bytes argument is only in the __call__ method, where
> it is used to improve efficiency within mpl. There is no facility for
> using ints in the lookup table, and no recognition of 0-255 ints within
> mpl colorspecs, hence no way to feed your ctab in directly. Even if
> there were, though, I don't think it would make much difference in
> plotting time.
>
>
> Eric
>
> >
Eric,
That did the trick! The images are displaying properly now. Thanks
for the quick help.
Delbert
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