Ok, I'll answer my own question. I got the answer from
http://groups.google.com/group/de.comp.lang.python/browse_thread/thread/5d9a315c3d3ac0f5
, already 2 months old, but I wasn't aware. Code snippet below.
----------------
import Image
from pylab import *
#generate intensity map
a=meshgrid(arange(256),arange(256))[0]
f=figure()
for map in [eval("cm."+t) for t in dir(cm) if "N" in dir(eval("cm."+t))]:
#generate false color map
color_image=(map(a)*255.).astype(uint8)
#rasterize colormap
gray_to_color=(map(linspace(0,1,256))[:,:3]*255).astype(uint8)
#create reverse LUT
color_to_gray = zeros((256, 256, 256), dtype=uint8)
color_to_gray [gray_to_color[:,0], gray_to_color[:,1],
gray_to_color[:,2]] = arange (0, 256, dtype=uint8)
# use reverse LUT
new_gray_image = color_to_gray[color_image[:,:,0], color_image[:,:,1],
color_image[:,:,2]]
gray()
subplot(131)
imshow(a)
title("intensity")
subplot(132)
imshow(color_image)
title("intensity->%s"%map.name)
subplot(133)
imshow(new_gray_image)
title("intensity->%s->intensity"%map.name)
f.savefig("c:/temp/%s.png"%map.name)
-----------------
By the way, the resulting plots show that all the colormaps except cm.prism
are bijective (even for example mc.flag):
http://www.nabble.com/file/p25891118/jet.png
http://www.nabble.com/file/p25891118/flag.png
http://www.nabble.com/file/p25891118/prism.png
cheers
Thomas
thkoe002 wrote:
>
> Hi,
>
> following problem: Let's say I have a couple of .png images that were
> produced with imshow() and the jet() (==default) colormap. The original
> data was 8 bit intensity data, now the images are 24 bit with false colors
> / pseudocolors.
> Now, is there a (simple?) way to calculate back from those 24 bit images
> to the 8 bit intensity information? Of course this can only work with
> bijective colormaps and if the colormap is well known; both is the case
> here.
>
> Any ideas?
>
> Thanks in advance, cheers
>
> Thomas
>
--
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