Re: xvloadimage with imagesc and colormap

[Stefan van der Walt]

Hi Mika

On Tue, Mar 07, 2006 at 08:13:14PM -0600, Mike Miller wrote:
> I'm finding it tricky to make an imagesc() plot with a particular colormap 
> and save it in a file (I want PNG but can get to that from some other 
> format if necessary).  I think maybe imagesc() doesn't respect the 
> colormap, but if I try to use image(), or imshow(), the image is just 
> solid black.  My colormap is grayscale.  I could just use 
> colormap("default"), and I tried it but it didn't work.  The commmand 
> "imagesc(X,1)" makes a reasonable image but it isn't using my
> colormap.

I find it strange that `imshow' displays a black image.  Imshow tries
to determine what kind of picture you supply, since they come in so
many shapes and forms.  Your image values can be in [0,1], or in
[0,255], or in [0,2^anything].

If the largest value of the image is slightly larger than 255, imshow
will probably assume it is in [0,512].  If `I' is your image, you
should therefore have

min(I(:)) >= 0
max(I(:)) <= 255 or max(I(:)) <= 1

You are right about imagesc: it does look like the colormap specified
is ignored.  A quick workaround is to set the colormap before calling
it, i.e.

colormap(jet(256));
imagesc(...)

> If I want to save an imagesc() image to a file, what do I have to do?  It 
> isn't using gnuplot, so the usual __gnuplot_set__ methods don't
> work.

Use the `ind2rgb' function to convert your indexed image to RGB, after
which you can use `imwrite' (`jpgwrite' or `pngwrite') to store the
image to a file.

> In case it matters, I'm running Octave 2.1.71 under Linux with Octave 
> Forge and I don't have xv or ImageMagick installed, but I do have 
> xloadimage.

Without ImageMagick you will have to rely on `jpgwrite` or `pngwrite`.

Regards
Stéfan



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