ind2rgb and pngwrite (was "xvloadimage with imagesc and colormap")

[Mike Miller]

On Wed, 8 Mar 2006, Stefan van der Walt wrote:

> Mike Miller wrote:
>
> > 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`.


This was excellent advice, but I think I should add something to help 
people like me who are dealing with these functions for the first time. 
Here are the two help docs:

 - Function File: [R, G, B] = ind2rgb (X, MAP)
     Convert an indexed image to red, green, and blue color components.
     If MAP is omitted, the current colormap is used for the conversion.

pngwrite writes a png file to the disk.
    pngwrite('filename',R,G,B,A) writes the specified file
    using the Red, Green, Blue and Alpha matrices.

    Data must be [0 255] or the high bytes will be lost.


My MAP used [0,1] values, so the R,G,B values were all [0,1], but it seems 
that pngwrite requires [0,255] instead.  Also, my X matrix was [0,1], and 
it seems that it has to be [1,256].  So I did this and it seemed to work 
for me:

[R, G, B] = ind2rgb( round( 255*X ) + 1, round( 255*MAP ) );

But I didn't know what an "Alpha matrix" was for pngwrite, so I tried a 
few things and ended up finding that this seemed to work:

A = round( 255 * ones( size(X) ) );

pngwrite( 'file.png', R, G, B, A)

The resulting png file looked good, but I'm not sure that I was following 
correct procedure and I would enjoy hearing from someone who knows better.

Thanks again for the helpful tips.

Mike



-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------