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Re: Miscellaneous questions

  • To: mathgroup at
  • Subject: [mg36797] Re: Miscellaneous questions
  • From: Jens-Peer Kuska <kuska at>
  • Date: Thu, 26 Sep 2002 04:56:51 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <amrjfe$5av$>
  • Reply-to: kuska at
  • Sender: owner-wri-mathgroup at


> I would appreciate any info on these issues.
> 1. I have a symbol "slback." I was getting
> "tag times lback protected " (or something) errors.
> (A typically uninformative error message.)
> I suspected there was a separation between the s
> and l, and they looked too far apart, but when I
> spaced past them with the arrow keys, there did
> not appear to be anything like a space in between.
> I retyped the symbol and all was ok. This has
> happened before. What's going on ???

As long as you don't send us the *exact* input we can't
help you. You should also send the mathematica version
you are using. But typical this error comes from a equation


where the user has mixed up Equal[] and Set[]

> 2. I'm using Raster to plot data points
> in a 256x256 array. Two problems: First,
> regardless of the ImageSize setting (I need the
> display as big as possible), the individual data cells
> when examined closely at 300% vary in size by
> almost 2:1. This makes detailed inspection of the
> data values difficult. Surely there is some setting
> of something which would make each data cell an
> exact number of screen pixels? Second, I'm
> using
> Show[ Graphics[Raster[ rescol,
>                           ColorFunction -> Hue]],
>                           AspectRatio -> Automatic,
>                           ImageSize -> 700];
> but the Hues don't allow enough easily distinguishable
> shades to visually recognize even 6 data values easily.
> (I'm slightly colorblind.) 

The most humans can distinguish 160 gray levels

>It would be nice to have black
> and white available for two of the data values, but Hue
> does not allow this.

And something like:

mycolor[i_] :=
                  0, RGBColor[0, 0, 0],
                  1, RGBColor[1, 0, 0],
                  2, RGBColor[1, 1, 0],
                  3, RGBColor[0, 1, 0],
                  4, RGBColor[0, 1, 1],
                  5, RGBColor[0, 0, 1],
                  _, RGBColor[1, 1, 1]]

    Raster[Table[Random[Integer, {0, 6}], {16}, {16}], 
      ColorFunction -> mycolor, ColorFunctionScaling -> False]]]

does not help ?

>  I don't understand what the
> documentation says about RasterArray.

If you can't be more specific *what* you not understand
we can not help you.

> 3. I wanted to get this raster image into a format
> where I could dissect it with Photoshop or equivalent.

And ? waht does Export[] do ? it write the expression
in a desired format, TIFF, PNG, PPM are all lossless
compressed bitmap formats, that Mathematica can export
and that can be imported into PhotoShop

> After much fooling around, I found that I can export the
> selection as an html file, read it into Navigator, do File>
> Edit Page, which brings up Netscape Composer, right
> click the image which, allows saving it as a GIF, which
> I can finally work on with a photo editor. Maybe
> there is an easier way, or maybe this description will
> help someone with the same need.

May be that thhis description help someone who can't read
the fancy documation on Import[] and Export[].


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