Re: ListDensityPlot and GraphicsArray

• To: mathgroup at smc.vnet.net
• Subject: [mg65531] Re: ListDensityPlot and GraphicsArray
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Thu, 6 Apr 2006 06:52:45 -0400 (EDT)
• References: <e0r06j\$li\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

I can't reproduce your problem and as long as I can't see
what your RGB to Lab conversion does I can tell you anything.
But

ftoxyz = Compile[{{rgb, _Real}}, 100*If[rgb > 0.04045, ((rgb +
0.055)/1.055)^2.4, rgb/12.94]
];

RGBtoXYZ[rgb_List, max_:255] :=
With[{m = {{0.4124,  0.3576,  0.1805},
{0.2126, 0.7152 , 0.0722},
{0.0193 , 0.1192, 0.9505}}},
Module[{varRGB},
varRGB = ftoxyz /@ (N[rgb/max]);
m.varRGB
]
]

ftolab = Compile[{{xyz, _Real}},
If[xyz > 0.008856, xyz^(1/3), (7.787*xyz) + (16/116)]];

Clear[XYZtoCIELab]
XYZtoCIELab[xyz_List] :=
Module[{vx, vy, vz},
{vx, vy, vz} = ftolab /@ (xyz/{95.047, 100.000, 108.883});
{116*vy - 16, 500*(vx - vy), 200*(vy - vz)}
]

RGBToCIELab[rgb_List, max_:255] :=
Round /@ XYZtoCIELab[RGBtoXYZ[rgb, max]]

img = Import["G:/images/leopard.jpg"];
labimg = img /.
Raster[bm_, args___] :> Raster[Map[RGBToCIELab, bm, {2}], args];

ListDensityPlot[Map[#[[2]] &, labimg[[1, 1]], {2}], Mesh ->
False, AspectRatio -> Automatic]

work fine.

Regards
Jens

buttgereit at biomediclab.de wrote:
> Dear Group,
>
> using Mathematica 5.0 on WinXP I have a strange problem with
> ListDensityPlot.
>
> I have imported a picture (PNG) using
>
> img=Import["mypic.png"];
>
> I can show it, all fine so far.
>
> I then extract the pixel data (rgb in this case) using
>
> raster=img//First//First;
>
> I extract the red channel using
>
> rc=Map[First,raster,{2}];
>
> and can display it with
>
> ListDensityPlot[rc];
>
> However, when I convert the raster to Lab color space and extract, say
> the illumination
>
> ListDensityPlot[lc];
>
> fails with
>
> "DensityGraphics::gmat: ... is not a rectangular array larger than 2 x
> 2"
>
> But the size is identical to rc according to
>
> Dimension[rc] is equal to Dimension[lc] : {400,300}.
>
> Just Reals instead of Integers -- and though, rounding/scaling does not
> make a difference.
>
> I have checked the array using
>
> Union[Length/@lc]
>
> which evaluates to {400} in this case -- doesn't this say it _is_
> rectangular? Do I miss something?
>
> I can show part of the array using Take and define a region of at most
> 200x200 pixels -- expanding it again pops the error. I can move that
> "window" over the image without errors.
> Just expanding it does not work.
>
> Any idea what could cause the problem? ( yes, I did restart windows ;)
>
> Best Regards,
>
> Peter
>

```

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