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Re: Alpha blending with object graphics - different color palettes do not work [message #75784 is a reply to message #75782]

Wed, 04 May 2011 09:17

Karl[1]
Messages: 79
Registered: October 2005

Member

On May 4, 6:30 am, David Fanning <n...@idlcoyote.com> wrote:
> LNpellen writes:
>> Why isn't the rainbow (LoadCT, 13) working for me?
>
> I would guess because you are using indexed color mode.
> You do not EVER want to use indexed color when you are
> working with object graphics.
>
> Cheers,
>
> David
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming:http://www.idlcoyote.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")

The IDL docs for IDLgrImage say this for the image data parm:

"An n x m greyscale image, or a 2 x n x m, n x 2 x m, or n x m x 2
greyscale image with an alpha channel. (The alpha channel is ignored
if the destination device uses indexed color mode.)"

So this is considered a Luminance-Alpha image. The first channel is a
luminance (greyscale) channel. Greyscale images do not perform color
lookup through a palette. That is why setting the palette to rainbow
had no effect.

Is the dose map going to be displayed with a constant alpha, or a per-
pixel alpha, where the alpha value could be different for each pixel?

It *looks* like you want a constant alpha because you are filling the
alpha channel of the dose image with a constant (alpha * 255). You
don't need alpha data in the image if you want to apply constant alpha
with the ALPHA_CHANNEL property.

If the alpha is constant, you should just go back to a single channel
image and use that as an "indexed image" in conjunction with the
palette and use the ALPHA_CHANNEL property to set the global constant
alpha to 0.5 or whatever.

That should display your dose image with the color palette and half-
transparent.

As far as being in indexed mode goes, I don't think that the
DECOMPOSED setting affects the way IDLgrWindows are created. (not
sure). But you don't need to have an Indexed destination to use
palettes in images.

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[Message index]

		Re: Alpha blending with object graphics - different color palettes do not work By: djh on Wed, 04 May 2011 09:43
		Re: Alpha blending with object graphics - different color palettes do not work By: David Fanning on Wed, 04 May 2011 09:33
		Re: Alpha blending with object graphics - different color palettes do not work By: Karl[1] on Wed, 04 May 2011 09:17
		Re: Alpha blending with object graphics - different color palettes do not work By: David Fanning on Wed, 04 May 2011 05:30
		Re: Alpha blending with object graphics - different color palettes do not work By: LNpellen on Wed, 04 May 2011 22:50

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