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Re: Creating Transparent PNG Images [message #68444 is a reply to message #68443]

Sat, 24 October 2009 05:33

devin.white
Messages: 50
Registered: March 2007

Member

You can write out transparent 24-bit PNGs by supplying a 4-channel
array to WRITE_PNG (4 x samples x lines). The first three channels
are RGB, the fourth is the alpha channel. Fully visible pixels should
have their corresponding alpha channel elements set to 255,
transparent pixels should have theirs set to 0.

On Oct 23, 2:53 pm, David Fanning <n...@dfanning.com> wrote:
> Folks,
>
> I have a question about transparent PNG images. If you
> go somewhere on the Internet and look for a transparent
> PNG image, you will typically download an image that
> has a "4" in one of its dimensions. One could think
> of this as a "typical" 24-bit or true-color image with
> an extra alpha channel. In IDL you have to go to some
> effort to display such an image correctly:
>
> http://www.dfanning.com/ip_tips/transparentpng.html
>
> Now, for reasons that are obscure and don't have much
> to do with IDL, I need to create such a PNG image from
> within a piece of IDL code. Unfortunately, it doesn't
> look like WRITE_PNG is up to the job.
>
> I can easily create a 24-bit true-color image. But this
> doesn't allow me to use the TRANSPARENT keyword. The only
> way to make a transparent PNG is to have an 8-bit byte
> image and RGB color vectors.
>
> Am I missing something here in the documentation? Or, if
> not, does anyone have a favorite method for creating
> a true-color PNG file with an alpha channel? (I am
> afraid this is a firm requirement for downstream
> software to work correctly.)
>
> Cheers,
>
> David
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming:http://www.dfanning.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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[Message index]

		Re: Creating Transparent PNG Images By: David Fanning on Sun, 25 October 2009 09:32
		Re: Creating Transparent PNG Images By: greg.addr on Sat, 24 October 2009 10:13
		Re: Creating Transparent PNG Images By: David Fanning on Sat, 24 October 2009 09:01
		Re: Creating Transparent PNG Images By: David Fanning on Sat, 24 October 2009 07:59
		Re: Creating Transparent PNG Images By: devin.white on Sat, 24 October 2009 05:33
		Re: Creating Transparent PNG Images By: Vince Hradil on Fri, 23 October 2009 13:05
		Re: Creating Transparent PNG Images By: David Fanning on Fri, 23 October 2009 12:18

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