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Re: Creating Transparent PNG Images [message #68442 is a reply to message #68441]

Sat, 24 October 2009 09:01

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

David Fanning writes:

>
> Devin White writes:
>
>> 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.
>
> OK, well, then the documentation REALLY needs to be improved!
>
> But there is still a problem. Consider the transparent PNG
> image you find here:
>
> http://www.dfanning.com/ip_tips/example.png
>
> I tried this:
>
> IDL> image = read_png('example.png')
> IDL> help, image
> IMAGE BYTE = Array[4, 162, 150]
> IDL> write_png, 'test.png', image
>
> Then, I opened both example.png and test.png in my
> browser (FireFox). They appear to be two different
> images! The test.png file appears in MUCH darker colors.
>
> Any ideas on what this could be about?

The mystery deepens. When I look at these two images in
Photoshop, they appear to be identical, and look much more
like the image I created in IDL then the image I downloaded
from the Internet. And yet, the *data* in the two images
appears to be identical:

IDL> image = read_png('example.png')
IDL> img = read_png('test.png')
IDL> Print, Total(image - img)
0.000000

Even Internet Explorer displays the two images differently.

Very strange. :-(

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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