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Re: PNGs with alpha channel? [message #33822]

Mon, 03 February 2003 16:01

Rick Towler
Messages: 821
Registered: August 1998

Senior Member

"Paul Woodford" wrote in message
> David Fanning wrote:
>
>> What do you mean by "do not see" it? It doesn't have a fourth
>> dimension, or you can't get the alpha channel to do anything?
>
> It doesn't have a fourth dimension.

It may not. Depending on how the image was created you may have to create
the 4,n,m image yourself using the data from the TRANSPARENT keyword . The
PNG spec allows for full alpha support and a sort of 1-bit support where
specific pixels are considered transparent.

FWIW, I read 4 channel .png files regularly going back to at least 5.3 with
a simple:

image = READ_PNG('myimage.png')

-Rick

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Re: PNGs with alpha channel? [message #33829 is a reply to message #33822]

Sun, 02 February 2003 12:49

Paul Woodford
Messages: 43
Registered: June 2000

Member

In article <MPG.18a519b4b9809640989abe@news.frii.com>,
David Fanning <david@dfanning.com> wrote:

> What do you mean by "do not see" it? It doesn't have a fourth
> dimension, or you can't get the alpha channel to do anything?

It doesn't have a fourth dimension.

> What are you trying to do with the alpha channel?

Overlay images with transparency; some of the images are generated
within IDL, and some elsewhere.

Might be time to check out 5.6...

Paul

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Re: PNGs with alpha channel? [message #33832 is a reply to message #33829]

Fri, 31 January 2003 22:40

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Paul Woodford (cpwoodford@spamcop.net) writes:

> Can IDL read and/or write PNGs with an alpha channel?

It would appear so in IDL 5.6, at least.

> I've tried
> reading a PNG that I believe has an alpha channel using IDL 5.4, but I
> do not see the alpha channel.

What do you mean by "do not see" it? It doesn't
have a fourth dimension, or you can't get the alpha
channel to do anything? What are you trying to do
with the alpha channel?

Cheers,

David

--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155

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Re: PNGs with alpha channel? [message #33884 is a reply to message #33822]

Tue, 04 February 2003 20:35

Paul Woodford
Messages: 43
Registered: June 2000

Member

In article <b1n03l$33es$1@nntp6.u.washington.edu>,
"Rick Towler" <rtowler@u.washington.edu> wrote:

> Depending on how the image was created you may have to create
> the 4,n,m image yourself using the data from the TRANSPARENT keyword.

Aha! That does it, at least for color images. It doesn't seem to work
for grayscale images.

Paul

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