Bizzare .png behavior

[Elwin Estle]

This is with Gimp 2.4.6 on Windows XP professional.

I loaded an image into gimp and selected various aspects of it and cut away, i.e. rendered parts of the background using layer masks.  What I ended up with were several layers, each with a different part of the original image, but still the same size as the original, i.e. with a subject surrounded by transparency.

I saved each layer out as an individual image using edit>copy and edit>paste as new to save each one as a .png file.

These were then loaded into Blender as texture files.

Now for the weird part.  If I turn off "use alpha" in the Blender texture file options, the entire, original image appears, it's as if the .png file was saved out just like the original picture, without any renders cut from it, etc.

I viewed the images using Windows Explorer in filmstrip mode.  They looked as they should.  I opened one of them in Windows Picture and Fax Viewer.  They looked as they should.

Then, just for the heck of it, I loaded one into Microsoft Paint.

What I got was not a subject against alpha, but the original, completely unmodified picture!

What in heck is going on?


     
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[Elwin Estle]

Okay, I did a further experiment.  I did alpha to selection on the layer.  Saved the selection to a channel.  Flattened the image, then re-added the alpha channel and used the selection to cut away the white background.

Now, when I save out the image, it works as it should and the filesize is way smaller.  Still won't do what I want in Blender, but that is another matter.


--- On Fri, 7/11/08, Elwin Estle <chrysalis_reborn@...> wrote:

> From: Elwin Estle <chrysalis_reborn@...>
> Subject: [Gimp-user] Bizzare .png behavior
> To: gimp-user@...
> Date: Friday, July 11, 2008, 10:35 AM
> This is with Gimp 2.4.6 on Windows XP professional.
>
> I loaded an image into gimp and selected various aspects of
> it and cut away, i.e. rendered parts of the background using
> layer masks.  What I ended up with were several layers, each
> with a different part of the original image, but still the
> same size as the original, i.e. with a subject surrounded
> by transparency.
>
> I saved each layer out as an individual image using
> edit>copy and edit>paste as new to save each one as a
> .png file.
>
> These were then loaded into Blender as texture files.
>
> Now for the weird part.  If I turn off "use
> alpha" in the Blender texture file options, the
> entire, original image appears, it's as if the .png
> file was saved out just like the original picture, without
> any renders cut from it, etc.
>
> I viewed the images using Windows Explorer in filmstrip
> mode.  They looked as they should.  I opened one of them in
> Windows Picture and Fax Viewer.  They looked as they should.
>
> Then, just for the heck of it, I loaded one into Microsoft
> Paint.
>
> What I got was not a subject against alpha, but the
> original, completely unmodified picture!
>
> What in heck is going on?
>
>
>      
> _______________________________________________
> Gimp-user mailing list
> Gimp-user@...
> https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-user

     
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[David Hodson]

Elwin Estle wrote:

> Now for the weird part.  If I turn off "use alpha" in the Blender texture file options, the entire, original image appears, it's as if the .png file was saved out just like the original picture, without any renders cut from it, etc.

As far as I know, the alpha channel in both Gimp and png files is
non-premultiplying. (In other words, it's what many people would refer
to as a matte channel rather than an alpha channel.) The colour channels
are not affected by its presence. If you turn it off, the colour is
still there.

--
David Hodson  --  this night wounds time
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[Elwin Estle]

I found a workaround.  I simply loaded the image into Gimp, added a transparent layer above the image, merged down, then saved.  This time, what was supposed to be transparent stayed that way.


--- On Fri, 7/11/08, David Hodson <hodsond@...> wrote:

> From: David Hodson <hodsond@...>
> Subject: Re: [Gimp-user] Bizzare .png behavior
> To: gimp-user@...
> Date: Friday, July 11, 2008, 11:19 AM
> Elwin Estle wrote:
>
> > Now for the weird part.  If I turn off "use
> alpha" in the Blender texture file options, the
> entire, original image appears, it's as if the .png
> file was saved out just like the original picture, without
> any renders cut from it, etc.
>
> As far as I know, the alpha channel in both Gimp and png
> files is
> non-premultiplying. (In other words, it's what many
> people would refer
> to as a matte channel rather than an alpha channel.) The
> colour channels
> are not affected by its presence. If you turn it off, the
> colour is
> still there.
>
> --
> David Hodson  --  this night wounds time
> _______________________________________________
> Gimp-user mailing list
> Gimp-user@...
> https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-user

     
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