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 Post subject: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 2:43 am  (#1) 
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Excuse my ignorance and it is not meant as a criticism of Gimp.
I would just like to understand somthing.
If I copy the background layer and want to erase part of the top layer I have to add Alpha channel. You have to add Alpha channel for a lot of things. In photoshop it's just the default you dont have to add it. Can someone explain the difference?

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 4:15 am  (#2) 
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When you duplicate a layer without alpha channel, there is no good reason for Gimp to decide that you want an alpha channel in the duplicate. It could sometimes be a valid assumption, but in many cases it wouldn't.

Now, if we look at the sources of layers without an alpha channel:
  • Gimp won't add an alpha channel when the image is loaded from a format that doesn't support transparency (JPEG...). Adding an alpha channel to that by default would sent the wrong signal.
  • The default background in new images is filled with white. But this can be changed in Edit>Preference>Default image>Advanced options>Fill with to leave it transparent instead (you can also do that in the advanced options in File>New).

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 4:25 am  (#3) 
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I think photoshop seems to add one to jpgs too?
At least if I open a jpg and then duplicate the background layer then use the ereaser tool on the top layer it erases too transparancy not white like in gimp.
I must admit I am confused abt all this and probably just need to use gimp more to realize the differences and how it works.

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 4:53 am  (#4) 
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Born2BeMild wrote:
I think photoshop seems to add one to jpgs too?
At least if I open a jpg and then duplicate the background layer then use the ereaser tool on the top layer it erases too transparancy not white like in gimp.
I must admit I am confused abt all this and probably just need to use gimp more to realize the differences and how it works.


Well this is a choice made by Adobe... and in this case it doesn't make much sense, because when you have two layers that are an exact copy of each other above one another, when you erase the top one you see exactly the same thing from the bottom one :) In Gimp you would do that to keep a "backup" of the original layer, and erasing in the top layer, producing white, would give you what you will obtain when you export back to JPG.

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 6:15 am  (#5) 
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Born2BeMild wrote:
I must admit I am confused abt all this and probably just need to use gimp more to realize the differences and how it works.

Basically, GIMP retains the "alpha-ness" of the source layer, whether you are duplicating a layer, using "File->Open As Layer(s)", or using drag-n-drop. The only caveat is that using the paste buffer ("Edit->Copy", "Edit->Paste") always results in a layer with an alpha channel.

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 6:18 am  (#6) 
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ofnuts wrote:
Born2BeMild wrote:
I think photoshop seems to add one to jpgs too?
At least if I open a jpg and then duplicate the background layer then use the ereaser tool on the top layer it erases too transparancy not white like in gimp.
I must admit I am confused abt all this and probably just need to use gimp more to realize the differences and how it works.


Well this is a choice made by Adobe... and in this case it doesn't make much sense, because when you have two layers that are an exact copy of each other above one another, when you erase the top one you see exactly the same thing from the bottom one :) In Gimp you would do that to keep a "backup" of the original layer, and erasing in the top layer, producing white, would give you what you will obtain when you export back to JPG.


As I said I probably just have to get used to the differences but so far Photoshop made more sense - It's kind of stuck in my head that if I erase anything from a layer above then the info from below will show through to the upper layer.

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 Post subject: Re: Having to add alpha channel - why is this not default?
PostPosted: Thu Jun 19, 2014 7:51 am  (#7) 
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Believe it or not, it use to be the default to add alpha channel on duplicate layer. I use to question the same thing you are now when GIMP went to the 2x series, but got use to the fact that you have to add alpha layer and, as time progressed, glad that it's not the default anymore for many reasons (and you have to discover for yourself why as you use GIMP). :)

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