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carol.gimp.orggimptransparency |
There are times when you try to run a filter and the option that you desire is not available (grayed out in the menus), or you are using the eraser tool but it seems to be painting a color instead of removing colors.
This ability that an image has to display nothing is called the alpha channel (like the color channels: red, green and blue). You can see it in the channel dialog when the image has this channel and you will not see it if it is not there.
adding alphaJpegs and other formats do not have the means to support this color of transparency, so almost every photograph taken with a digital camera or scanned and saved as a jpeg will not have this channel or ability to handle transparency when you first open it up in TheGIMP. It can be easily added, however. One way to add the alpha channel is to right click on the image and follow the menus <Image> -->Layer -->Transparency -->Add Alpha Channel. The Layer Dialog <File> -->Dialogs -->Layers also has a menu you can get to by right clicking next to the thumbnail (<Layer > -->Dialogs -->Layers). The option can be found near to the end of the list. |
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removing the alpha channelWhen you are finished with your task and want to save the image again as a jpeg, you will need to remove the alpha channel. Easily done by right clicking on the image and "Flattening" your image. This can be accomplished by following the menus to <Image> -->Image -->Flatten Image or also from the Layers Dialog (at the end of the list). |
Maybe beyond the scope of a beginner -- definately more information than a beginner needs to know, the mathematics of transparency is not real obvious.
Usually, just due to the way the earth rotates, the Europeans are drinking beer while I am drinking coffee. I am glad that jimmac is there so that I was not the only idiot.
11:18 < jimmac> excuse my ignorance, but If i place two black pixels
above each other, in normal blend mode, both with
50% opacity, how come the reult is not a black pixel?
11:19 < jimmac> ie, doesn't opacity add up?
11:21 <@mitch> jimmac: no it doesn't
11:22 < carol> it averages?
11:22 <@mitch> it subtracts from the remaining transparency
11:22 <@mitch> 50% transp minus another 50% yields 25% transparency
11:23 < carol> who came up with this mess?
11:23 <@mitch> it's logical
11:23 <@mitch> put two 50% transparencies against the light, you
can hardly expect to see full black
11:24 < carol> this is how my money works. i fail to see any logic
11:24 < carol> mitch: yes i would
11:24 <@mitch> you have a problem then :)
11:24 < carol> like two polarized lens
11:24 < carol> maybe
11:24 < jimmac> mitch: aha!
11:25 <@mitch> jimmac: it's like in that transparency metaphor
11:25 < carol> two glasses of fluid. both half full. the contents
should fill a same volume glass perfectly.
11:25 <@mitch> that's another part of physics i'm afraid
11:26 < carol> aww, come on. stuff is stuff
11:26 <@mitch> take away half the light, from the rest, take away
the half again
11:26 < carol> this is stuff, on the image area ...
11:27 < carol> oh, a percent of a percent
11:27 <@mitch> yea
11:27 < carol> do
11:27 < carol> doh, even. sorry. dense this lifetime i think ...
11:27 <@mitch> that's how layer opacity "adds" up
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 Most all of my software is gnu. thanks! |
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