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I've selected an area, decided to add an alpha channel. When I do that, for some reason the transparency spreads to unselected areas as seen in the pictures. Same happens with bucket fill. If I fill a selection, it spreads in a faded manner throughout the picture. How do I fix this?

enter image description here

  • Alpha channel or alpha mask? It looks like you're wanting to mask but I'm not seeing the mask in the toolbox [I'm used to photoshop, not gimp, so it may display differently] – Tetsujin Apr 25 '17 at 7:24
  • @Tetsujin i think I want an alpha channel, but idk what are the behind the scenes gimp operations that make it happen. Potentially i'm doing a mask without knowing it. – SalmonKiller Apr 25 '17 at 7:26
  • If you are trying to 'make the background invisible' you want a mask. It looks like you got a channel instead. In PShop an alpha mask looks like this - gyazo.com/34fda62840ea5dc4eb57d4a6049512f8 & I would expect Gimp to be similar. – Tetsujin Apr 25 '17 at 7:55
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Let me guess, you did a color-to-alpha to remove background and this makes the light parts of you subject transparent. This is normal, the aim of C2A is to produce a layer, which, over a layer filled with the removed color, recreates the original layer. But there are several ways to do it: while removing white, a gray could be either an opaque gray or a partially transparent black. V2A's algorithm aims for the maximum transparency and uses the partially transparent black.

Solution:

  • Select the background: typically, Fuzzy select with the usual threshold (~15), unless you subject has parts that blend smoothly in the background in which case you will have to be a bit more "manual".
  • `Select>Grow' by two pixels, so that the selection includes the pixels on the edge of the subject
  • Apply color-to-alpha

That way, color-to-alpha only apply were it counts (the edge pixels)

  • Tried this, same issue persisted. – SalmonKiller Apr 25 '17 at 15:41
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    You must have missed something, I (and others) have been using this for ages. C2A only applies to the selection, if pixels get partially transparent, then either they were in the selection, or there was no selection. Can you make a screenshot showing the marching ants just before you apply C2A? – xenoid Apr 25 '17 at 19:24
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It looks like you are sliding the "opacity" control for the layer. This will change the opacity of the whole layer, regardless of selections and any pixel information - it is meant more for making a content semi-visible behind the current layer than any otherthing.

If you want to change the selected areas to transparent, after adding an alpha channel, and creating your selection, just do edit->cut - this will modify the information on the pixel level, and effectively turn all pixels to transparency.

If you want varying degrees of opacity instead of total transparency, use the colors->curves tool or colors->levels and modify the alpha channel in there: againt he changes are on the pixel level, not on the layer level.

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After doing color to alpha, do layer > transparency > threshold alpha. It performs an "all or nothing" operation on the transparency. Works for me.

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