Layer transparency

Layer transparency

Post by JOE » Sun, 12 Nov 2000 04:00:00



Hiya all!
I was wondering how I could create an image with 2 layers, both with
similar images on them, and have the top layer semi-transparent in *some*
areas, but not all?
I had assumed that I just needed to select the region that I wanted to
alter, then adjust the opacity, but thats not what I want, because it
changes the entire layer.

any ideas?

thanks!

 
 
 

Layer transparency

Post by Lars Clause » Sun, 12 Nov 2000 04:00:00



> Hiya all!  I was wondering how I could create an image with 2 layers,
> both with similar images on them, and have the top layer semi-transparent
> in *some* areas, but not all?  I had assumed that I just needed to select
> the region that I wanted to alter, then adjust the opacity, but thats not
> what I want, because it changes the entire layer.

You need to work on the layer mask instead of the entire image.  On the
layer, do 'Add Layer Mask'.  There'll be a grayscale box next to the layer
box in the layers dialog.  Select that, and you're painting on the alpha
layer.  Dark is transparent, white is opaque.  Have fun -- this is one of
the most useful things in Gimp.

-Lars

--
Lars Clausen (http://shasta.cs.uiuc.edu/~lrclause) | H?rdgrim of Numenor
"I do not agree with a word that you say, but I    | Retainer of Sir Kegg
will defend to the death your right to say it."    |   of Westfield
    --Evelyn Beatrice Hall paraphrasing Voltaire   | Chaos Berserker of Khorne

 
 
 

Layer transparency

Post by Don Palme » Fri, 24 Nov 2000 04:00:00


Need to easily make a layer have transparent regions?

Just dealt with that very same problem for wargame graphics I'm creating.  I
first created a bitmap of a hexgrid using Visual Basic (yes I'm a Windozer).
I loaded it into Gimp, did a magic selection of one of the hexes, and used
"clear" in the "edit" menu.  Voila! I had a transparent area in the layer
and I could now create another layer below this one to put my terrain
graphics into. (Obviously I had to clear all the other hexes too for this to
be useful.)

Hope that helps.

 
 
 

Layer transparency

Post by Lars Clause » Fri, 24 Nov 2000 04:00:00



> Need to easily make a layer have transparent regions?

> Just dealt with that very same problem for wargame graphics I'm creating.
> I  first created a  bitmap of  a hexgrid  using Visual  Basic (yes  I'm a
> Windozer).  I  loaded it into Gimp, did  a magic selection of  one of the
> hexes, and used "clear" in the  "edit" menu.  Voila!  I had a transparent
> area in the layer and I could  now create another layer below this one to
> put my  terrain graphics into.  (Obviously  I had to clear  all the other
> hexes too for this to be useful.)

It would probably have been easier to magically select the grid, inverse
the selection, then clear.

-Lars

--
Lars Clausen (http://shasta.cs.uiuc.edu/~lrclause) | H?rdgrim of Numenor
"I do not agree with a word that you say, but I    | Retainer of Sir Kegg
will defend to the death your right to say it."    |   of Westfield
    --Evelyn Beatrice Hall paraphrasing Voltaire   | Chaos Berserker of Khorne

 
 
 

Layer transparency

Post by Don Palme » Fri, 24 Nov 2000 04:00:00


Quote:

> It would probably have been easier to magically select the grid, inverse
> the selection, then clear.

> -Lars

Doh!

Yup, I'm a newbie with gimp and you just proved it!  I appreciate the tip.
Thanks.

Don.

 
 
 

1. PS7: Layer Transparency vs Alpha Channel(s)?

In photoshop, besides the color channels (usually R,G and B or C,Y,M
and K) a layer can have transparency and multiple alpha channels.
Obviously, the transparency can be regarded as an "extra channel"
which you just don't get to see or edit explicitly.

I have 3 questions about transparency and alpha channels:

1. How do I "convert" one to another? To use an alpha channel as
transparency, I ctrl-click on the alpha channel I want, invert
selection and delete that selection from the particular layer (i.e.
from its RGB or CMYK channels). The data from the alpha channel is now
exactly the transparency. However this does not work correctly if the
layer is already transparent at some spots: areas in the layer which
were transparent, will stay transparent after this trick - no matter
what the alpha channel looks like in those areas.

Is there a correct way to achieve this, I mean something that
'overwrites' the original transparency with the data from the alpha
channel? And how about the reverse: getting the transparency data into
an alpha channel?

2. When saving images, some file formats (such as BMP) can only save
alpha channels and discard transparency, other formats only store
transparency and throw away alpha (like PNG). There's also formats
which are able to store both (only TIF afaik, besides PSD of course).
Is there a way to know which format stores what?

3. I noticed that Photoshop tends to blend (partially) transparent
pixels to white. For example I make an RGB image with a single layer,
which has transparency (i.e. it's not a background layer). When I
flatten it, transparent pixels become white (and partially transparent
pixels partially blend to white). I'm totally clueless about why it
does this, especially where it gets the color white from. I understand
it has to become _some_ color, but why white, even if my currently
selected background color is not white? Wouldn't it make more sense if
the pixels just keep their original RGB values when flattening?

Even more strange is when I save the image (before flattening it, so
it's still a single layer with transparency) as RAW: photoshop does
not save alpha channels in .raw images, so it just saves 24 bit RGB
data, and then the pixels which where semi-transparent also blends to
white! Why doesn't it just leave the RGB data as it is, and simply
throws away the alpha or transparency if it can't save that?

Thanks a lot!

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