"EagleSun" <nomail@nomail> wrote:
> "Slime" <fak### [at] emailaddress> wrote:
> > If you take the +UA version and put it on a black background in Photoshop,
> > it's nearly identical to the original (some pixels around the edge are off
> > by a very slight amount (7 or 8 out of 255), which might be caused by how
> > POV-Ray handles transparency on emissive media).
>
> Hmm... I have GIMP... perhaps you can show me how in GIMP?  (I don't have
> Photoshop.)

Open your alpha image in The Gimp, add a layer and move it underneath your
alpha image, fill the blank layer with black. Now open your other Earth
picture, either through dragging its icon to the layer stack window or by
opening it in its own window and then dragging its single layer onto the
other alpha picture. Now you have three layers, put the
Earth-with-black-background layer on top of the layer stack. Just above it
there's a drop-down menu with "Normal" written on it; click on it and
select "Difference" (make sure the Earth with background layer is
selected), you should see almost all black. For ease of use, highlight the
alpha layer, select the magic wand and click anywhere on the 'space', so
that you have a selection outlining the Earth. Now pick the eyedropper and
check the RGB values. There's a zone with over 30 in R and G in the bottom
of the Earth (huh I can't really call it South Pole, can I?). The rest is
either correct or 7-8 off in places. I don't know what causes these
discrepancies but my money is on assumed gamma stuff.

> In this "game" I was playing, I was hoping to produce 2 identical images,
> where 1 would have an alpha channel.  But I'm not convinced that I have 2
> identical images.  Would like to see someone take the image with alpha,
> remove the alpha and see if it looks identical to the first... and tell me
> what software you used to do it (and I hope you mention "GIMP" :) )
>
> Here's what I get when I attempt to remove the Alpha.  I get this using
> XnView and NetPBM.  Obviously it's not identical to the original.  Maybe I
> need more explanation?

The Alpha channel makes part of the color transparent, by filtering whatever
it is applied on, not unlike transmit in POV-ray. If you remove it from your
pic, unsurprisingly, you'll get the full color that is currently filtered.

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