creating an alpha channel based on darkness

[markuskidd]

I have a bunch of images that are blended to black that I'd like to convert, if possible, to PNGs that are blended to the alpha channel.

I have access to Photoshop 7 and Paint Shop Pro 7, is there any way for me to do that in either program?

[pepe]

look for  a "luma keyer" filter in those or other programs.
They will convert luminosity into alpha.

[markuskidd]

Searches through help reveal no information on a built-in luma keyer in either software, though I have googled some sites on plugins you can purchase. Any way to do this with what I've got or can get for free?

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[Orangy Tang]

Psp had be miffed with this one for a while, but its not too tricky when you've got it sussed:

• For starters, you'll probably be best painting colour and alpha images separately (or copy/paste and use greyscale for an easy alternative).
  
• On the colour image, choose Masks->new->from image..
  
• Once you've got your mask loaded, choose Masks->save to alpha
  
• Finally, delete the mask Do not merge with the current layer though.
  

Hope that gets you on your way..

[markuskidd]

Thanks for the help   Trying to get this to work..

• For starters, you'll probably be best painting colour and alpha images separately (or copy/paste and use greyscale for an easy alternative).
  

Hrm... what do you mean by color and alpha images?

• On the colour image, choose Masks->new->from image..
  

I do this, and I get something that looks convincingly like the luminosity (sp?) of the image. Good so far.

• Once you've got your mask loaded, choose Masks->save to alpha
  

Nothing happens at this point, except that I have to name a new selection.

• Finally, delete the mask Do not merge with the current layer though.
  

And.... it goes back to looking just as it did before I started.

Hope that gets you on your way..

I'm so close... maybe just one more push, eh?  :-/
I was going to take screenshots to illustrate what I'm doing; then I found out that PSP doesn't want to take shots of itself for some reason.

[Orangy Tang]

It will go back to looking the same, but the alpha channel is still stored inside the image. You can use the alpha channel in your own apps, and if you're running XP then it'll affect the thumbnail view. You can reload the alpha into a mask again to check that its still there, just don't try and save more than one in a single file, odds are it will pick one randomly and just save that.

By separate colour and alpha, I mean paint a solid, no transparency colour image for the base, and a greyscale image that represents the alpha intensity (full white being solid, black being invisible). This gives you greater control before merging them together as a single image file.

[markuskidd]

{dons newbie suit}
erm... so how would i go about painting the sperate channels so that the dark portions of my RGB are translucent?

[markuskidd]

Troublingly, if I export the PNG (making sure to keep 16.7 colors and using existing alpha transparancy) and then reopen it, when I click on Masks->Load from Alpha Channel it tells me that "No current documents have alpha channels."

[Abuse]

urg, giving step by step instructions on how to use psp or photoshop is never gonna be ez.  

I'll just keep it short - it does work, cos i've done it (in psp7)
(and after loading the png back in, the alpha mask is still there)

silly question, but have you read the help doc  

[markuskidd]

I've found that if I save it first as a TGA file, and then export *that* as a PNG, I can recover the mask from the PNG.. I've tested both ways half a dozen times and that's what I get each time. I'm not entirely sure *why* whis would be, but it's at least progress. Hopefully I can get it working without the tedious intermediate step eventually

The only thing I guess I need to know is how to paint these PNGs so that I get the correct translucency

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[Abuse]

same way as any other  

[markuskidd]

Thanks to all who have pitched in. Final verdict: using PNG export in PSP (rather than Save As and then PNG format) has been eating my alpha channel. Everything seems to be working pretty well at this point.
(Well, my framerate use has gone to sh#t, but that's the fun, right?)

[leknor]

In my experience PhotoShop has had weak PNG support but that was 2+ years ago. I've had better luck with the Gimp which you can get for windows here: http://www.gimp.org/win32/ It's crunchy but whatever, it works.

[zparticle]

PhotoShop is the only graphics program I use, and I always use PNG format. However I have heard good things about Gimp.