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Subject: Using alpha channels as masks in Photoshop...


Lostboy ( ) posted Fri, 01 February 2002 at 11:30 AM · edited Tue, 26 November 2024 at 4:26 AM

Can anyone help me convert an Alpha Channel into an honest to God mask? I've looked everywhere, and the only documents I find show me how to load it as a selection. But a selection can't erase subtle edge details like hairs of feathered edges. The alpha channel that Lightwave (or other 3D software) renders should be able to mask these areas. Comping is a snap in AfterEffects...why is it so damn difficult to load an alpha channel as a true mask (NOT as a selection) in Photoshop?! Any help would be greatly appreciated.


Varian ( ) posted Fri, 01 February 2002 at 12:07 PM

Moving this to the Photoshop forum so you can get specific assistance. Good luck! :)


Jim Burton ( ) posted Fri, 01 February 2002 at 3:05 PM

Hi Lostboy! A mask is the inverse of a selection in Photoshop, anyway. But bear in mind a mask doesn't do anything in itself, it just lets you operate on selected parts of an image (or to look at it the other way, it protects parts of the image from what you are doing.) Anyway, load the selection, then use Select, Inverse to change the selected to a mask if you want. CTRL H (Apple H) toggles the marching ants on and off if they are in the way. Bear in mind th emarching ants indicate 50% or more selected, but a pixel can have the full range of 0 to 100% selected (or masked)


dlm ( ) posted Fri, 01 February 2002 at 6:19 PM

If you want a visible mask,go to the quickmask button at the very bottom of the tools pallette and click on it.It looks like a circle inside a square.One is quickmask on the other off. By default the selected area will turn up as a red coloured mask.You can change colour & opacity) You can then alter the mask with any of the tools by selecting the default black & white colours in the tool pallette.Painting in black will add to the mask,white will cut away.You can use airbrush ect to feather the edge. When you convert back to selection it will act as a mask retaing the information of the painted one.


Lostboy ( ) posted Fri, 01 February 2002 at 8:14 PM

AHHH!! Finally, the answer that no one else could provide! My alphas always showed up as red, and I could never figure out why! I played with the controls, but obviously didn't know what I was looking at. I can't do anything until I finish work today, but I can't wait to get home. Thanks so much!


Lostboy ( ) posted Wed, 06 February 2002 at 12:06 PM

Sorry to be a bother after the help... OK, when I load in an image rendered as a 32 bit Photoshop file (or tiff, or png, etc...), and I click on the Alpha channel, the "masked" area shows up red. I can play with the color value, and play with the opacity. But after converting the background to a layer, if I set the opacity to 100% - the masked area is black! What I'm trying to do is to load in a rendered image and delete the background. I want to use the Alpha channel like a quick mask - where the background 'disappears.' Loading it as a selection isn't good enough because sometimes my images have very fine edges (hair) or semi-transparancy (glass). Am I missing a step? Or is my understanding of how selections work a bit lacking? Thanks!


dlm ( ) posted Wed, 06 February 2002 at 1:33 PM

By rendered image I take it you mean rendered in another programme,such as poser or bryce. Both of these programmes support alpha channels in tiff or photoshop formats. You do as your doing and load the Alpha channel as a selection,then go to the quick mask button and click on quickmask.If you double click the quickmask icon you should get an options pallette that allows you to set the mask as covering selected or none selected areas. If you want to refine the mask you now need to do some panting around areas like hair in close up.Once your happy with the painted quickmask,turn it back into a selection.If your main figure is selected I find it best to copy then paste the selection onto a new layer,then I,m free to do what I want with the background.If not invert the selection. Varying opacity is easily handled by duplicating the image on another layer.Removing the semitransparent section from the top layer allowing the duplicate to show through and altering the opacity of the duplicate layer to allow some background to show through.


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