drop shadows and alpha channels

M
Posted By
Matt
Sep 14, 2004
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1128
Replies
3
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Closed
I’m using Photoshop 7 to create graphics for video.

If I’ve used the layer style to add a drop shadow to an element is there a quick and easy way to give that element an alpha channel that includes the shadow? All I can ever seem to get is the item itself or if I do get the shadow then I also tend to get a white halo around it and I need it to fade to nothingness. I have achieved this effect with older versions of Photoshop in the past but my steps were too time consuming and I’m hoping that my newer version will have a simple easy way that I just haven’t found yet.
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MR
Mike Russell
Sep 14, 2004
Matt wrote:
I’m using Photoshop 7 to create graphics for video.

If I’ve used the layer style to add a drop shadow to an element is there a quick and easy way to give that element an alpha channel that includes the shadow? All I can ever seem to get is the item itself or if I do get the shadow then I also tend to get a white halo around it and I need it to fade to nothingness. I have achieved this effect with older versions of Photoshop in the past but my steps were too time consuming and I’m hoping that my newer version will have a simple easy way that I just haven’t found yet.

The white halo happens when a Photoshop image with alpha channel is intrepreted incorrectly. If your video editor’s transparency option has a checkbox for "white matte", click on it and the halo will go away.

Otherwise, if you don’t mind a little extra work, do what you did to get the shadow with the white halo, and then color the shadow solid, so that the shadow transparency comes only from the alpha channel.

For more info, do a google on "premultiplied alpha". —

Mike Russell
www.curvemeister.com
www.geigy.2y.net
M
Matt
Sep 14, 2004
"Mike Russell" wrote in message
Matt wrote:

The white halo happens when a Photoshop image with alpha channel is intrepreted incorrectly. If your video editor’s transparency option has a checkbox for "white matte", click on it and the halo will go away.
Otherwise, if you don’t mind a little extra work, do what you did to get the
shadow with the white halo, and then color the shadow solid, so that the shadow transparency comes only from the alpha channel.

For more info, do a google on "premultiplied alpha".

Well perhaps I’m doing something wrong. I’ve always thought that the white halo came from the fact that Photoshop doesn’t fully select the drop shadow, not from a bad key. I know what you’re talking about in terms of a white matte and this isn’t exactly the same thing… but I’m sure that the fault lies in my process. In the past I’ve always used the layer style to apply a drop shadow, then rendered the layer so that the shadow is selectable, then ctrl-clicked the layer to select the object, then went to channels and created an alpha channel and deleted the selected area… at this point Photoshop never gives me a true feathered/gradient delete of the drop shadow and I usually go in with the air brush tool and paint back in extra white as lightly as possible to give the desired effect. Or if the object is a simple shape then I’ll feather the selection and then delete and this always gives me a smooth drop shadow in the alpha channel.
MR
Mike Russell
Sep 15, 2004
Matt wrote:
[re copying an accurate drop shadow mask to the alpha channel]
… I’ve always thought that the
white halo came from the fact that Photoshop doesn’t fully select the drop shadow, not from a bad key.

The main thing is you want the shading info to be in the alpha mask, and pure black in the shadow, not gray.

I know what you’re talking about in
terms of a white matte and this isn’t exactly the same thing. but I’m sure that the fault lies in my process. In the past I’ve always used the layer style to apply a drop shadow, then rendered the layer so that the shadow is selectable, then ctrl-clicked the layer to select the object, then went to channels and created an alpha channel and deleted the selected area… at this point Photoshop never gives me a true feathered/gradient delete of the drop shadow and I usually go in with the air brush tool and paint back in extra white as lightly as possible to give the desired effect. Or if the object is a simple shape then I’ll feather the selection and then delete and this always gives me a smooth drop shadow in the alpha channel.

I’d suggest an addiitonal merge. Create a new layer under the rendered drop shadow and merge your rendered drop shadow on that to get a more accurate rendering of the drop shadow’s transparency. Then ctrl click on the Layer and use that selection to add white to your alpha channel. This will make it unnecessary to touch up the shadow manually.

If you ctrl click on the layer to load that as a selection, and copy/paste pure black to the layer containing your object, the appearance of the drop shadow will be improved. It will match the darkness of the original drop shadow, and it will always darken what is under it, as a shadow should.

This procedure doesn’t involve any manual touching up, so you can pack it into an action.
—

Mike Russell
www.curvemeister.com
www.geigy.2y.net

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