Newbie – alpha channel help

PM
Posted By
Phil_McFarlane
Aug 12, 2008
Views
267
Replies
9
Status
Closed
Hi everyone,

I have a scan of my signature in a TIFF file. I would like to add an alpha channel, so that when I paste it into documents the black signature will be visible but the white background will be transparent, allowing the document behind to show through. I have changed the signature scan into greyscale mode, and added an alpha channel. I know how to adjust the mask using paint tools, but there must be a simple way to select only the white background of the signature and apply the mask to this portion.

Any help much appreciated.

Cheers!
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alpha channel help newbie

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JJ
Jim_Jordan
Aug 12, 2008
What type of documents are you trying to paste into?

Most signature files can be saved in the ‘bitmap’ color mode, one more step past ‘grayscale’. Most page layout programs treat 1-bitmap files as transparent with no need for an alpha channel.
JM
J_Maloney
Aug 12, 2008
Press command (clover) and then click the gray channel in the channels palette. You should be able to go to the ayers palette and delete the background. You might need to double click the background layer to make it an actual "layer".

J
JM
J_Maloney
Aug 12, 2008
Jim’s got a good point. Or PNG24, which will save the alpha channel for you, if you just create a transparent layer. Works great in the Office suite (yes!).
NK
Neil_Keller
Aug 12, 2008
Bitmap works very well (as previously pointed out) — but — there are no grays. Bitmaps are either solid color or none. So, no antialiasing is available. Unless the signature is high resolution, you will see jagged edges.

However, if you’re working in InDesign (or XPress), you can easily colorize the black-and-white bitmap signature from within the application, so you can have, say, blue or any other color you wish assigned to it. (Actually, the same holds true from grayscale images as well.)

Neil
PM
Phil_McFarlane
Aug 12, 2008
Hi everyone,

Thanks for the quick replies!

I have my image in bitmap mode (even tried saving it as a BMP file), but when I paste it on top of another TIFF either in Photoshop or Word, the white background remains opaque, obscuring the document beneath.

Cheers!
NK
Neil_Keller
Aug 12, 2008
Phil,

Save it as a bitmap .tif. I wasn’t even thinking of a .bmp.

Neil
JJ
Jim_Jordan
Aug 12, 2008
If using Word you have two options as outlined in this thread.

1) follow the advice of post #3 and use a PNG file with transparency

or

2) after placing the bitmap TIFF, use the ‘set transparent color’ tool found in the Picture toolbar to select the white in the placed signature image.
PM
Phil_McFarlane
Aug 12, 2008
Thanks Jim,

I have tried with the file saved as BMP, PDF and TIFF. For some reason, the "Set transparent color" tool is greyed out when I select my signature picture, but not when I select the background picture.

Cheers!
PM
Phil_McFarlane
Aug 12, 2008
Interesting, I went back to the original scan file, and copied the signature out of that. Using that copy of the signature and the "Set transparent color" option works perfectly. I guess something bad happened along the way to my signature file, so I will recreate it from the original scan file.

Thanks for everyone’s quick and useful replies!

Cheers!

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