Creating a 32-bit Alpha Key Mask File For Video In Photoshop
This is how you create a 32 bit targa file (24 bit RGB file with an 8 bit mask
channel) in Photoshop for alpha keying logos or graphics onto video productions.
- Clean up & colorize logo or graphic if it isn't already. Make sure all
edges of graphic are clean so you can get a nice key slice with the magic
wand.
- If your graphic isn't already a 24 bit file (i.e. if it's an 8 bit GIF),
then convert it over to 24 bits by menuing under Image/Mode/RGB color.
- Scale logo to the size you want it to be in pixels.
- Menu under Image/Canvas size, & set to a 4X3 ratio for your output
(i.e. 720X540 or 320X240).
- Cut & paste graphic to position in it's proper spot on the canvas,
keeping in mind that safe title/graphic area is no closer than 10% from each
edge. Flatten the layers so it's one image (Layer/Flatten Image).
- Flood any keyed areas (areas you want background video to be seen in) with
zero black or 255 white, whichever isn't in the edge of the graphic.
- Pick the magic wand tool & click on the keyed area (shift click to
add to a previous selection). In magic wand options you may have to adjust
the wand tolerance (I often use 32, but you should experiment) & turn
on anti-aliased to get a clean key slice. With some graphics, you may have
to manually adjust the outline setting of the magic wand to get a more accurate
edge on your graphic.
- Menu under Select/Inverse.
- With the channels tab selected, click on "save selection as a channel"
to create an 8 bit key channel, OR menu under Select/Save selection, create
a new channel or alpha channel (depending on whether you are using Photoshop
4 or 5. They mean the same thing, just different terminology). This channel
will be used to create the keyed area in video editing.
- Menu under File/Save As, select Targa (tga) as the file type, give it a
new name and click OK. Then select 32bits/pixels and OK. You will also see
a dialogue box that says "some image data will not be saved" for
printing...Click OK, it won't adversely affect the final file.
- If your video output hardware doesn't use square pixels, scale to the final
target size for video (i.e. 720X486). To do this, menu under Image/Image size,
unselect constrain proportions, enter new size in pixels & click ok.
- Save. File is now ready for use in video editing program.
By Doug Hembruff.
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