Referenced from Joining Images in Photoshop
THIS IS A SIMPLE WALK-THROUGH for producing drop shadows by hand. In some cases you'll want to know and understand this process because you'll need a shadow that does not "drop" from the entire object, or one that you can manipulate as a separate object rather than part of the layer as in the Drop-Shadow layer effect.
This is an object, rastered on its own layer.
The first thing we'll do is duplicate that object to another new layer.
* Drag the layer to the "New Layer" icon at the bottom of the layers palette, or,
* Use the "Float" command: Command/J, or Control/J for Windows.
Next: Select the layer original object layer
* Command/click the layer thumbnail (Control/Click for Windows)
Now your object is selected on the bottom layer.
Tap the letter D to change colors to default
Tap Option/Delete (Alt/Delete for Windows) to fill the object
(Racing ants, the dotted line around the object signifies it is selected)
In this capture, you'll see the object is now filled with the foreground color (Black) even though you can't see it because it's behind (or beneath) the White object on the next layer.
Tap Command/D (Control/D) to drop any selections
Choose: Filter > Blur > Gaussian Blur
and in the resulting dialog box, set the amount of blur to result in the desired amount of shadow.
Here, we've created the blur than selected the Move tool (Tap V) to "nudge" the shadow to its new location. (In this example, we went 12 pixels right, and 12 pixels down. Your own project may be different. )
You've completed the drop shadow just as the Layer Effects would have. We've actually been doing this for years-- even long before Photoshop had layers!
However, you now own the shadow as a separate object and can act upon it as you wish. For instance, perhaps the shadow needs to run across multiple surfaces...
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