There are cases where a tiny detail is all that’s required for some cut-out object or layer to look realistic in a composition. In many of these cases, that detail is a plain drop shadow.
Layers, the Magic Eraser Tool, and the Blur effect are used in this tutorial.
Open an image in Pixelmator, preferably one with an object that could use some shadow.
If the object you’re applying the effect to is still not cut out of the background, use any of the selection tools to cut it out so that the object appears on a separate, transparent layer.
Duplicate the layer that contains the object to which you’re applying the effect (in our case, the leaf) by selecting it in the Layers palette (View > Show Layers) and choosing Layer > Duplicate Layer.
In order to turn the recently duplicated object into a shadow, fill the duplicated layer by choosing Edit > Fill, then in the Fill palette set the Color to black. Then, select the “Preserve Transparency” checkbox to fill the object only and not the entire layer. Click OK.
Once the shadow is created, move it underneath the main object by dragging the duplicated layer below the original object’s layer in the Layers palette. To position the shadow, click to select the Move Tool in the Tools palette and drag the shadow to move it where you want it to appear. Use the example in the image below as a reference.
Tip: To cast a true-to-life drop shadow, move the shadow in the direction directly opposite from the main light source. In many cases you may decide where the main light source should be.
To smooth the shadow, double-click the Gaussian thumbnail in the Effects Browser (View > Show Effects Browser) and set the Radius to approximately 15.
To make the shadow look even more realistic, make it a bit transparent by reducing the shadow layer’s Opacity to approximately 20-40% in the Layers palette.
And that’s how the drop shadow effect is applied to an object. However, in order to cast the best shadow for a particular object, feel free to experiment with different values for the smoothness and the transparency.