You will need
  • - Photoshop;
  • image.
Download editor Photoshop image file to which you want to add shadows, use the option Open from the File menu. As a basis you'll need a copy of the layer with the cut out from a background object that casts a shadow. If you are working with a layered document, select the layer with your object and duplicate it with Ctrl+J.
If you have the image composed of one layer, and casting a shadow object is not separated it from the background, trace the outline of the subject with the Lasso tool. Option Save Selection menu Select save selection to a new channel. By default it will be called “Alpha1”. Copy the selected object to a new layer.
Turn the copy of the subject into a dark silhouette. If the layer is hiding the background mask, just fill the object with black color applying the Paint Bucket tool. If the layer has no mask, limit the scope of the fill, loading the selection option, Load Selection menu Select. As a source of information on the allocation, select from the Channel list, select the Layer Transparency. The loaded area can be filled with color.
Change the blending mode of the workpiece to the shadows with the layer below, from Normal to Multiply. If necessary, dilute the shadow option Gaussian Blur band Blur Filter menu. The radius of the blur depends on the brightness of the light source in the collage. To create the effect of very bright light will require a minimum blur radius, a weak source will give soft shadows with very soft edges. If the shadow was very dark, reduce the opacity setting in the layers palette the Opacity parameter.
Using the Skew or Distort options group menu Edit Transform warp shadow, putting it on the surface. If the collage contains other shadows, a new position at the same angle.
If the shadow falls on several different planes, highlight the layer, shading every surface. Option Edit menu Cut cut the selection and paste it on a new layer with the Paste option. Deform plot the shadow for each surface and collect the processed fragments, using the option Merge Down the Layer menu to all layers with parts of the shadow.
Move treated a shadow under the layer with the object that casts it. In a layered document you can just use the mouse. If you created the shadow from the land of single-layer file, click on the background layer and load a selection saved in a channel. Return to the copy layer and apply the Clear option of the Edit menu. This way you will remove part of the shadow, which should be behind the object.
For future work, save the document with Save option of the File menu. If you don't want to lose the previous version of the file, use the Save As option.