I have just corrected my mistakes about the the axis you need to rotate about, I got X and Y mixed up in the original version of this post)
@ZantievIn general creating shadows that properly align with those in a background image is very difficult, but there are some simple special cases where the shadow is directly behind or in front of the item casting the shadow.
Let us start by considering a clip as if it was a rectangular playing card standing upright on a horizontal surface. A shadow cast by this by a very distant point light will be a dark parallelogram lying on the horizontal surface and will meet the rectangle casting the shadow at the base of that rectangle. The horizontal width of the the shadow will be equal to that of the rectangle but its height and the angles between its sides will vary depending on the direction of the light source.
To transform the original rectangle into an image of its shadow can be regarded as comprising
a rotation by 90 degrees (to get it into the horizontal plane)
a vertical scaling (to lengthen or shorten it)
a shear transform (to convert it to a parallelogram)
and these transforms are those that would need to be applied if the rectangle is replaced by any flat image (such as that of an iStripper clip).
The first two of these transforms are simple (e.g. Rot: 90,0,0) and a scaling (e.g. scale: 1.0,0.5,1.0) but there is no simple way to do the shear transform.
If the light source casting the shadow is directly in front of or behind the flat object casting the shadow then no shear transform is needed and we can generate a shadow just by applying a vertical scale factor and rotating about the base of the clip by plus or minus 90 degrees about the clip's X axis.
In a 2D scene you can't rotate about the X axis (or about Y, only about Z) but the lengthening or shortening of the shadow can be adjusted to have the same visual effect.
If the light is coming from a slightly different direction then reasonable shadows can still be created by applying a small rotation about the Z axis, but this can never be perfect (think of the playing card approximation where dong this would men that bases of the shadow and the original object would not meet except at a single point).
The above only considers the very simplest cases. To generate realistic looking shadows you also need to consider what happens if the light source is not very distant and is not a point source. To model the effects of these you really need to make the shadow have soft edges and to taper.
The soft edges can be approximated by using a blur filter but the tapering is more difficult. Like the shear there is no way to do it using just .scn file features and you have to resort to using a filter.
I have produced a couple of "shadow" filters - they can be found in the EmuLib "toolbox" - but they are rather difficult to use (at least I have difficulties using them). As with all the EmuLib shaders there are some simple test scenes and I use them in a very small number of realistic scenes - e.g. the Woodland Border scene (which is in the Misc Scenes zip)
Addendum:
The user interface to my Shadow shaders is ***** becaue the parameters controlling it interract with each other in ways that mean that if you set the parameters for one aspect so that it looks OK then change a parameter for different aspect you need to go back and tweak the first set - which is the main reason why using it is not easy. At some point I hope to go back and find a better, more user friendly, way of doing all this - but I have been saying that to myself for a few years now. I also should look into using vertex shaders to perform the required shape transformations.
