In this tutorial, I will attempt to explain how to make a spotlight project a shadow of a specific size and shape. I will do this with textured lights and negative light. (I've used thumbnail images below for people with slow internets, but you can enlarge them by clicking.)
Here's about what I want it to look like:
So, first things first; in order to make a shadow projector, you need a shadow! My approach for this is going to be to use an image with some alpha, and to map it to a negative spotlight in such a way that the negative light only shines through where there is alpha. I used this image as a shadow. It's not very exciting, but it's a gradient with alpha in the middle, and gray on the edges.
I made it in paint.net using the gradients tool. It was fairly simple. You should make the outside of the image black, I discovered, because this is going to 'color' the negative light.
Negative light, in Blender, is light that cancels out other light that is being reflected off of a surface. So, if I shine a red-colored negative light at a white surface, the surface turns greenish. Because the red 'real' light is canceled out by the red negative light, only the blue and yellow 'real' are left to reflect. What we want, is a stencil that subtracts no light in the middle, and slowly fades in to subtracting all light on the edges. So make the outside black, and the inside clear (alpha) or white.
Now that we have our image, we need to use it in Blender. So, go ahead and start it up. I've designed this to be used on a spotlight, so add one to the scene. I also added a cube and a plane, so we'll have something to project the shadow onto, and something to 'cast' the shadow. I pointed the spotlight down, and parented the cube to it so that the spotlight will follow the cube. My setup looks like this.
Once that's set, select the spotlight, and go into materials. Select the Lamp buttons, by pressing the button with a lamp on it. On the far right, is the texture settings. Click Add New, and then click the Texture button up top to access the texture settings. From here, click the drop-down menu to select the texture type 'image'. Load the texture you're planning to use. Now, there are only two buttons to push, and then we're done with this panel. The first is 'premul', and it's in a group of four buttons directly below the 'load' box. This does something with the alpha, and makes the edges of our shadow nice and smooth. The other is 'extend', and it's in the middle left of the 'map image' tab. What this does, is extend the color at the edges of our image out, so we only have a single 'hole' for the negative light to shine through, and the rest of it is blocked off by the black color.
I've highlighted the buttons here:
Ok, now that's set up, we can go back to the Lamp settings. This won't take long; first off, make sure that the texture is set to affect the color of the lamp. Then, turn off shadows; both the ray shadow and buf shadow buttons should be off, because we don't need this lamp casting normal shadows at all. Last, click the negative and layer buttons on the left. This makes the spot cast negative light, and only illuminate the objects on the same layer as it.
Here are the buttons highlighted:
So, we're almost done; simply move the spotlight and the ground plane into their own layer, and your shadows should be good to go! Here's what mine looked like, with aprox AO turned on:
Sorry if this is a bit hard to read; I've thrown it together, and it's past my bedtime. >.< Hopefully, I'll be back to proofread this later. Oh, and if you want to use my shadow, you ought to be able to download any/all of the pics through clicking on them.
Oops, almost forgot: the .blend.