This tutorial came about when I tried to find a way to render just shadows in Softimage. The method explained here is only one approach, if you know of a better one then let me know.

Scene Set-up -

1) Load your background footage using the rotoscope view port in the perspective window. If your background footage is fairly static you can just use a still frame to save disk space and replace it later on with an image sequence. However if you're likely to have any interaction between foreground and background you'll need to use a sequence.

2) Using all those measurements you made on set build a CG version of the background image (called match move objects), you'll only need to build those background elements that will receive shadows or reflections from your CG objects. This process will take some time but if you have sufficient reference from the set you should be able to build the background fairly easily.

3) This is the backbreaker - you'll need to match your CG camera with the real camera so your match move objects align perfectly with your background footage, I tend to line everything up best I can and then go through and tag a few points on the match move objects and touch them up a little - If you're careful no one will notice any SMALL changes!.


RGB Pass -

4) Set up your scene make sure all the lights are postioned carefully so they match the lights you had on set (this is an art not a science pay close attention to falloff, intensity and colour).

5) Now set all your match move objects to only appear in reflections and to cast shadows onto our CG object, you can do this by selecting each of them and choosing info->selection->render setup and turning off the little tick box for primary rays. Now the match move objects won't render but they will cast shadows and reflections onto your CG object.

6) Render out the RGB pass of your animation, that is only the CG objects you want to composite into the scene, don't include any of the match move objects you'll be using to cast shadows on.


Shadow Pass -

7) The trick now is to render out only our shadows without any trace of our match move objects, so that we can composite the shadows in nicely behind the CG objects. We'll need to select the match move objects and give them a Lambert Material. Also go to info -> selection -> render setup and turn off Shadow casting for the match move objects.

8) In Atmosphere make sure the Ambient light is set to 0.0, to avoid any unwanted shading.

9) For all the lights that will cast shadows set their colour (or luminance using HLS rather than RGB) to a negative number, for example if your light has been set to HLS 0,0.8,0 then just change it to 0,-0.8,0. Also change the Umbra intensity of the shadows to be a negative value.

10) Now one by one select all your CG objects and go to info -> selection -> render set-up and turn off Primary Rays, now you'll see only the shadows your objects leave. (if you don't have Mental Ray you can use the shadow material but you'll have a hard time completing the following steps without it).

11) Now render out the animation again and you should get something like this a black frame with a grey or white blob where the shadow should be If you negate this image and layer it over your background and then composite your RGB pass on top you should have a good composite, you can now blur shadows and make any alterations you wish to match the original footage.

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