Once the basics have been blocked out using the simulation and particle systems, all of the details are planned out. This is what makes the difference! Without all these exact details in place, the shot would look realistic but would still feel wrong, causing you to realize intuitively that the scene was using CG effects. You have probably seen this in other movies -- maybe the shadows were not quite right, or the coloration of the CG portion didn’t exactly match the real part.
It is amazing to look at all the different layers that ILM created for each shot:
In the all-CG shots, nothing else needs to be done. But many of the shots combine CG and live action. Most of the live action was filmed in a water tank with a blue screen surrounding it, on Stage 16 at Warner Bros. Studios (the largest indoor tank in the world). The blue screen has a grid of red dots in white squares, called targets on it, which are used to align the virtual camera’s viewpoint with the live camera. Most of the targets had to be removed manually frame by frame, making for a long and arduous process.
The CG elements are color-corrected and blended with the live footage. Some scenes require a little CG sleight of hand -- adding CG waves or swells -- to mask unwanted live elements. Additional mist and foam patterns are composited onto each frame. Finally, all of the live and CG elements are composited together to form the complete shot.
The shots are edited together and the film is released. You go to the theater, or you rent the VHS or DVD release of "The Perfect Storm," and you are stunned by the most amazing and realistic weather effects ever created, unable to tell what is real and what is computer-generated. And that is the greatest compliment ILM could ask for.