The amazing story of Secretariat's Triple Crown victory in 1973 required special vfx treatment by Pixel Magic  (under the supervision of Ray McIntyre Jr.), since the production was not allowed to shoot racing footage on location.
"We shot all five races at the same facility in Louisiana: Evangeline Downs, located in Lafayette," McIntyre recounts. "The issue right from the beginning is you shoot the footage [you need] to make it look like you're at the different locations you're supposed to be at."
In addition to the Triple Crown that comprises The Kentucky Derby in Louisville, The Preakness in New York and The Belmont in Baltimore, Secretariat featured qualifying runs in New York at the Aqueduct and Saratoga.
"I went to each of the locations and shot visual effects plates, and then we composited in or created both the backgrounds and stadiums," McIntyre continues."We filled the stadiums with people; changed the background because every race has the hero camera following the horses around the last turn into the homestretch. When we shot the racing footage in Louisiana, you see the same background in every shot, so it became a visual effects shot so that it represented a different track."
Pixel Magic, whose team also included Brad Moylan (compositing supervisor), Rif Dagher (CG supervisor), Victor DiMichina (production supervisor), Dione Wood (VFX producer) and Ray Scalice (general manager), then went about making the environment fit the appropriate track: changing the backstretch or the area behind the corner as well as on the infield when the camera pans with the horses and you see the grandstands and the rest of the environment.
The stadiums were empty except for Churchill Downs, which holds about 65,000 people, so they populated the stands with CG spectators. "In all cases, if they were close to the camera, I shot greenscreen people as a separate element to populate the front two or three rows," McIntyre explains, "and then, after that, shot plates for the various angles and then the rest of the people are CG. We greenscreened real people in the front rows where close detail matters, but immediately behind or far enough away they become CG."
"When we were in Kingland, the decision was made that that represented Belmont, the final race of the Triple Crown. Then, when I went back to recreate Belmont, I went to Kingland, where production could shoot with extras inside.
"The first race is Saratoga and that was shot at the training track Evangeline Downs in Louisiana. Every time you see the grandstands, it's a composite and we also populated the grandstands. In that case, it's seen at a distance, so there are CG people all the time. But there CG horses in some scenes, too."
The work involved a tremendous amount of roto, not only for the environment but also for every single horse "because we were putting grandstands and people beyond them. We have big panning shots at the start of Saratoga. And two-year-old horses race from the backstretch around the front stretch and only race half the track. There is also CG dirt kicked up."
"The difference with Churchill Downs is not only do they fill the grandstands with that race every year but they also fill the infield. There will be 150,000 people. For that track I had to populate a lot more people. The actual horserace footage, again, was shot in Louisiana, so we did have some extras there. But I had to replace the infield with the one from Churchill Downs. I had to rotoscope the horses and the people and add around 70,000 people in the infield. I shot additional greenscreen extras with same camera angles and populated the near ground with real people and the rest are all CG, depending on the shot.
"But the advantage of shooting there was real lighting reference, scale and size, placement reference and motion blur reference. Since I was doing so much rotoscope anyway -- the horses and the their manes, all the dirt being kicked up -- I figured a little more roto for the people at the railing didn't add that much work; it gave me some reality.
"The biggest thing in this movie was the amount of rotoscope to replace everything because we were shooting in a single environment with only 50 people."
Bill Desowitz is senior editor of AWN & VFXWorld.