Bruce Manning drags a mouse wired to his Mac G4 through this new tool and reveals that no person wearing a blue shirt against a blue screen is too tough for this matte creating program. Happy compositing is here again.
When blue and green screen images are not perfectly shot or lit, the background reflects blue or green spill on the subject. Often there is not enough time to trick out each shot. Factors such as budgetary cut backs, grips and gaffers at the craft service table munching on cashews, not enough 10k's or sky pans to even the background or whatever the cameraman's excuse is this time, it's your job as a digital compositor to fix it. That's when you pull Puffin's Primatte Keyer out of your bag of tricks and tools and become a hero.
I had been working on a sci-fi short film called Lizards. There were several blue screen shots in the movie. As always some shots went together easily, but there were a couple of real lulus. Bad spill, not enough light -- you know the story. One of the subjects was a little boy wearing a blue shirt. I had tried to fix the scene with another brand of blue screen keyer, but had limited success. That blue shirt was causing real trouble. It would change brown every time I would composite it. It was time to try something new.
What It Is
I slid the Primatte disk in my MacG4. Primatte works with Adobe After Effects. I found the Primatte Keyer in the After Effects clips effects menu. The interface is comprised of four main components: view buttons; Primatte tools; sliders; and Help, Undo and Redo. The Primatte buttons are user friendly and visually easy to work with. The Primatte tools allow you to select the keying operation you wish to perform. These operations include such actions as sampling colors to be keyed, adjusting properties of sampled foreground color to remove color spill, defining transparency properties of the matte, specifying foreground colors, removing noise and other matte creation steps. The Primatte tools also include the slider, Decolor, Clip high and Clip low. The View buttons allow you to choose which component is displayed in the composition window. Sliders allow you to fine-tune your matte by applying blur and shrinking the matte. The Help, Undo and Redo buttons allow you to correct mistakes and obtain on-line help. But the star of this software package is the process known as sampling. Sampling involves selecting an area of the composite with the mouse by dragging a line. As you drag, a snail trail will follow the path of the cursor to show you what area of the image you have selected. After releasing the mouse button, Primatte Keyer will start the compositing process and quickly generate a matte. A good matte is the secret of effective compositing.
In Lizards' scene 10C, the little boy had a blue shirt and there was uneven lighting in the blue background. When creating a matte the foreground elements should be a perfect white and the background elements perfect black. The technology behind the Primatte Keyer that makes perfect mattes possible is a process called the Primatte polyhedral slicing algorithm. I used the tools offered by this software to solve the problem.
How It Works
The first step in fixing the blue shirt/blue background problem was to (Select BG Color) indicate the selected background backing color on the original foreground image. The sample should usually be taken from a 'medium shaded' area near the foreground object. By 'medium shaded' area, I mean that if blue is the backing color and that area of the foreground image has blue shades ranging from very pale blue to almost black, a shade of blue in-between these extreme ranges should be chosen. If good results are not obtained using this sample, Primatte should be reset and another sample taken using a slightly darker or lighter shade of blue. A single pixel may be selected or a range of pixels (snail trail or rectangular sample). If a range of pixels is taken, the sample will be averaged to get a single color sample. Don't worry, Primatte Keyer has 99 levels of undo so take advantage of this and sample this and that and this and that.
Before BG Noise Removal. © Photron USA/Puffin Designs. After BG Noise Removal. © Photron USA/Puffin Designs.
The second step I used in fixing the blue shirt/blue background problem was to clean up the backing color area by adding additional shades of blue. This second step (Clean BG Noise) is usually executed while viewing the black and white (Matte View).
While in the (Clean BG Noise) sampling mode, the user samples the white milky regions as shown in the left-hand image above. As the user samples these regions, they turn to black as shown in the right-hand image above.
The third step in fixing the blue shirt/blue background problem (Clean FG Noise) was to sample and eliminate gray areas in the 100% foreground area of the image.
Before FG Noise Removal. © Photron USA/Puffin Designs After FG Noise Removal. © Photron USA/Puffin Designs
Again, I made several samples on the dark, grayish areas on the foreground object until it was solid white in color. Care should be taken in both this and the previous steps to not sample too close to the edges of the foreground object. Getting too close to the foreground object's edges will result in hard edges around the foreground object. At this point, the matte or key has been created and would allow the foreground objects to be composited into a new background image. I then changed the display mode to the color (Composite View). There was 'color spill' on the edges of the foreground objects. When on the edges of the foreground object, this spill comes from where the edges of the foreground object blended into the backing color. If it is on the center of the foreground object, it usually results from reflected color from the backing screen. The next Primatte step (Spill Sponge) can be used to eliminate this spill color.
If you have a choice, try not to shoot anything that is blue against a blue background. Use a green background instead. But if you get stuck like I did, apply Primatte Keyer. Bingo! Perfecto! Primatte Keyer is sold through Pinnacle Systems for Macintosh and Windows 95-98 NT systems.
Bruce Manning can be found on his Website. Bruce shoots with a Mitchell Fries 35mm film camera. He composites with a Mac G4 and all the Adobe software he can jam into it, which now includes Puffin's Primatte Keyer.
