Previzion’s new color correction interface is broken up into four sections: color matching, color grading, graphs, and test patterns.
Foreground and Scene Matching
The color matching section consists of the two sets of sliders on the left. These sliders are used to match the look of the real foreground, and the cg background render to one another. The controls are pretty simple, but powerful for matching foreground and background. The “Foreground” and “Background” controls are identical to one another except for the fact that they control the foreground and background independently from one another.
Your goal when using these controls is to make the foreground and background look like they can be in the same world. Usually it is a good idea to try and have the two worlds meet in the middle by doing subtle adjustments to both the foreground and background. However there are reasons why an operator would want to leave the foreground camera alone, and adjust only background CG, or the other way around.
Let’s take a look at what each one of these sliders does. Below are the base line with black to white gradient, as well as a 3d rendered scene. Both of these have RGB Overlay turned on so you can see how the adjustments affect the levels.
First let’s adjust the Gamma. Gamma works a bit like exposure or brightness making an image darker or lighter. The big thing that a gamma adjustment does differently is that it mostly leaves the brightest and darkest parts of an image in place while moving the middle. Think of a slinky that is nailed to the floor and ceiling. The floor represents pure black, the ceiling represents pure white. Gamma moves the middle up and down between the floor and ceiling.
Contrast is the next control. Going back to the slinky analogy this would be like moving the end of the slinky below the floor and above the ceiling. Things that go above the ceiling are cut off and turned white, while things that are below the floor get cut off and turned black.
Saturation control has no effect on the gradient test pattern since it is black and white. As you can see for the room scene it can make the room black and white, or exaggerate the colors.
Temperature control is 1/2 of the white balance equation. This control shifts the image between a very yellow/orange image and a very blue image, simulating what would happen if the camera had its white balance temperature shifted.
Tint is a control which shifts the image between a magenta and green. This control is the other 1/2 of the white balance equation.
Composite Color Grading
The composite color works differently from the foreground and background. This set of controls works most similarly to a traditional video color system. This part gives the user controls over shadows, midtones and highlights individually. We have a basic primary color corrector. Each control has a reset button next to it to reset it to the default center value.
The three color wheels affect the color of different ranges of luminosity. The farther the cursor is from the center the more saturated it becomes, while the color you pull the cursor towards is the color it will cast over the image. Holding shift while adjusting color allows you to adjust saturation without hue, and holding ctrl allows you to adjust hue without saturation.
The Shadows wheel has an extreme effect on the shadows, moderate effect on the midtones, and almost no effect on the highlights.
The Midtones wheel has an extreme effect on the midtones, and moderate effects on the highlights and shadows.
The Highlights wheel has an extreme effect on highlights, moderate effect on the midtones, and almost no effect on the shadows.
Going back to the slinky analogy, adjusting the Shadows slider will bring the part of the slinky at floor level up, raising the darkest point your image above black. Or by pulling additional information below the floor, cutting it off as black. While adjusting this you will notice that it has little to no effect on the white point of the image.
The Midtones slider behaves just like the gamma slider in the color matching section. This adjusts the middle while leaving the white and black in the same place.
The Highlights Slider works just like the “Shadows,” except for the brights parts of the image instead of the darkest. It will allow the user to bring the brightest parts of the image down to a grey, or cut off information by bringing it above the white threshold, while leaving the black point of the image exactly where it is.
Saturation slider controls the saturation for the composite, and can take the colors of the composite from black and white to vivid and over-saturated.
Here is an original composite:
Here are the Foreground and Scene elements matched:
Here is the same shot with looks applied to it: