Colour Grading
Colour grading is the process of altering and enhancing the color of a motion picture or television image, either electronically, photo-chemically or digitally.
Modern colour correction, whether for theatrical film or video distribution, is generally done digitally.
Primary and secondary colour correction
Primary colour correction affects the whole image utilizing control over intensities of red, green, blue, gamma (mid tones), shadows (blacks) and highlights (whites). Secondary correction brings about alterations in luminance, saturation and hue in six colours (red, green, blue, cyan, magenta, yellow). The main objective of secondary controls is to adjust values within a narrow range while having a minimum effect on the remainder of the colour spectrum. Using digital grading, objects and colour ranges within the scene can be isolated with precision and adjusted. Colour tints can be manipulated and visual treatments pushed to extremes not physically possible with laboratory processing. Special digital filters and effects can also be applied to the images.
Masks, Mattes, Power Windows
The evolution of digital colour correction tools advanced to the point where the colourist could use geometric shapes (like mattes or masks in photo software such as Photoshop) to isolate color adjustments to specific areas of an image. Called Power Windows by Da Vinci Systems, these tools can highlight a wall in the background and colour only that wall — leaving the rest of the frame alone — or color everything but that wall. Subsequent colour correctors (typically software-based) have the ability to use spline-based shapes for even greater control over isolating colour adjustments. Colour keying is also used for isolating areas to adjust.
Motion Tracking
When trying to isolate a colour adjustment on a moving subject, the colourist traditionally would have needed to manually move the mask to follow the subject. In its most simple form, motion tracking automates this time-consuming process using algorithms to evaluate the motion of a group of pixels. These techniques are generally derived from match moving techniques used in special effects and compositing work.
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