This chapter is from the book
This chapter is from the book
This chapter is from the book
Luma Adjustments
Image adjustment is the process of using controls, similar to those in image-editing applications, to change aspects such as brightness and contrast. Digital video is described in either RGB or in Y'CbCr (aka YUV) color space. A color space (or model) is a way of describing and specifying a color. RGB and the Y'CbCr color space formulas contain three variables, also known as components or channels. RGB's variables are red, green, and blue, while Y'CbCr breaks down as follows: Y is luma (or black and white or lightness), and CbCr is chroma or color (Cb is blue minus luma, and Cr is red minus luma).
This representation addresses the human eye predisposition to green-light sensitivity, which is why most of the information about the proportion of green is in the luma (Y), and only the deviations for the red and blue portions need to be represented. The Y values have twice the resolution of the other two values, Cb and Cr, in most practical applications, such as on DVDs.
Because different color spaces are used by different codecs and video playback devices, image adjustment may be required as part of the preprocessing.
Luma Range Expansion
The RGB color space breaks the steps from black to white into 256 even steps (0 to 255, with 0 being black and 255 being white). Standard TV has only 220 steps from black to white, using black as 16 and white as 235 within the same scale (Figure 4.6). Generally, modern postproduction applications automatically keep black and white consistent. However, if blacks and whites appear either crushed (too black) or faded (too white), the luma range may need to be remapped.
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Figure 4.6 Television and computer-based video use different luma ranges to depict the range between black and white. When not compensated for in preprocessing, this distortion of the luma range can leave video either washed out or too dark.
The Digital Color Meter
Apple users have some built-in help when figuring out luma and chroma values. In the Utilities folder (Application > Utilities) is the Digital Color Meter (Figure 4.7). This little application launches a small magnifier and a table that shows the specific RGB value of the samples part of the screen. This application is a handy way of checking values, rather than just trusting that your display is calibrated correctly.
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Figure 4.7 Apple's Digital Color Meter makes it easy to check luma and chroma values.