Image Processing Reference
Headroom & overshoot
Legal video values
Figure 35-1 Luma range.
If you have some video and intend to broadcast it on the web or via a broadband link
to eventually be viewed on a computer screen, you must bring the black level down. You
must also increase the range.
Some color correctors allow you to simply grab the black and white levels on a visual
histogram graph or via some dial wheels on a color-correction user interface. On a simpler
system, you will have to use the brightness and contrast controls. If you are using
QuickTime-based tools, a lot of this compensation takes place automatically and you need
not worry about it.
We adjust the various properties of the video by applying an operator. This term just
describes a tool that modifies a particular part of the video information, usually without
affecting any other property. For example, color and lightness are separate operators,
although they are actually adjusting the separate red, green, and blue values to accomplish
their respective tasks.
To get the appropriate value for the background, first adjust the brightness level.
Figure 35-2 shows that reducing the brightness will cause the loss of some information.
It is an offset operator, not a scaling operator. Contrast correction is a scaling operator.
Figure 35-3 illustrates the levels palette from Photoshop. It presents a histogram of pixel
values in the image and lets you pull up the black value, white value, and mid-tone set-
ting. It is a little more complex for video because you must integrate over a series of
frames. Similar controls are available in After Effects, and because they are key-framed,
the settings can be altered while the movie is playing.
Note in this example that two colors in particular show up as spikes in the histogram
Final Cut Pro provides some very sophisticated color-correction and monitoring
tools. Figure 35-4 is a screenshot of the vector scope, histogram, and waveform monitors