Image Processing Reference
In-Depth Information
This preprocessing step takes place here because if luma correction were applied earlier,
the scaling algorithm would alter some individual pixel values. The new red-green-blue
(RGB) values generated would change the intensity values. Altering the luma at this point
leaves it unchanged by subsequent processes unless you apply large amounts of color-cor-
rection and hue-rotation effects.
It is this interplay between the different preprocessing stages that tends to make it
appear more complex for the beginner. Once you break down the process into a series of
discrete steps, everything becomes easier and more predictable. At that point you are well
on the way to becoming a proficient compressionist.
The Range of Legal Values
Lowering the overall brightness of an image adds contrast and color saturation. That
makes it appear stronger and less washed out. It also corrects the black levels. Black and
peak-white levels must be checked when adjusting the luma range.
The video standards define a range of legal color values. That range forces the blacks
to be somewhat higher than 0 and the peak-white values to be somewhat less than 100%.
It is important to preserve this legal range of colors when broadcasting video. All adjust-
ments of the luma must remain within that range. The values of black below the legal level
are called footroom and the white values above peak are called headroom.
The exact values you will find will depend on where the image-sampling values are
taken. For most purposes, you will be working with an 8-bit value. Very-high-quality sys-
tems use a 10-bit resolution, but it is rare to encounter that outside of a studio setting.
The 8-bit values range from 0 to 255. You will normally only see these values if you
are recording the serial digital interface (SDI) video values. These are not the same as the
analog values. The 0 and 255 are reserved to carry synchronization pulses. So any video
information will be range-limited to between 1 and 254. The analog black level is defined
as 16 increments higher than the SDI 0 value. The blacks will be a very dark grey, but not
absolute black. The peak-white values are limited to 219 increments above the base-black
level. In terms of the interface, this is a byte value of 235. The full range at the extreme of
the legal values within the interface values is 254. If the 16 increments of black-level offset
are subtracted, the maximum white value is 238. The space between 219 and 238 in the
analog-video domain is the headroom. This way, any filtering operations that cause a pixel
value to exceed the peak-white level of 219 do not get crushed because they fall into the
headroom and are corrected.
This is a lot easier to see in a picture (Figure 35-1).
You don't have to adjust these settings if you have a clean source and the video looks
good. If you are working wholly within a broadcast system, these level corrections will be
taken care of for you.
It is in the area where your footage is being moved between the video world and the com-
puter world that you must compensate the luma values.
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