 |
| Contrast correct*
|
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| Contrast too high
(clipped)*
|
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| Contrast too low*
|
Why Contrast Is Important
Contrast is central to perceived quality in all forms of image
reproduction. Your eyes can detect a ratio of 10,000:1 in the
same scene under ideal conditions, and at least 1000:1 under
less ideal conditions. It’s not absolutely necessary to meet
that level of contrast to produce acceptable images. Printed
photographs, for example, typically have a contrast no greater
than 100:1, which most people find reasonable. That said,
increasing the contrast further makes images look punchier and
more realistic. People talk about images “popping off the
screen,” and most of that effect is contrast. At some point
there is a law of diminishing returns, but broadly speaking a
higher contrast ratio is better.
This immediately raises the question of why you’d ever want to
do anything other than cranking Contrast all the way up. If
more contrast is better, why not take it all the way up to the
maximum possible? The basic reason is simple: because most
displays will distort in some way well before the Contrast
control reaches maximum.
Prior to the rise of fixed-pixel displays like LCD, Plasma,
and DLP, the CRT was the most common display technology. The
most common result of setting Contrast too high on a CRT is
“blooming,” where small areas of picture get larger as the
electron beam loses focus. Thin lines get thick and small
bright dots turn into larger bright dots. In addition, the
power supply can get overtaxed as the contrast goes up, in
some cases resulting in geometry distortion, where straight
lines on screen become curved.
So setting Contrast on a CRT is usually a matter of putting up
a test pattern with thin lines or boxes that make blooming and
geometry easy to see, then raising Contrast until the image
starts to bloom or you see curving of straight lines, then
lowering Contrast a notch or two below the level where the
problem(s) occur.
Figuring out exactly where blooming occurs can be tricky in
practice; an excellent display might never bloom, or at least
not bloom even with Contrast all the way up. If the power
supply is good, you might never see geometry distortion.
Alternatively, the display might be badly designed and there
might be blooming or geometry distortion even at the lowest
Contrast level. There’s also a feature found on many CRTs
called “Scan Velocity Modulation,” or SVM, that is intended to
improve the sharpness of edges, but also tends to distort the
picture geometry. Among other issues, it can make thin white
lines thinner and thin black lines thicker. This makes it
harder to see blooming and other distortions. Certainly if you
can turn off SVM, we recommend that you do so. Search the
internet for the model number of your display and “SVM,” and
you’ll often find forums or web guides to tell you how to
enter the service menu and turn it off.
Nowadays with digital displays the most common problem caused
by setting Contrast too high is clipping. Modern fixed-pixel
displays typically have an absolute maximum brightness they
can produce. That maximum is a hard limit, so once the
brightest parts of the image are being displayed at the
brightest level the display can produce, going any further
just causes the upper ranges of brightness levels to collapse
so they all appear to be the same (very high) brightness.
Setting Contrast on a fixed-pixel display, then, is simple.
Put up a pattern that has patches of near-maximum white on a
background of maximum white. Raise Contrast until some of the
patches disappear and blend into the background, then lower
Contrast until they are visible again.
One important thing that we haven’t yet mentioned is that the
video system has two different white levels. The white level
that is talked about most often is “reference white.”
Reference white is the top of the nominal video range.
However, there is room in the video standard for
above-reference values, which are sometimes called “whiter
than white” or “super white.” This area of the video range is
supposed to be reserved for “overshoots” and occasional
excursions outside the nominal range. Very little of most
video frames strays outside the reference range, but often the
stuff that does stray outside the range makes a difference in
the image. Most notably, when displaying bright saturated
colors and/or highlights on bright white and near-white
objects, one or more of the red, green, or blue channels may
stray into the overshoot area. For this reason, professional
video monitors are always calibrated such that they have room
to display the above-reference range.
This is the basic quandary: if you calibrate so that reference
white is the brightest level the display can produce, you
maximize the contrast ratio, which is measured by comparing
reference white to reference black. But you may be missing
some amount of picture information, albeit small, that the
professionals working on the video saw.
How exactly to resolve this quandary involves considering what
you consider most important and what your viewing conditions
are. If what you want is the punchiest possible video, or you
routinely watch video in a room with a significant amount of
light, you may want to consider deliberately calibrating to
maximize reference white and clipping the above-reference
range. If you want to make your display look as much like the
pro monitors used to master video, and you’re viewing the
video in a dim or dark room, you’ll almost certainly want to
preserve the above-reference range.
You can split the difference if you like. Each additional
chunk of brightness above reference white adds a little bit
less to the image than the previous chunk, so you might decide
to preserve some of the above-reference range but not all.
This isn’t inherently wrong. However, it’s worth noting that
we nearly always calibrate our displays to preserve the entire
above-reference range. We think that most modern displays have
enough inherent contrast that sacrificing a small part of it
to the above-reference range is worthwhile.
Display Controls vs. Player
Controls
In general we recommend
calibrating using the controls on the display. That ensures
that all the other devices you connect to the display will get
the benefit of the calibration. Be aware that not all devices
will necessarily output the same levels; if you get the BD
player looking great but other devices look too dark or too
bright, that suggests that you’ll need to use different
settings for those devices, or search the menus of the other
devices to see if they have options for changing the output
color space or levels. In some cases you just need to ensure
that all your devices are set to the same mode. For example,
if one device is outputting 4:2:2 Y’CbCr, but another is
outputting RGB, that might cause the picture to look different
for multiple reasons. The RGB device might be using different
levels (such as the so-called “PC levels” of 0 for black and
255 for reference white). The display might have separate
calibration settings for Y’CbCr and RGB. Take a look in the
menus of both devices looking for settings like “Color Space”
or “Black Level” or “Super White” or “Extended Range” or
variations on those themes.
The other problem with using the controls on the player is
that the player may not be using high enough precision to
accurately map the changes you’re making into the output code
space. If you’re outputting 24 bit (standard) video (8 bits
per channel), then adjusting Contrast on the player will lead
to some of the codes either being clipped off or collapsed.
If your player offers a 30-bit or 36-bit mode (10 bits or 12
bits per channel), and your display can accept one or both of
those modes, then we recommend using them. If you use one of
those higher-bit modes, adjusting video settings on the
player is probably going to be workable, though we still
recommend starting by getting the display settings as
calibrated as possible. When calibrating with the player
controls, make sure to view the ramps on the Contrast
pattern to look for bands or streaks, just in case the display
or player isn’t actually maintaining the extra precision all
the way to the display panel.
Calibrating to Absolute Light
Level
When broadcast video CRT monitors are
calibrated, they are set so that a field of reference white
has a specific absolute brightness (usually around 100 cd/m2
(candelas per square meter) or 30 footlamberts). Note that
broadcast video monitors are viewed in a dim, but not
completely dark, environment. In a completely dark room, half
that brightness (50 cd/m2) is fine. If you have a light meter
that measures luminance directly, you can do this yourself,
but it’s not really the recommended strategy for modern
fixed-panel displays. Most of them have limited precision
available in their digital processing chips, so usually you
want to set Contrast to the highest available level that
doesn’t clip. If the final screen brightness is too bright, if
your display is a front projector you can often knock it down
with a neutral-density filter on the lens or a low-gain
screen. If your display has an iris control, you can use it to
reduce the overall light level to an acceptable brightness.
How much light is too much from a display is subjective and
depends on the ambient light level of the room. In a dark
room, above 150 cd/m2 is probably pushing it, and in a
moderately-lit room above 300 cd/m2 may be too much. In
general, the key is that the display shouldn’t appear dazzling
or hard to look at when bright scenes are shown. If the screen
looks better when you’re wearing sunglasses, it may be too
bright.
Using the Contrast
Pattern
This pattern is designed to
calibrate the display to show all of the levels up to peak
white (254). If you only want to calibrate up to reference
white (or your display or player won’t reproduce values above
reference white), you can adjust so all the bars above level
234 disappear. If you want to calibrate to some middle level,
pick a bar somewhere between 235 and 253 and calibrate so that
bar is visible but the bar above it is not.
The pattern is divided into several sections. On the top is a
row of bars at levels ranging from 2 to 24 on a background of
minimum black (1). If your brightness is set correctly, you’ll
probably just barely be able to see bar #18 against the
background, and nothing lower. This is good. You can use these
bars to see if the black level changes when you change the
Contrast control. If it does, you may need to move back and
forth between the Brightness and Contrast controls to get both
black and white level set correctly.
 |
Brightness too high
|
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Brightness too low
|
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Brightness correct
|
In the middle are two ramps. These are useful for watching for
banding, also called “contouring,” while you adjust the
Contrast control. See the section below called Other
Issues To Watch For for tips on using the ramps.
 |
| Ramps with banding
(contouring)
|
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| Ramps with smooth gradient
|
The bottom section is a row of white bars at levels from 231
to 253 on a background of peak white (254). When nothing is
clipped, you will probably be able to just barely make out the
252 bar against the background. If the 253 bar is clearly
visible, that suggests that your display doesn’t use the
appropriate gamma curve, or possibly that it’s doing some kind
of odd sharpening or edge enhancement.
To set white level, put up the pattern and turn down Contrast
until all of the white bars are visible, or as many are
visible as possible. If not all the bars can be made visible,
you may want to check through the settings in your display or
player to see if there is a mode switch that can put the
device in the proper mode to show or send the entire video
range. These modes are often called things like “Level
Expansion,” “Super White,” or “Headroom.” You might try
changing the output mode on your player, if it offers multiple
modes such as “4:2:2,” “4:4:4,” or “RGB.” You may also want to
search the internet to see if other people have encountered
the problem and fixed it. If you cannot make the
above-reference bars appear, then either the display or the
player is clipping the levels in a way that can’t be
recovered. Until you can fix or replace the problem component,
continue calibrating using the levels that are visible.
 |
| Bars on display that clips
above reference white
|
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| Bars with no clipping
|
If you are calibrating a DLP, LCD, Plasma, LCOS, or
other fixed-pixel digital display, turn up Contrast
just until the highest visible bar disappears. Then turn it
down again until the bar reappears. This is probably your
final calibrated setting, but you should also read the
Other Issues To Watch For section below to see if further
tweaking is necessary.
If you are calibrating a CRT, you’ll want to
look at the needle pulses, which are the thin white and black
lines on the sides of the screen. Turn Contrast up until the
white lines get thicker and the black lines get thinner, or
until you see thin halos of color (usually blue) on either
side of the white lines (both of which are signs of blooming),
or you see visible curving of the lines. At that point, lower
Contrast at least two notches below the point where you first
see the blooming or curving. If you never see any blooming at
any contrast level, and no clipping seems to be happening, you
can leave Contrast at maximum. Read the Other Issues to
Watch For section, below to see if further tweaking is
necessary.
Other Issues to Watch For
• Verify on the Contrast
pattern that the black bars at levels 16 and below continue to
be invisible, and the levels 18 and above are visible (though
18 may be very faint). If they are not, go back to calibrating
Brightness, and then come back to this pattern to readjust
Contrast. Continue moving between the two calibration
operations until both patterns appear correct, or a reasonable
happy medium is achieved.
• You may want to also check the Dynamic Range High
pattern, which is a larger version of just the white bars
from the Contrast pattern, with the white bars
blinking for better sensitivity. It doesn’t have the ramps and
below-reference bars to check for other issues, so we don’t
recommend using it as the primary Contrast-setting pattern
unless you have already verified that you don’t have any
issues with banding or with the black level changing when you
change the Contrast control.
•
One problem we see reasonably often in displays is that the
Contrast control is implemented using fairly low-precision
math. The result is that the various levels in between 0 and
maximum are rounded to the nearest displayable level. This can
result in contouring in smooth gradients. To check for this,
it’s worth looking at a continuous ramp (such as the one in
the Contrast pattern) and ensuring that it looks like
a gradient and not a series of thin bars. You may want to move
Contrast up and down and see if you see bars, bands, or
stripes appear or disappear. If there is one setting that is
close to the optimal setting and produces a smoother ramp,
it’s probably worth compromising on perfect contrast in order
to get smoother gradients. If possible, use a Contrast setting
lower than optimal rather than higher than optimal, to avoid
clipping.
• It’s possible to be able to
see all the contrast bars and still be suffering from clipping
in one or two of the R, G, and B color channels. To check for
this, bring up the Clipping pattern and check that
you can see concentric squares of progressively brighter color
in each of the large colored squares. If any square or
subportion of any square is a solid, even shade of red, green,
or blue, then that channel is being clipped. You should check
to see if lowering Contrast will bring back all of the
subsquares. If it won’t, or you need to turn down Contrast an
excessive amount to get all the squares back, then all is not
lost. There are other reasons for clipping in individual color
channels, such as having the “color” or “saturation” control
set too high. We’ll cover that control, and more about the
Clipping pattern in future articles.
