HP LP2480zx Understanding the HP DreamColor LP2480zx Professional Display's 30

HP LP2480zx - DreamColor - 24" LCD Monitor Manual

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© 2008 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice. The only warranties for HP
products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as
constituting an additional warranty. HP shall not be liable for technical or editorial errors or omissions contained herein. July 2008
Gray banding
(left,
exaggerated) is
eliminated by the
30-bit panel
(right)
Mini White Paper
Understanding the HP DreamColor LP2480zx Professional Display’s 30-bit panel
Introduction:
The HP LP2480zx DreamColor Professional Display has a 30-bit panel. What does this mean, and what are the benefits?
What is the bit-precision of a display?
All LCD monitors operate by presenting just three primary colors to the
eye: red, green, and blue. Each pixel on the display is implemented as
three sub-pixels, one for each primary color (see the
illustration on the left). The color of the pixel is
controlled by liquid crystal cells (of which there is one
per sub-pixel) which modulate the amount of light
which passes through the sub-pixel to the user’s eye.
For example, if the pixel is red, the red sub-pixel passes maximum
light, while the green and blue sub-pixels pass no light. If the pixel is
mid-gray, all three sub-pixels pass an equal, medium amount of light.
The bit-precision of the display determines how many steps of
brightness are possible. A display which supports 6 bits per sub-pixel
will provide 64 (2
6
) steps from darkest to brightest; a display which
supports 8 bits will provide 256 (2
8
) steps. The HP DreamColor
LP2480zx display supports 10 bits per sub-pixel, giving 1024 (2
10
)
steps. The bit-precision is a result of the design of the electronics which
control the liquid crystal cells in the panel.
Since there are three sub-pixels, the maximum number of colors that a
pixel can present is
2
n
x 2
n
x 2
n
where n is the bit-precision of a sub-pixel. Therefore, an 8-bit design
gives
2
8
x 2
8
x 2
8
= 16.7 million colors
The HP DreamColor LP2480zx display, with its 10-bit design, gives a
palette of
2
10
x 2
10
x 2
10
= 1.07 billion colors
By the way, while many people talk about “an 8-bit panel” or “a
10-bit panel”, it’s also common to refer to the total number of bits
needed to define a red-green-blue pixel. Therefore, it’s valid (and
preferred) to refer to the HP DreamColor LP2480zx display’s panel as
“a 30-bit panel.” 30 is 10 + 10 + 10, which takes account of the 10
bits for each sub-pixel.
Many consumer displays have low-cost 18-bit panels. Most
professional displays (such as the HP LP2465 or Apple’s Cinema HD
23” display) have 24-bit panels. The HP DreamColor LP2480zx
display is one of a small number of high-end displays with a true 30-bit
panel. The higher the bit-precision of a display, the better able it is to
represent colors accurately.
The benefits of 30-bit
It might seem that a 24-bit panel, which offers 16.7 million colors,
would be sufficient. For most purposes, that’s true. However, there are
cases where 8-bits per sub-pixel is not enough. Consider a grayscale
image. Gray (including white and black) is produced when the three
sub-pixels (red, green, and blue) are equally bright. This means that the
values for the three sub-pixels are the same: 35/35/35, for example.
With 8-bits per sub-pixel, gray can go from 0/0/0 (black) to
255/255/255 (white). Therefore, there are only 256 levels of gray
possible. This can lead to “banding,’ which is an effect that arises
because the step between adjacent levels of gray is big enough for the
eye to detect. It can be a problem in certain kinds of
visualization, such as 3D rendering for automotive
styling. With a 30-bit panel, there are 1024 levels
of gray, and it’s almost impossible for the eye to
detect the step between adjacent levels.
Also, there are cases where images can have
greater bit-precision than 24 bits, especially where
subtle detail is important. Examples are: satellite
imagery for intelligence agencies, or medical
imagery for, say, mammography.
30-bit world
The HP DreamColor LP2480zx display’s 30-bit
panel can display 30-bit content accurately, without
losing precision. However, to achieve this you must
have a complete 30-bit chain, with all components able to handle
30-bit pixels:
application
operating system
graphics driver
graphics card
DisplayPort cable
LP2480zx display
At the time of writing this paper (July 2008), few commercial
applications are able to display 30-bit images, and 30-bit-capable
graphics cards are in the prototype stage. The DisplayPort connection
is already 30-bit capable. Over time, applications, cards, and drivers
will reach the market and provide a full 30-bit path from application to
display.
24-bit world
For most users, color will remain at 24-bit precision. For example, if
your graphics card doesn’t have a DisplayPort output, then DVI will
probably be in use. DVI is limited to 24 bits.
The good news is that the HP DreamColor LP2480zx display’s 30-bit
panel delivers a benefit even when displaying 24-bit pixels. The
internal electronic system in the display, known as the HP DreamColor
Engine, adjusts pixel colors and luminances to map them accurately to
the user’s selected standard color space (sRGB, Adobe RGB, etc). This
engine operates at very high (36-bit) precision, and the results are
displayed at 30-bit precision on the panel. This means that the 30-bit
panel improves the ability of the display to show exactly the correct
color for every pixel. A 24-bit panel would introduce larger deviations
from accuracy, which can give rise to banding and other undesirable
effects.
Further reading
: