Specifications

CHAPTER 3
Application Program Interface
32
VPTEngine Component
The colors shown on different displays may vary because of minor variances that occur
during manufacture. If you are using multiple displays, you will want the colors on all
displays to be as closely matched as possible.
AppleVision displays have the ability to correct for manufacturing variances through a
process called Virtual Photometry Technology, or VPT. Using VPT, the AppleVision
displays can be set to a wide variety of white points and can correct for CRT aging, and
for ambient light conditions. Refer to the following sections, “White Point,” “Virtual
Photometry Technology (VPT),” and “Ambient Light,” for background information on
these subjects.
Normally, the VPTEngine component is used by the Accurate Color panel that is in the
software provided with AppleVision displays. The information presented in this section
is for applications that need specific information about the current color characteristics of
the displays. The information returned by thisVPTEngine component interface is unique
to the particular AppleVision display being used. It does not supply so-called “nominal”
values that may or may not reflect reality.
Like other Engine Components that belong to the AV Architecture, the VPTEngine
component is based on the Component Manager. The VPTEngine component has the
following component description:
componentType = ‘avec ’
Indicates that the component is a video panel.
componentSubType = ‘vpte’
Identifies the specific engine.
componentManufacturer = ‘aapl’
Identifies Apple Computer, Inc. as the manufacturer.
componentFlags = cmpWantsRegisterMessage
Makes sure the component is suitable for the machine.
White Point 3
The image on your display is a combination of red, green, and blue signals. All lights,
including display light, have a white point, which is the measure of the color content of
the light. The AppleVision displays allow you to change the intensity (or white point) of
the R (red), G (green), and B (blue) signals. White point is measured in degrees Kelvin,
and it sets the foundation for the other colors on your display. If you have a high white
point, colors have a bluish tinge. If you have a low white point, colors have a slightly
reddish tinge.
Virtual Photometry Technology (VPT) 3
VPT is a proprietary Apple technique used to calculate a theoretical white point. It does
this by measuring the currents for the R, G, and B electron guns, while displaying a
white screen. The current values are correlated to factory calibration data that is
programmed into the display during production. An extremely accurate white point can
be determined by compensating for the aging affects of the display. The current sensing