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Description  |
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REFERENCE TO APPENDIX
A microfiche appendix is attached to this application. The appendix, which
includes a source code listing of an embodiment of the invention, includes
nineteen frames on one microfiche. The programs thereof are written in C
and designed to run with Microsoft Windows 3.1.
A portion of the disclosure of this patent document contains material that
is subject to copyright protection. The copyright owner has no objection
to the facsimile reproduction by anyone of the patent document or the
patent disclosure as it appears in the Patent and Trademark Office file or
records, but otherwise reserves all copyright rights whatsoever.
BACKGROUND OF THE INVENTION
The invention relates to calibrating the colors produced by a
digitally-controlled display device. ("Color" is used in a general sense
to mean the hue, saturation, and value for light sources, or hue, shade,
and value for objects. The term may also be used to encompass black,
white, or grayscale.)
In digital photographic editing, photographs (or other images) are scanned
and digitized, and then displayed on a computer monitor. A user can then
alter certain attributes of the displayed digitized image, and the
alterations appear on the computer monitor. Finally, the altered image can
be printed on a hard copy output device.
SUMMARY OF THE INVENTION
The invention provides methods for calibrating the colors displayed by a
digitally-controlled display device in response to digital inputs.
Calibration according to the invention improves the subjective agreement
between an image displayed on the calibrated device and the same image as
displayed by a different display device. The invention finds particular
use in editing of digital images, so that an image edited on a monitor
calibrated by the method of the invention can be stored and later used to
reproduce the digital image on, for instance, print media with a closely
corresponding color balance.
In general, in a first aspect, the invention features a method for
calibrating a display device, for instance, a computer monitor, that
transduces digital inputs to regions of perceived color. The method
includes the steps of: (1) supplying digital inputs to produce a white
area and one or more color patch areas each having a perceived color other
than white; (2) overlaying on the white area one or more filter chips that
each produce a desired reference color; and (3) conforming the perceived
color of each color patch and the perceived color produced by the
corresponding filter chip, by adjusting the digital input for the color
patch or the filter value of the filter chip.
In a second aspect, the invention features an optical filter that is
particularly useful with the method described above. The filter has a
substrate configured to allow the filter to be affixed to the face of a
computer display monitor, and a plurality of filter chips, each having a
known optical density and arranged on the substrate to facilitate color
comparison of light transmitted through the filter chips with light from
corresponding color patches displayed on the display monitor.
In a third aspect, the invention features a method for calibrating a
brightness control of a display device, including the steps of: (1)
supplying to the display device digital inputs that produce a black area
and an adjacent dark gray area; (2) adjusting the brightness control of
the display so that the black area and the gray area are distinguishable;
and (3) slowly reducing the brightness control to the point where the
black area and the gray area are indistinguishable.
Preferred embodiments may include any of the following features. Each
filter chip has an associated density value denoting a light-filtering
character of the filter chip, and this density value is received at a
source of the digital inputs; the source produces the nominal color patch
digital inputs according to the received density values. The color patch
digital inputs are each adjusted to conform perceived hue, brightness, and
saturation of each color patch to the corresponding reference color. The
filter chips and color patches are-of an essentially hueless neutral gray,
even for full-color display devices. The position of the color patches can
be adjusted to bring them into alignment with the corresponding filter
chips. The filter chips are formed by printing half-tone areas on a
transparent or translucent film. If the filter chips are on transparent
film, a strip of translucent tape can be adhered to the film as a
diffuser. The filter chips have an adhesive member to removably attach the
optical filter to the display monitor face. Digital inputs supplied to the
display device are adjusted to conform a white display region to a
reference source of white light, thereby producing a calibrated white
digital input for use in forming the white area. This white reference can
be either a light box or a sheet of white paper. The adjusted digital
inputs are stored in a form for later use in displaying a digital color
image: when the image is displayed, color values of portions of the image
having digital colors other than the nominal digital inputs are adjusted
for display by interpolating between the nominal and adjusted digital
inputs.
Among the advantages of the invention are the following. A display device
can be calibrated quite precisely. The invention provides this calibration
with a minimum of equipment: instead of an expensive photometer, the
invention uses an inexpensive film overlay to produce a reference color,
and a human eye to compare the reference color to the color produced by
the display. The invention is easy to use; ordinary users can calibrate
their display devices without requiring either the equipment or the
knowledge of an expert.
Other objects, advantages and features of the invention will become
apparent from the following description of a preferred embodiment, from
the drawing, and from the claims.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1, 2, and 4a are computer screen displays.
FIG. 3a is a perspective view of a novel light filter.
FIG. 3b is an edgewise view of the filter of FIG. 3a.
FIGS. 4b-4d show computer screen displays with the filter of FIG. 3a
affixed.
FIG. 5 is a table showing the memory of a computer.
FIGS. 6a and 6b are pseudocode descriptions of routines that convert a
digital color expressed in HSV color space to a color in RGB color space,
and vice-versa.
DESCRIPTION OF A PREFERRED EMBODIMENT
The invention provides several techniques that allow precise calibration of
a color display device. The techniques allow a user to precisely adjust
the display device's brightness and contrast controls, white point, and
gamma curve. In each of the techniques, a computer (or other controller)
displays a screen (a collection of figures) on the display device, and the
user provides adjustments to the display device or computer to achieve a
particular color relationship between the displayed figures or between
displayed figures and a reference. After calibration, information is
stored in a memory of the computer. As future images are displayed, this
stored information is used by the computer to adjust the nominal digital
colors of the image to actual digital values for use by the calibrated
device, so that the subjectively perceived color balance of the image
as-displayed by the calibrated device will conform to the color balance of
the image as it will be displayed on a standard monitor.
The gamma curve of a display monitor describes the relationship between the
digital color values sent to the display and the brightness of the light
emitted by the monitor in response. While the invention can be adapted to
match any reasonably smooth gamma curve, the default is to use the gamma
curve widely adopted by the television industry:
L=cv.sup..gamma.
where L is a measure of the intensity of the light emitted by the monitor,
v is the digital color value (expressed in a range from 0 to 1) sent to
the display adapter, c is a scale factor, and .gamma. (gamma) is a
constant, with value typically set at 2.22. This curve approximates the
response of the human visual system--a unit change in the value of v will
be approximately equally perceptible over any part of the useable range
for v.
Many computer display systems represent colors internally as three
eight-bit numbers, one number for each of red (R), green (G), and blue
(B). Eight bits allows each of the three values to range between 0 and
255. For such monitors, black is represented internally as the triple
(R,G,B)=(0,0,0). The brightest possible red is represented internally as
the triple (255,0,0), and the brightest possible white as the triple
(255,255,255). Because this method for digitally representing colors is
commonly used, it will be used as the example representation in this
disclosure. Similarly, the discussion below describes using the invention
to calibrate a display monitor, typically a CRT (cathode ray tube). Those
of ordinary skill will appreciate that the invention can easily be adapted
for use with display devices that use other digital color representations
and other display technologies, for instance color printers, etc.
Because a viewer's impression of an image is influenced by various external
factors such as room lighting and the color of room walls, it is desirable
to establish normalized viewing conditions before beginning the
calibration procedure. Room lighting should be very subdued, but not
totally dark. No direct light should fall on the monitor, so that the
monitor is free from glare or reflections. This can be achieved by
shrouding the top and sides of the monitor with a viewing hood. The user
sets the monitor into a desired display mode: for instance,
24-bit-per-pixel color (three eight-bit numbers for each of red, green,
and blue, as discussed above), and dithering disabled. It is also
desirable to allow the display monitor to warm up for a time, for instance
fifteen minutes. The user turns the contrast control to maximum contrast.
Software for a preferred embodiment of the invention displays on the
monitor a screen from which the user can select one of three calibration
steps, brightness, white point, and gamma curve, which are preferably
performed in this order.
Referring to FIG. 1, the user has clicked on the "brightness" button 102 of
the menu screen. The invention displays two rectangles, a very dark gray
rectangle 110 of color value (11,11,11) nested within a completely black
rectangle 112 of color value (0,0,0). The user turns the brightness
control up until gray rectangle 110 is clearly visible against the black
background 112, and then down to the point that the difference between
gray rectangle 110 and black rectangle 112 is barely perceptible at the
border between the two rectangles. Then, the user decreases the brightness
control until gray rectangle 110 just disappears against black background
112. The brightness control is now set at a calibrated level.
The goal of adjusting monitor brightness is to find a setting of the
brightness control that yields the brightest possible image while leaving
blacks as black as possible. Too low a brightness setting loses shadow
detail; too high a setting makes the image washed out and reduces overall
contrast.
Referring to FIG. 2, the invention allows the user to adjust the white
point of the monitor. Typically, the full white value (255,255,255) of a
computer monitor has a slight blue cast compared to the illumination under
which the final printed images will be viewed. The white point setting
step allows a user to remove this blue by shifting this full white
slightly toward the yellow-red. When the user clicks on button 204 to set
the white point button, the invention displays a white rectangle 210 at
full intensity and a color picker window 250. Color picker 250 displays in
a display plane 260 the colors of the three full-intensity sides of the
RGB color cube, with white (255,255,255) at the center, fading toward red
(255,0,0) at the top, toward green (0,255,0) in the lower right, and
toward blue (0,0,255) in the lower left. The point 280 in the color
picker's color plane corresponding to the current white point is indicated
with a circle, and shown in three panels 282 at the lower left, with
respectively, black, gray, and white borders. The user compares white
rectangle 210 with a reference white, for instance a daylight-corrected
light box, or a piece of white paper held under a lamp of known color
temperature, preferably corresponding to that in which final prints will
be viewed. If white rectangle 210 has a noticeable hue cast compared to
the white reference, the user uses a mouse to select a different point in
the color picker's color plane 260 to use as full white. The color picker
has two modes, a full mode in which the colors displayed in the color
plane extend out to full red, green, and blue, and a pastel mode that
displays a narrower set of colors in the same display space, allowing
selection from a finer gradation of colors.
The white point step determines the highest possible values that produce a
hueless white (for example, leaving the red value at 255 but reducing the
green value to 253 and the blue value to 234).
Because color (255,255,255) typically is a bit too blue, it may be
preferred to set the initial default white point at a value that takes
this into account, for instance (255,255,235).
Referring to FIGS. 3a and 3b, the preferred system according to the
invention uses an overlay film 300 to generate reference colors to which
color patches displayed by the monitor will be conformed. Overlay 300 is
about 4".times.3/4", made of plastic film 302, with a pattern of five
filter chips 310-318, each having a different optical density. Preferably,
the overlay film is manufactured by phototypesetting a series of halftone
screens onto plastic film. To reduce moire interference between the
monitor's shadow mask and the line frequency of the halftone screen, the
overlay pattern may be printed on translucent film, or alternately, a
translucent diffuser may be applied after photoprinting, for instance by
applying a layer of clear plastic adhesive tape 304, Scotch 810 Magic Tape
for example, to the emulsion or printed side of the film. Alternately,
overlay film 300 can be produced by photographing a set of gray filter
chips onto transparency film; the developed film can then be used directly
as the overlay. While the present embodiment uses five filter chips
310-318 and fiv | | |
