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FAQs about Canvas 5 color management
This FAQ discusses how Canvas 5 converts and displays colors, and how
Canvas compensates for differences in monitors and printers to reproduce
colors as accurately as possible. The information here includes some basic
color theory you might find useful when preparing documents for color output.
Q: How does Canvas perform color conversions,
and how can you calibrate Canvas to accurately convert and display colors? |
A: To understand how Canvas 5 deals with
colors in your document, you need to know:
- the difference between device-dependent and device-independent color
spaces
- the properties of the three main color models used in computer graphics:
RGB, CMYK, and LAB (also known as CIELAB).
Device-dependent color spaces, such as RGB and CMYK, appear slightly
different on every output device. For example, monitors are RGB devices;
each monitor uses different phosphor shapes, arrangements, or mixtures,
and responds to the same RGB values with sometimes astonishingly different
results. Therefore, the same image on two different monitors won't look
exactly the same, even if the brand and model are identical.
In contrast, LAB color is a purely mathematical construction that is
completely independent from output devices; in fact, there is no such thing
as a LAB output device. Unlike RGB and CMYK, which are greatly affected
by the physical properties of an output device (phosphor brightness, ink
amount, and so on), LAB colors have no direct physical correlation. The
LAB color space defines colors using a luminance value and two color components,
ranging from red to green and yellow to blue. Using these three components,
LAB defines colors so that two devices will interpret a LAB color exactly
the same way.
For this reason, Canvas 5 uses LAB color space as a "hub" for
performing color conversions. For example, when converting an RGB image
to CMYK, Canvas first converts the image to LAB, then to CMYK. This has
the effect of "normalizing" the color data; even though devices
must eventually convert the LAB colors to RGB or CMYK for display or printing,
at least the original data conform to a universal standard, unaffected by
the physical properties of a specific monitor or printer.
Canvas also lets you specify certain important information about a monitor
or printing press to make conversions between color spaces more accurate.
The Calibration submenu commands (Monitor Setup, Printing Inks, and Color
Separations) in the Edit menu tell Canvas how to compensate for your particular
system when converting from LAB color mode. For example, you can use the
Monitor Setup command to tell Canvas what type of phosphors your monitor
uses, and Canvas will adjust the onscreen RGB colors to compensate. Similarly,
the Printing Inks dialog box lets you compensate for different types of
inks, paper, press, and other aspects of the printing process. (In general,
this type of calibration is necessary for professional printing jobs only.)
Canvas uses the Printing Inks information when converting colors between
CMYK and LAB, and when creating CMYK separations from LAB color data. |
Q: How does Canvas simulate CMYK colors on RGB
monitors? |
A: Because monitors are RGB devices and colors
used in commercial printing are CMYK, Canvas does some color conversion
to provide you with an RGB approximation of CMYK colors onscreen so you
can get an idea how the final product will look. Canvas converts CMYK values
to LAB using the Printing Inks dialog box settings and a proprietary formula,
called the ink recipe. Using the Monitor Setup settings and a standard formula,
Canvas then converts the LAB values to RGB for display. This process is
also referred to as "soft proofing," because it lets you preview
CMYK colors without printing. |
Q: How does Canvas convert RGB colors to CMYK?
How does "black generation" affect this process? |
A: To convert RGB colors to CMYK, Canvas
reverses the previous procedure, converting RGB to LAB (using Monitor Setup
settings), then LAB to CMYK (using Printing Inks settings). In addition,
Canvas uses settings in the Color Separations dialog box to determine how
much black ink to use in generating CMYK colors. In theory, 100 percent
cyan, magenta, and yellow results in black; however, in practice, these
three inks can't produce real black or dark tones. To produce darker colors,
you need to add black, and reduce the amount of cyan, magenta, and yellow.
The amount of black you should use depends on the content of a document.
In general, you should use less black for high-contrast images with a variety
of colors, and use more black for text, line drawings, and images, where
the colors are less intricate and darker shades are required.
To determine what ratio of black to color inks to use when separating
colors, Canvas uses the Black Generation settings in the Color Separations
dialog box. To open the Color Separations dialog box, choose Calibration
> Color Separations in the Edit menu.
Keep in mind, however, that the onscreen appearance of colors can never
reproduce the printed result with complete accuracy. The only way to be
sure the colors are correct is to print proofs using the actual negatives
for the final product. |
Q: What are "separation tables," and
how does Canvas use them? |
A: Canvas doesn't do the math to convert
RGB, LAB, and CMYK color modes on the fly; that would be agonizingly slow!
Instead, Canvas builds three separation tables that it uses for screen redraws
and color conversions:
- The CMYK to LAB table is based on changes in the Printing Inks dialog
box settings.
- The LAB to CMYK table is based on changes in the Printing Inks or Color
Separations dialog box settings.
- The out-of-gamut table tracks which colors cannot be reproduced using
CMYK. Canvas recalculates this table whenever the LAB to CMYK table changes.
Some vendors provide separation tables with output devices and color
conversion software. The Calibration > Separation Tables command in the
Edit menu lets you save and load Adobe Photoshop-compatible separation tables,
so you can get the best performance from these devices and software products. |
Q: Does Canvas support ColorSync? |
A: Canvas does not currently support ColorSync,
an Apple Computer system for color matching. The Canvas ink recipe is specifically
optimized to produce good results for color offset printing, inkjet printing,
and dye sublimination printing. |
Q: Why does Canvas define HSL colors with RGB values? |
A: The HSL color system is based on hue, saturation, and lightness
values. Canvas provides the HSL system as an alternative to viewing and
defining colors with CMYK or RGB values only. However, unlike CMYK and RGB
colors, HSL colors cannot be outputted. For this reason, Canvas defines
HSL colors with RGB values. |
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