Similarly, our computers cannot reconcile black and white areas in a digital image that are smaller than the size of the pixels in the image, so when you resample the image, they create—
Trang 1size, check for visible artifacts in your print, and if there’s visible corruption, go back
to your original, or seriously consider reshooting the image
One of the methods CorelDRAW, PHOTO-PAINT, and other programs that can import bitmaps use to lessen and occasionally eliminate visible artifacting from
resampling a photo is called anti-aliasing, and you have some control over this
method, discussed later in this chapter Anti-aliasing is a math calculation that performs averaging in a resulting photo that’s been altered in areas where there is visual ambiguity (some of the pesky pixels are traveling under an alias) For example, suppose you could photograph a checkerboard plane that extends from your feet way out to the horizon You look down toward your feet and clearly see black squares and white squares You look at the horizon, and you do not see clear edges of the black and white squares: actually you’ll see a lot of gray, as your mind blends black and white together because your eyes don’t have photoreceptors fine enough to resolve the very distant black and white squares Similarly, our computers cannot reconcile black and white areas in a digital image that are smaller than the size of the pixels in the image, so when you resample the image, they create—inaccurately—little black and white squares where they shouldn’t be The inaccuracy means the black and white
squares are traveling as an alias and presenting themselves falsely.
Anti-aliasing comes to the designer’s rescue by averaging pixel colors when you
resample such a checkerboard photo, or any photo The anti-aliasing technique examines, in this example, areas that include both a white square and a black one, understands that both colors can’t be assigned to only one pixel, and so writes a blend
of colors—gray—to the new pixel color value At left in this illustration, you can see some unwanted patterning toward the horizon—this image was not anti-aliased when
it was resampled to produce a larger image At right, the same image was resampled using anti-aliasing; at 1:1 viewing, you can see the smooth transition as the checkerboard extends into the distance—and the close-up shows the result of good anti-aliasing (some applications anti-alias poorly)—gray is substituted for black and white when it’s a tossup for a single pixel color
Ill 23-1
Aliased image Anti-aliased image
Trang 2many of us cannot invest the time and talent to “make photographs” using CorelDRAW.
Fortunately, this chapter shows you the easy way to make your CorelDRAW more
photographic in nature: you just import a photograph!
Bitmaps and Pixels
Programmers and users alike are accustomed to calling a pixel-based image a bitmap.
However, the term “bitmap” is a little like the term “dial phone.” Telephones haven’t had
dials in nearly two decades, and similarly, a bitmap—literally a map of bits of information—
is inadequate to describe a pixel-based image, but “bitmap” is used as the term for
non-vector graphics in this chapter anyway We’re comfortable with the term, the term is
described in the next paragraph, and “bitmap” is shorter to write than “pixel-based image.”
Let’s say you have onscreen a JPEG fresh off your camera What are you seeing? Of
course, you’re probably seeing a friend or relative, but what you’re really seeing is a finite
number of placeholders for color—the number of placeholders for color is so large that your
mind integrates the placeholders into a familiar image That’s the “map” part of the term
“bitmap”; this map could also be called a “mesh,” a “grid,” or a “canvas.” All the bitmap
images you take with a camera or paint in a paint program are composed of information
units all lined up in a grid You don’t see the grid (or the map part of a bitmap); it’s only a
figurative thing, intangible—it’s the structure for the visual information The finer the grid,
the less likely you are to see the individual color elements, instead of your mind blending
the elements into a photograph The “bit” part of the term “bitmap” is actually a byte of
color information: a bit of information can only have two possible values (usually on or off);
the graphics that artists work with today have a byte (8 bits) of information per color channel
with which to express a color value The term bitmap was coined in the days when a monitor
could truly only display a color or no color; thus the term bitmap, and the term has stuck
with us for more than 30 years
To extend this explanation further, this unit of information lodged in a map is called a
pixel, short for picture element, the smallest unit of color you can see in a bitmap image A
pixel is only a placeholder for a color value; it is not a unit of measurement It doesn’t even
have to be square (digital movie cameras take rectangular-proportioned movie pixels), and it
has no fixed size Other things a pixel is not include:
● A dot Occasionally even professionals will lapse into describing the resolution of
a digital image in dots per inch This is okay if they’re using the term “dot” as slang
to mean a pixel, but this is confusing jargon Printers print dots of toner and other
pigment onto a surface (usually paper); a 1,200 dpi printer, for example, renders
1,200 dots of toner per inch of paper, but it is not rendering 1,200 pixels per inch
of toner! In fact, a 1,200 dpi laser printer is incapable of rendering 1,200 pixels per
inch (ppi) A pixel is not a dot of toner or ink, nor is a dot of ink equal to a pixel—
pixels alone have no size
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Trang 3● A screen phosphor or LED Pixels that make up an image do not correspond 1:1 to whatever the elements on your monitor are made of With high-quality images, there are many more pixels per inch than there are light units (phosphors, LEDs, and so on)
on your screen This is why CorelDRAW and paint programs such as PHOTO-PAINT and Adobe Photoshop offer zoom tools, so you can get a better look at image areas, mapping small amounts of pixels to your screen, which has a finite number of light-emitting elements Because resolution is discussed later in this chapter, it’s good to know that the most frequently used resolution for web graphics is 96 ppi (pixels per inch) Therefore, if the resolution of an image is also 96 ppi, this means that when you view it at 100 percent viewing resolution, what your screen’s light-emitting elements are mapping corresponds 1:1 to the image resolution This means you’re viewing a bitmap graphic exactly as the creator of the bitmap intended it
● Any sort of ratio The measurement commonly used in bitmap evaluation is pixels per inch, which is a ratio, like mph is a ratio—miles (one unit) per hour (a different unit) A pixel is a unit, but not a ratio, and therefore if someone says they have an image that’s 640×480 pixels, they’ve told you how many pixels are in the image, but
not its resolution and not its size A pixel is a unit, and needs to be contextualized—
for example, 120 pixels per inch, or 300 pixels per centimeter—before the unit becomes meaningful and useful to a printer or designer If you told friends you were driving your car down the autobahn at “200 miles,” they probably wouldn’t be impressed, because you haven’t contextualized this unit into something meaningful such as a measurement But “200 miles per hour” tells your friends something—that they probably don’t want to ride with you
Color Depth
In addition to being color value placeholders, pixels also have “depth”—not “depth” as we’d measure a swimming pool, but rather a color “density.” For example, GIF images have a maximum color depth of 256 unique values; grayscale images have a brightness depth of
256 shades
Because 256 unique colors can’t truly express the beauty we capture with a digital camera (even dull scenes can contain tens of thousands of unique colors), programmers decided early on
in the digital-imaging game to structure high-quality images into components, the most common structure being red, green, and blue, like your computer monitor is based on the RGB color
model We usually call these three components color channels: by adding the brightness values
of these three channels together, we get the composite view of digital photos and other bitmaps Channels are a very efficient method for storing bitmap color information—in contrast, GIF images store image colors as explicit color table values, and this is one of the reasons why GIF images are limited to 256 unique colors By assigning the red, green, and blue color channels in
a bitmap image an 8-bit-per-channel color capability, the color capability equals 28, meaning the red, green, and blue channels can each have one of 256 possible brightness values with which to contribute color to the RGB composite image Eight bits per channel times three color channels
Trang 4adds up to 24-bit images—BMP, PNG, TIFF, TGA (Targa), and Photoshop PSD being the most
common file formats that can hold this color information So, 24-bit images have a maximum
unique color capability of 16.7 million colors
However, color depth doesn’t stop at 24-bit (8 bits per channel) Although monitors today
can only display 24-bit image depth, the camera manufacturers anticipate that this will change
soon, with the increasing popularity of high dynamic range (HDR) displays and higher-definition
monitors Today, many of the middle-range digital cameras can write photos to the RAW file
format, whose specifics (including file extensions) vary from manufacturer to manufacturer, but
happily CorelDRAW can import most digital camera RAW files RAW files (as covered later in
this chapter) are “unprocessed film”: they contain exposure settings, f-stops, and other camera
data, but they also provide a lot of flexibility and leeway when you import such an image
CorelDRAW has a little utility called Camera RAW Lab where you can color-correct and change
image exposure—all after the photo was taken You can do this because RAW images can
contain 16 bits per color channel, to offer a 48-bit image—more than 281 trillion possible
colors…indeed this would require a very large crayon box
Consider it a given that because CorelDRAW can handle such mega-information and has
some very good processing tools for imported bitmap images, the compositions you create
using bitmaps along with vector designs will print splendidly Now it’s time to discuss
image resolution as it relates to outputting your work
Resolution and Resolution-Dependent Images
As mentioned earlier, resolution is expressed as a fraction, a ratio between units (pixels) and
space (inches, usually) As you’ll see later in Table 23-1, a few image file types such as PSD
and TIFF can store image resolution information, and this is good For example, let’s say
you need to inkjet-print a brochure, and the front page needs a photograph A photograph of
insufficient resolution is going to print lousily, pure and simple However, if the photographer
saved the digital photo to PSD, TIFF, PNG, or RAW camera file format (and knows about
image resolution), you can import the image and know before you print whether the image
needs to be resized (resizing is covered later in this chapter) The rule is that an image’s
resolution should be in the neighborhood of your printer’s resolution Therefore, let’s say
that you’ve imported a photo and you know (by looking at the Bitmap page of Object
Properties, covered in detail later in this chapter) that it’s 4" wide, 3" tall, and 250 pixels per
inch in resolution Your next move is to check the printer manufacturer’s documentation:
although manufacturers tend to tap dance around specific printer resolutions, a good working
guide is that an inkjet prints about one-third of the stated overall resolution on its box If the
box says the inkjet is a “720 dpi enhanced resolution,” disregard the hype about “enhanced”
and cut to the chase: a 720 dpi (dots per inch, not pixels per inch) inkjet can render about
240 pixels per inch Therefore in this example, you can indeed faithfully print this 4"×3"
image with no loss in image detail
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Trang 5There is a way to tell the resolution of image file formats that cannot hold resolution information, so don’t worry if you have a bunch of JPEG images you want to use in a composition you need to inkjet-print As you progress through this chapter, working tutorial files will demonstrate what you want to do and when
Resolution vs Image Size
Another digital-image reality that makes many designers pull their hair out is that image resolution is inversely proportional to dimensions: this is another cold, hard fact about bitmap resolution When you make an image larger in dimensions, its resolution decreases Viewing resolution and image resolution display the same thing onscreen, but changing
viewing resolution—zooming in and out of an image—is nondestructive, whereas changing image resolution is destructive editing and often irreversible Here’s an example that
demonstrates the resolution-dependence properties of bitmaps Figure 23-1 shows a desktop icon; it’s 48×48 pixels and the widely adopted resolution convention is that screen pixels are
96 per inch At 1:1 viewing resolution, this icon looks fine, but when you zoom into it to 10:1 viewing resolution, it begins to look coarse The same effect would be visible if you actually were to change the resolution of the image Bitmap images are resolution dependent; the pixels you capture of a scene with a camera can’t be added-to later to increase detail—no application can guess what the extra detail and extra pixels would be At right in Figure 23-1 you can see an extreme enlargement of the icon, and the pixels are so clearly visible that you can’t make out what the design is!
per inch
48×48-pixel icon viewed at 1:1 and
96 ppi resolution
48×48-pixel icon viewed at 10:1 and 96 ppi resolution 48×48-pixel icon viewed at20:1 Pixels are clearly visible.
Trang 6The lesson here is that to take advantage of the unique property of bitmap images—that
they accurately portray a photographic scene—you need to take a photo that is high resolution—
3,264 pixels×2,448 pixels is average for an 8-megapixel camera, a little larger than 10"×8"
at 300 ppi CorelDRAW can resize an image; for example, this same 8-megapixel image
could also be expressed as 20"×15" at 163 ppi without changing any visual information
CorelDRAW can also resample an image, and this is the destructive type of editing; you
change pixels when you resample, so generally it’s a good idea to resize and to resample
only as a last resort when adding photos to a CorelDRAW composition
Importing Bitmaps into a Document
As a CorelDRAW user, you have at your fingertips a vast collection of bitmap import filter
selections Although import commands are discussed in Chapter 3, some of the import options
apply specifically to bitmaps and are explained here; you’ll find them useful if your work
requires photographs and graphics from the Web Table 23-1 does not list the bitmap types
CorelDRAW can import the way that the Files Of Type drop-down list does in the Import dialog
Although it’s terrific to have a billion different file types available for import, particularly if you
have legacy file formats, you’ll probably only use a handful of image types in everyday work
Therefore, the table lists the most common file types first; more exotic and legacy file formats
appear toward the bottom of this table The asterisk after the file extension indicates that a file
type can retain resolution information; this is a capability—it doesn’t necessarily mean the person
who saved the file actually saved resolution information
If you intend to print a CorelDRAW composition you’ve created that features a bitmap
photo or painting, step 1 is to set up the color management of the document You can access
settings for your document’s color profile through Tools | Color Management | Default
Settings Read Chapter 17 for the details on color management: what it does and why you
should use it to increase your chances of faithful color output to print and to the monitor
The following steps aren’t a tutorial but rather a checklist, a workflow based on your need to
bring a copy of a photo into a CorelDRAW composition:
1. After launching CorelDRAW, pressCTRL+N(File | New) if you’ve set up your copy
not to show the Create A New Document dialog after launch
2. Define the Color Settings after you’ve specified page size, resolution, and other
parameters Your color settings—the color space within which everything on the
page “resides”—are not “Oh, yes! I know the answer!” sorts of decisions you make
lightly Generally, you’re safe choosing sRGB IEC61996-2.1 because many digital
cameras and scanners use this color profile If, however, after importing a photo, the
photo looks dull or lacks contrast, the color space of the photo doesn’t match the
color settings of your CorelDRAW document This can be changed later; let’s
continue the workflow here…
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Trang 7Bitmap Type File Extension
JPEG and JPEG 2000 bitmaps JPG*, JP2, JFF, and JTF
Adobe Portable Document File PDF
RAW image file format CRW, CR2 (Canon); MRW (Minolta); NEF (Nikon); ORF
(Olympus); DNG (Adobe); PTX (Pentax); ARW, SRF, SR2 (Sony)
Corel (and Micrografx) Picture
Publisher
PPF, PP4, and PP5
Trang 83. Before importing a photo, choose Tools | Color Management | Default Settings.
Check both the “Warn on color profile mismatch” and the “Warn on missing color profile” boxes in both the Open and the Import And Paste areas in Color Management Policies You’re all set to import a photo now
Ill 23-2
4. When you choose File | Import (or pressCTRL+I), you then have the opportunity to scale an image before placing it on the page More experienced users might want to simply drag an image file into the workspace when CorelDRAW is not maximized
Using either technique, when you import an image, if it has a color profile that doesn’t match your current document—or has no color profile at all—you’ll get an alert box where you have the opportunity to choose the color space for the imported image
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Trang 9Ill 23-3
5. Generally, you’ll want to choose the same color space as the document’s color space However, if you set up your document incorrectly—for example, your client specified Adobe RGB, you had soap in your ears, and thought they said “sRGB”—import the photo using the Adobe RGB choice from the drop-down list Then choose Tools | Color Management | Document Settings, and choose Adobe RGB from the RGB drop-down list in the Edit Document Color Settings area Choose “Convert document colors
to new color profiles”—do not use “Assign different colors profiles,” because this will
introduce color-shifting
Because the method of dragging an image file into the workspace doesn’t afford scaling options, you might want to stick withCTRL+Ifor importing File | Import also gives you the chance to filter for the file types you seek in a Windows folder, and you have other options
in the Import dialog (discussed in Chapter 24) Choose your bitmap format from the Files
Of Type menu
Drag from folder into workspace,
or press CTRL+I (File | Import).
Color profile alert
Trang 10After the Import dialog closes, your cursor will change to an import cursor that has two
functions: with it you specify the upper-left corner of your new bitmap using a single-click,
which in turn imports the image onto a document page at its original size—whatever
dimensions, by whatever its original saved resolution PressingENTERinstead of using a
mouse click imports the image to the center of the page
Placing and Modifying an Image
The best way to get the hang of inserting an image into a CorelDRAW composition is by
example: open Wally’s Wheel’s.cdr, and in the tutorial to follow you’ll place a picture of an
auto and then perform a little manual cropping
In addition to clicking or pressingENTERwith a cursor that’s loaded with an image you
import, you can also place and proportionately scale the imported image by click-dragging
diagonally After you specify the size this way, the bitmap is imported and automatically
resized to closely fit the defined area with the original proportions of the bitmap preserved,
but the resolution will not be the original’s As you drag, the cursor changes orientation and
the image’s bounding box appears, showing the space the new image will occupy While
importing during either operation, the original filename and the image dimensions are
displayed beside the cursor Your goal in the next steps is to place Expensive car.jpg at the
top of the 5"×7" riser card layout As you work through the steps, you’ll note that the
document’s color space isn’t the same as the JPEG image, but you already know how to
correct this The native dimensions of the JPEG are also larger than the CorelDRAW page
layout, which affords the perfect opportunity to try out this importing and scaling stuff
You don’t have to import large bitmap images to use them in a CorelDRAW
composition Instead, you can link to images externally, and CorelDRAW displays
a low-resolution version of the image on the page After you choose to Import a
photo, click the name of the file in the Import dialog box, and then click the
drop-down list arrow on the Import button Choose Import As Externally Linked Image
(in Windows 7 and Windows Vista) or enable the Link Bitmap Externally check box
(in Windows XP) You can review externally linked photos in your CorelDRAW
documents at any time by choosing Window | Dockers | Links And Bookmarks.
Camera RAW images cannot be linked externally, and remember that if you change
the location of a linked photo or delete it, your document won’t print correctly The
purpose of externally linking to large bitmap images is to keep CorelDRAW file
sizes down while allowing the original bitmap to be modified—usually optimized
for high-resolution PostScript output.
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