It's
almost impossible to discuss graphics software without first
establishing an understanding of the differences between the two major
2D graphic types: bitmap and vector images. This is an important lesson
and often a tough one to grasp. If you work with graphics at all, it's
bound to come up, so it's an important concept to understand. Let's
start by talking about the more common type: bitmap images.
Bitmap images (also known as raster images) are made up of pixels in a
grid. Pixels are picture elements; tiny dots of individual color that
make up what you see on your screen. All these tiny dots of color come
together to form the images you see. Most computer monitors display
approximately 70 to 100 pixels per inch--the actual number depends on
your monitor and screen settings.
To illustrate this, let's take a look at a typical desktop icon such as
the one shown in the image here. The icons on your desktop are typically
32 by 32 pixels. In other words, there are 32 dots of color going in
each direction. When combined, these tiny dots form an image. The icon
shown in the upper right corner of this example is a typical desktop
icon at screen resolution. As you can see, when you enlarge the icon, as
I have in this example, you can clearly see each individual square dot
of color. Note the that white areas of the background are still
individual pixels, even though they appear to be one solid color.
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Bitmap
Bitmap images are resolution dependent. Resolution refers to the number
of pixels in an image and is usually stated as dpi (dots per inch) or
ppi (pixels per inch). Bitmap images are displayed on your computer
screen at screen resolution: approximately 100 ppi. However, when
printing bitmaps, your printer needs much more image data than a
monitor. In order to render a bitmap image accurately, the typical
desktop printer needs 150-300 ppi. If you've ever wondered why your 300
dpi scanned image appears so much larger on your monitor, this is why.
For more in-depth information about resolution, scanning, and printing
bitmap images, refer to my article Getting Started Scanning.
Because bitmaps are resolution dependent, it's difficult to increase or
decrease their size without sacrificing a degree of image quality. When
you reduce the size of a bitmap image through your software's resample
or resize command, you must throw away pixels. When you increase the
size of a bitmap image through your software's resample or resize
command, the software has to create new pixels. When creating pixels,
the software must estimate the color values of the new pixels based on
the surrounding pixels. This process is called interpolation.
Did you notice in the paragraph above, I specifically talked about
resizing images "through your software's resample or resize command"? I
want to make the distinction between this method of resizing versus
zooming in and out, or dragging the edges of your images in a page
layout program to resize it. This type of resizing is more accurately
called scaling. Scaling an image does not effect the image permanently.
In other words, it does not change the number of pixels in the image.
However, if you scale a bitmap image to a larger size in your page
layout software, you are going to see a definite jagged appearance. Even
if you don't see it on your screen, it will be very apparent in the
printed image. Scaling a bitmap image to a smaller size doesn't have any
effect; in fact, when you do this you are effectively increasing the ppi
of the image so that it will print clearer.
Common bitmap formats include:
• BMP
• GIF
• JPEG, JPG
• PNG
• PICT (Macintosh)
• PCX
• TIFF
• PSD (Adobe Photoshop)
Popular bitmap editing programs are:
• Microsoft Paint
• Adobe Photoshop
• Corel Photo-Paint
• Corel Paint Shop Pro
• The GIMP
All scanned images are bitmaps, and all images from digital cameras are
bitmaps.
Vector
Vector images are made up of many
individual, scalable objects. These objects are defined by mathematical
equations rather than pixels, so they always render at the highest
quality. Objects may consist of lines, curves, and shapes with editable
attributes such as color, fill, and outline. Changing the attributes of
a vector object does not effect the object itself. You can freely change
any number of object attributes without destroying the basic object. An
object can be modified not only by changing its attributes, but also by
shaping and transforming it using nodes and control handles. For an
example of manipulating an object's nodes, see my CorelDRAW tutorial on
drawing a heart.
Because they're scalable, vector-based images are resolution
independent. You can increase and decrease the size of vector images to
any degree and your lines will remain crisp and sharp, both on screen
and in print. Fonts are a type of vector object.
Another advantage of vector images is that they're not restricted to a
rectangular shape like bitmaps. Vector objects can be placed over other
objects, and the object below will show through. See the example images
on this page. The vector circle and bitmap circle appear to be exactly
the same when seen on a white background. But when you place the bitmap
circle over another color, it has a rectangular box around it, from the
white pixels in the image.
Vector images have many advantages, but the primary disadvantage is that
they're unsuitable for producing photo-realistic imagery. Vector images
are usually made up of solid areas of color or gradients, but they
cannot depict the continuous subtle tones of a photograph. That's why
most of the vector images you see tend to have a cartoon-like
appearance. Even so, vector graphics are continually becoming more
advanced, and we can do a lot more with vector drawings now than we
could a decade ago. Today's vector tools allow you to apply bitmapped
textures to objects giving them a photo-realistic appearance, and you
can now create soft blends, transparency, and shading that once was
difficult to achieve in vector drawing programs.
Vector images primarily originate from software. You can't scan an image
and save it as a vector file without using special conversion software.
On the other hand, vector images can, quite easily, be converted to
bitmaps. This process is called rasterizing. When you convert a vector
image to a bitmap, you can specify the output resolution of the final
bitmap for whatever size you need. It's always important to save a copy
of your original vector artwork in its native format before converting
it to a bitmap; once it has been converted to a bitmap, the image loses
all the wonderful qualities it had in its vector state. If you convert a
vector to a bitmap at a size of 100 by 100 pixels and then decide you
need the image to be larger, you'll need to go back to the original
vector file and export the image again. Also keep in mind that opening a
vector image in a bitmap editing program usually destroys the vector
qualities of the image and converts it to raster data.
The most common reason for wanting to convert a vector to a bitmap would
be for use on the Web. At this time, the most common and accepted format
for vector images on the Web is Shockwave Flash (SWF). Another standard
for vector images on the Web is SVG, a graphics programming language
based on XML. Due to the nature of vector images, they are best
converted to GIF or PNG format for use on the Web.
Common vector formats include:
• AI (Adobe Illustrator)
• CDR (CorelDRAW)
• CMX (Corel Exchange)
• CGM Computer Graphics Metafile
• DXF AutoCAD
• WMF Windows Metafile
• EPS
Popular vector drawing programs are:
• Adobe Illustrator
• CorelDRAW
• Xara Xtreme
• Serif DrawPlus
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Only vector image
can be used for cutter/ plotter. |
