Scientific
Communications EAS-450 – Spring 2004
Graphics
Formats
Bitmap vs Vector Images
Bitmap images, or raster images, contain many
single picture elements (pixels) in a grid (usually rectangular). Each pixel is
a dot of color that make up the image. Depending on the type of image, the
color is specified by the number of bits, from 2 bits (black and white) to 32
or 64 bits in a realistic color photograph. Some examples are:
1 bit per pixel refers to an
image with 2 colors.
4 bits per pixel refers to an
image with up to 16 colors.
8 bits per pixel refers to an
image with up to 256 colors.
16 bits per pixel refers to an
image with up to 32,768 colors.
24 bits per pixel refers to an
image with up to 16,777,216 colors.
The resolution of the image is the number of pixels per
inch (or dots per inch – dpi), e.g. 72 or 96 pixels for most computer monitors.
To give you an idea of a pixel, most program icons are 32 x 32 pixels. When
enlarged, you can see the individual dots. Images from scanners and digital cameras
are bitmap images.
To print a bitmap image on a printer requires between 150
and about 600 dpi for an accurate rendition (no graininess). Many printers now
go up to 1200 or higher dpi to produce ‘almost photographic’ images.
Resizing a bitmap image requires
throwing away pixels (reducing the size) or interpolating (adding new) pixels
to enlarge. Both of these operations require an algorithm (or program) that
does an approximation, but neither process gives a perfect rendition. It is
especially difficult to interpolate color during interpolation, and of course
impossible to improve actual resolution.
Resizing through software is not the same as changing the
image size on the screen by rescaling the image (zooming in and out
using various controls). Scaling is temporary and does not affect the way the
image is stored.
Some bitmap formats are: BMP, GIF, JPEG (JPG), PNG, PICT,
PCX, TIFF (TIF), and PSD. Two formats support transparency – GIF and PNG – but
only in their original format (not after a conversion). Characteristic are:
BMP –
standard Windows format, has a fixed color palette
GIF –
Graphics Interchange Format (Compuserve). This is an efficient bitmap that uses
a LZW compression algorithm; the images can be interlaced (4 passes to see the
full image). There is a limited color palette (1, 2, or 8 bits per pixel) – up
to 256 colors only.
MAC –
Mac paint file format – 1 bit per pixel (monochrome), 576 x 720 fixed
resolution
PCX – PC
Paintbrush format – an industry standard. Depending on the version supports
between 1 bit and 24 bits per pixel.
RAS –
Sun raster files, up to 32 bits per pixel
High compression formats:
JPG
(Joint Photographic Experts Group) - uses lossy compression – so discards
portions of the bitmap depending on the resolution. Compression factors are
typically between 5:1 and 15:1; compression is automatic on opening.
TIFF
(Tagged Image File Format) – used to exchange images between different
programs. Several different compression techniques are used, and images can be
BW, gray scale or color (up to 24 bits per pixel).
Some bitmap editing programs are: 
Adobe
Photoshop, Corel Photo-Paint, Micrografx Picture, MS Publisher, MS Paint. One can
convert between different formats using the Save As command, or Import /
Export.
Vector Images
Vector images contain lines and curves that are described
mathematically. They can therefore be represented at arbitrary resolution
without loss of detail. They cannot contain the same sophisticated detail as a
high resolution bitmapped image, and thus are best for figures and
illustrations (not photographs).
Two popular vector based programs are Corel Draw and Adobe
Illustrator that have special file formats. There are fewer vector file
formats, the most common are CDR, AI, WMF, and EPS (see later).
Object Based Software
Many high-end graphics programs blend bitmap and vector
image capabilities using the concept of objects. You are then free to manipulate
either kind of image. Still it is wise to be aware of the distinction between
programs:
·
Image editors are intended for outputting
bitmap-based graphics.
When working with vector objects in a bitmap editing program, they only
preserve their vector-like qualities as you work in that particular software,
saving your files in the program's native format. Once you want to
output your documents from an image editor to use in other software, you are
generally limited to bitmap-based formats.
·
Illustration software is intended for outputting
vector-based graphics.
When working with bitmap-based objects in a vector program, the bitmaps do not
become vector objects. Although you can sometimes output bitmap elements as
part of a vector-based file format, these hybrid graphics then become metafiles,
containing both bitmap and vector elements. In a metafile, only the
vector-based portions of the file are scalable and resolution independent.
Unlike bitmaps and simple vector graphics, metafiles cannot easily be converted
from one program's format to another.
Native File Formats
The native file format is the default file format used by a
specific software application. The native file format of an application is
proprietary and these types of files are not meant to be transferred to other
applications.
Usually special software-specific image properties can only be
retained when an image is saved in the software's native format. For example,
layer styles and text in Photoshop will only remain editable when it the image
is saved in the native Photoshop (PSD) format. Lens effects and PowerClips in
CorelDRAW can only be edited when the document is saved in native CorelDRAW
(CDR) format. Below are a few of the major graphics applications and their
native file formats:
CDR – CorelDRAW (vector)
CPP - Corel Photo-Paint (bitmap)
PSD - Adobe Photoshop (bitmap)
PDD - Adobe PhotoDeluxe (bitmap)
AI - Adobe Illustrator (vector)
PSP - Paint Shop Pro (bitmap)
When an image is being sent to another application it should be
converted or exported to a standard image format. The exception would be
if you are transferring an image between applications from the same publisher.
For example, you should have no problem sending Adobe Illustrator files to
Adobe Photoshop, or Corel Photo-Paint files to CorelDRAW. Also keep in mind
that you usually cannot use an earlier version of a program to open files saved
from a later version of the same software. In most cases, you'll loose image
properties that are specific to the later version.
Standard Image Formats
A standard image format is one that is cross-platform compatible
and supported by the majority of graphics applications.
The most common standard bitmap-based formats are TIFF, JPG, GIF,
and PNG. On the Windows platform BMP is a standard format and PICT is a
standard bitmap format on the Macintosh. Photoshop's PSD format, though
proprietary, is supported to some degree by most graphics applications, but be
aware that transferring PSD between non-Adobe applications may give unexpected
results.
The most common standard vector-based formats are EPS and AI. On
the Windows platform, most vector-based software also supports WMF and EMF
formats.
When you are sending files over the Internet or transporting them
between graphics applications, it is important to use one of these standard
formats, or you may not get the results you expect. When sending graphic images
via email and the Web, it is best to use JPEG or GIF format, which can be
displayed by any Web browser on any computer. When in doubt, ask the recipient
of your files which image formats they can accept.
Wherever possible use the vector format for all your type, line art and illustrations and only use bitmaps for photos or images with complex or non-uniform shading. If the graphics application recognizes native vector files then use them first.
The EPS File Format
If the graphics application you are using cannot read
native vector files the next best thing would be to save them as EPS
(Encapsulated PostScript) files. These are self-contained PostScript files
which contain the same mathematical descriptions as the vector files they are
made from. Even bitmaps can be saved in the EPS file format. EPS files are
supported by most all graphics applications. It is the most portable format for
this reason. It is best to use EPS files for all line art and illustrations
because they can be reproduced at any size or resolution and still display
exactly as they were drawn. Use them wherever native vector files cannot be
used.
Remember these 4 standard file formats:
For Web Pages:
GIF - This
format is best for images with solid colors or areas of uniform color such as
illustrations and logos.
JPEG - Used for photographic (continuous tone) images.
For Printed Documents:
TIFF - Used for bitmaps only. The TIFF format is supported by virtually
all graphics applications.
EPS - they are unique in that you can use them for vector graphics,
bitmap images, type or even entire pages.
Reference
Much of the above is a condensed version of the information
found in
http://graphicssoft.about.com/cs/graphicformats/index.htm