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Picture Formats: Raster Pictures
BMP: Windows Bitmap
Windows bitmaps store a single raster image in any color depth,
from black-and-white to 24-bit color. The Windows bitmap file format is
compatible with other Microsoft Windows programs. It does not support file
compression and is not suitable for Web pages.
Overall, the
disadvantages of the Windows bitmap file format outweigh the advantages. For
images of photographic quality, use a PNG file, a JPEG file, or a TIFF file.
BMP files are suitable for wallpaper in Windows.
Advantages:- BMP supports 1-bit through 24-bit color depth.
- The BMP format is widely compatible with existing Windows
programs, especially older programs.
Disadvantages:- BMP does not support compression, which results in very
large files.
- BMP files are not supported by Web browsers.
PCX: PC Paintbrush
PC Paintbrush pictures, also called Z-Soft bitmaps, store a
single raster image at any color depth. Paintbrush pictures are more widely
used in earlier Windows programs and MS-DOS-based programs. Paintbrush pictures
are compatible with many later programs. PCX pictures support Run Length
Encoded (RLE) internal compression.
Advantages:- PCX is a standard format across many Windows-based programs
and MS-DOS-based programs.
- PCX supports internal compression.
Disadvantages:- PCX is not supported by Web browsers.
PNG: Portable Network Graphics
PNG pictures store a single raster image at any color depth. PNG
is a platform-independent format.
Advantages:- PNG supports high-level lossless compression.
- PNG supports alpha channel transparency.
- PNG supports gamma correction.
- PNG supports interlacing.
- PNG is supported by more recent Web browsers.
Disadvantages:- Older browsers and programs may not support PNG
files.
- As an Internet file format, PNG provides less compression
than the lossy compression of JPEG.
- As an Internet file format, PNG offers no support for
multi-image files or animated files. The GIF format supports multi-image files
and animated files.
JPEG: Joint Photographic Experts Group
JPEG pictures store a single raster image in 24-bit color. JPEG
is a platform-independent format that supports the highest levels of
compression; however, this compression is lossy. Progressive JPEG files support
interlacing.
The level of JPEG file compression can be increased or
decreased. However, image quality is sacrificed for file size. The compression
ratio can be as high as 100:1. (The JPEG format comfortably compresses files at
a 10:1 to 20:1 ratio, with little picture degradation.) JPEG compression works
well with photo-realistic artwork. However, for simpler artwork with fewer
colors, sharp levels of contrast, solid borders, or large solid areas of color,
JPEG compression does not provide superior results. Sometimes the compression
ratio is as low as 5:1, with a high loss of picture integrity. This loss occurs
because the JPEG compression scheme compresses similar hues well. But the JPEG
compression scheme does not work as well with sharp differences in brightness
or with solid areas of color.
Advantages:- Superior compression is supported for photographic artwork
or realistic artwork.
- Variable compression allows control of the file
size.
- Interlacing (for Progressive JPEG files) is
supported.
- JPEG is a widely supported Internet standard.
Disadvantages:- Lossy compression degrades the original picture
data.
- When you edit and resave JPEG files, JPEG compounds the
degradation of the original picture data. This degradation is
cumulative.
- JPEG is not suitable for simpler pictures that contain few
colors, broad areas of similar color, or stark differences in
brightness.
GIF: Graphics Interchange Format
GIF pictures store single raster image data or multiple raster
image data in 8-bits, or 256 colors. GIF pictures support transparency,
compression, interlacing, and multiple-image pictures (animated
GIFs).
GIF transparency is not alpha channel transparency and cannot
support semitransparent effects. GIF compression is LZW compression, at a
roughly 3:1 ratio. Animated GIFs are supported in the GIF89a version of the GIF
file specification.
Advantages:- GIF is a widely supported Internet standard.
- Lossless compression and transparency are
supported.
- Animated GIFs are prevalent and easy to create with many
GIF animation programs.
Disadvantages:- GIF supports only a 256-color palette; therefore, detailed
pictures and photo-realistic images lose color information and look
paletted.
- Lossless compression is inferior to the JPEG format or the
PNG format, in most cases.
- GIF supports limited transparency and no semitransparent
effects or faded effects, such as those that are provided by alpha channel
transparency.
TIFF: Tagged Image File Format
TIFF pictures store a single raster image at any color depth.
TIFF is arguably the most widely supported graphics file format in the printing
industry. TIFF supports optional compression and is not suitable for viewing in
Web browsers.
The TIFF format is an extensible format. This means
that a programmer can modify the original specification to add functionality or
to meet specific needs. Modifying the specification can lead to
incompatibilities between different types of TIFF pictures.
Advantages:- TIFF is a widely supported format, especially between
Macintosh computers and Windows-based computers.
- Optional compression is supported.
- The extensible format supports many optional
features.
Disadvantages:- TIFF is not supported by Web browsers.
- Extensibility results in many different types of TIFF
pictures. Not all TIFF files are compatible with all programs that support the
baseline TIFF standard.
Picture Formats: Vector Pictures
DXF: AutoCAD Drawing Interchange File
The DXF format is a vector-based, ASCII format that the Autodesk
AutoCAD program uses. AutoCAD provides highly detailed schematics that are
completely scalable.
Advantages:- AutoCAD allows you to create highly detailed and precise
schematics and drawings.
- AutoCAD files are popular in the architectural, design, and
engraving industries.
Disadvantages:- AutoCAD has limited support in Office, which supports
AutoCAD versions up through R12.
- AutoCAD has a steep learning curve. Note that other
graphics programs are also capable of exporting DXF pictures.
CGM: Computer Graphics Metafile
The CGM metafile can contain vector information and bitmap
information. It is an internationally standardized file format that is used by
many organizations and government agencies, including the British Standards
Institute (BSI), the American National Standards Institute (ANSI), and the
United States Department of Defense.
Advantages:- CGM is an international standard format.
CDR: CorelDRAW!
The CorelDRAW! metafile can contain both vector information and
bitmap information.
Advantages:- CDR is widely used in the prepress industry and the
artistic design industry.
Disadvantages:- CDR has limited support in Office, which supports
CorelDRAW! version 6 and earlier.
WMF: Windows Metafile
The Windows Metafile is a 16-bit metafile format that can contain
both vector information and bitmap information. It is optimized for the Windows
operating system.
Advantages:- WMF is a Windows standard format that works well with
Office.
EPS: Encapsulated PostScript
The Encapsulated PostScript format is a proprietary, printer
description language that can describe both vector information and bitmap
information.
Advantages:- EPS produces accurate representation on any PostScript
printer.
- EPS is an industry-standard format.
Disadvantages:- The on-screen representation may not match the printed
representation. The on-screen representation may be low-resolution, a different
image, or only a placeholder image.
- EPS files are designed to be printed. They are not the most
suitable format to display information on the screen.
EMF: Enhanced Metafile
The Enhanced Metafile format is a 32-bit format that can contain
both vector information and bitmap information. This format is an improvement
over the Windows Metafile Format and contains extended features, such as the
following:
- Built-in scaling information
- Built-in descriptions that are saved with the
file
- Improvements in color palettes and device
independence
The EMF format is an extensible format, which means that a
programmer can modify the original specification to add functionality or to
meet specific needs. This modification can lead to incompatibilities between
different types of EMF pictures.
Advantages:- Extensible file format
- Improved features compared with WMF
Disadvantages:- Extensibility results in many different types of EMF
pictures. Not all EMF files are compatible with all programs that support the
EMF standard.
PICT: Macintosh Picture
The PICT file is a 32-bit metafile format for the Macintosh
computer. PICT files use Run Length Encoded (RLE) internal compression, which
works reasonably well. PICT files support JPEG compression if QuickTime is
installed
(Macintosh only).
Advantages:- PICT is the best file format for on-screen display on the
Macintosh computer.
- PICT is the best printing format to use when you print from
the Macintosh computer to a non-PostScript printer.
Disadvantages:- Fonts may be represented incorrectly when they are moved
cross-platform.
- QuickTime must be installed to view some PICT files
correctly.
Resolution and Color Depth
This section discusses the appropriate color depth and resolution
for raster pictures. If you save pictures with the correct resolution and color
settings, you create smaller files. Smaller files mean smaller, faster
documents and presentations. It is in your best interest to make a picture as
small as possible, within the requirements of your picture usage.
On-Screen Display
|
| 1 (black and white) | GIF at 72 pixels per inch
(ppi) | GIF at 72 pixels per inch (ppi) |
| 16 | GIF at 72 ppi | GIF at 72 ppi |
| 256 (simple picture)* | GIF at 72 ppi | GIF at 72
ppi |
| 256 (complex picture)* | JPEG at 72 ppi | JPEG at
72 ppi |
| More than 256 | JPEG or PNG at 72 ppi | JPEG,
PNG, or TIFF at 72 ppi |
NOTE: Microsoft recommends a resolution of 72 pixels per inch, because
most monitors have between 60 pixels and 80 pixels per inch. Saving at a higher
resolution does not result in a higher-quality display, because your monitor
cannot display more pixels than physically exist in the monitor. You should
calculate the points per inch according to the completed size of your picture,
not the original size. For example, if you are scanning an 8.5-by-2-inch
letterhead for use on a Web page with a completed width of 2 inches, you can
scan at 72 ppi for 2 inches, for a total of 144 pixels. The resulting file
looks great when it is sized to 2 inches and is displayed on a monitor.
*NOTE: At 256 colors, JPEG files offer a higher level of compression
than GIF files do. However, JPEG compression does not compress some simple
files as effectively as GIF compression does.
- If your picture is grayscale, has large areas of one solid
color, or has areas of high contrast (sharp differences between light areas and
dark areas), choose the GIF format.
- If your picture is in color and contains several different
colors (hues) that are similar in lightness or darkness (value), choose the
JPEG format, because JPEG offers better compression. JPEG compression works
according to hue and works well with different hues of a similar value. JPEG
compression does not work as well with similar hues at different
values.
Printed Output
How to create good printed output is a complex subject, because
of the vast number of printers that are available and the capability of each
printer to produce color output and grayscale output. The primary factor for
good printed output is the number of lines per inch (LPI) that your printer is
capable of printing.
To print in color or grayscale, a printer must
print in halftones. Halftones are arrays of dots that are arranged in a grid
and that represent each image pixel as a shade of gray. For a dark gray, most
of the dots in the grid are filled in. For a light gray, only a few dots on the
grid are filled in. The LPI setting for the printer determines the size of this
grid. The higher the LPI, the smaller the grid, and the fewer shades of gray
the printer can render.
To print in color, the printer must print
overlapping lines of colored dots. Each dot is set at a different angle from
the other dots and is slightly offset, so that the dots do not completely cover
each other. This measurement is known as the Screen Frequency and is
represented in degrees of rotation of the lines of dots that make up that
color.
The following table helps you to choose the optimum resolution
for scanning, in dots per inch (dpi).
|
| Laser printer | 300 | 55-65 | 120 |
| Laser printer | 600 | 65-85 | 150 |
| Ink-Jet printer | 300 | 50-60 | 110 |
| Dye-Sub printer | 300 | 55-70 | 125 |
| Imagesetter | 1250+ | 120-150 | 300 |
To calculate your target scanning resolution, you
can multiply the LPI of your printer by two. This is a general rule. To find
out the LPI of your printer, see your printer documentation.
NOTE: You must experiment when you apply the general rule of
multiplying the LPI by two. Some printers support very high resolutions. If you
save your picture at more than 300 ppi, larger pictures may take up large
amounts of disk space and may slow down other operations on your computer.
Multiple large pictures in a document can cause a program to stop responding or
can cause Windows to stop responding.
The only exception to this rule
is pure black-and-white images, or "line art" images. These images use 1 bit to
store color information. Scan these images at a 1-to-1 ratio. If you have a 600
dpi printer, scan these images at 600 ppi in Line Art mode.
If you
want your picture to be in grayscale or to have fewer than 256 colors, use
either the TIFF format or the GIF format. The TIFF format is the standard of
the printing industry for graphics, because the TIFF format does not use a
lossy compression scheme. Other formats, such as JPEG, use a lossy compression
scheme. TIFF also supports multiple levels of transparency, which few other
formats do.
If the picture has more than 256 colors, save the picture
in the TIFF format or the PNG format. Microsoft recommends the PNG format if
you want transparency. If you do not want transparency, use the TIFF
format.
Microsoft recommends that you save your picture at printer
resolution for the completed picture size. For example, assume that you have an
8.5-by-2-inch letterhead, and you want to print the letterhead at a size of 2
inches. If your printer supports 600 dpi and an LPI of 85, set the picture
resolution to 150 ppi at 2 inches, for a size of 300 x 71 pixels.
Glossary
- Alpha channel: An alpha channel describes an area of transparency in a picture.
This area of transparency allows a background to show through. An alpha channel
allows more than 64,000 levels of transparency, which makes it possible to use
semitransparent effects and blended effects.
- Color depth: The color depth is the number of colors in your picture. Color
depth is categorized by bit depth. If you use a deeper color depth, there are
more colors in the picture, but a deeper color depth also increases your file
size.
- 1 bit - Black and white only
- 8 bit - 256 shades of grayscale, or 256
colors
- 16 bit - High Color, 65,536 colors
- 24 bit - True Color, 16,777,216 colors
- 32 bit - True Color, 4,294,967,296 colors
- Compression: Compression is a mathematical scheme that makes a picture file
smaller by removing redundant information. There are two types of compression:
lossless and lossy.
- Compression, lossless: Lossless compression is a compression scheme that emphasizes
maintaining the integrity of the original picture. When the picture is
uncompressed, it maintains the same resolution and picture quality of the
original, uncompressed picture.
- Compression, lossy: Lossy compression is a compression scheme that emphasizes
producing a small picture file, even at the cost of picture quality. Lossy
compression can produce smaller picture files than lossless compression;
however, when you uncompress the picture, some of the original picture data is
lost and cannot be recovered.
- File size: File size is the ultimate limiting factor when you work with
picture files. File size is the most common cause of problems when you work
with pictures in Microsoft Office. File size is determined by the following
factors: picture size, resolution, file format, compression, and color
depth.
- Gamma correction: This is a method to correct the lightness or darkness of
pictures, so that the pictures appear with the same brightness on any
monitor.
- Hue: Hue describes the relative amounts of red, green, or blue in a
color. For example, both pink and crimson have a red hue.
- Interlaced: Interlacing is a method to send picture data over the Internet.
When a picture is interlaced, the following occurs: After one sixty-fourth of
the picture is downloaded, you can see a general image of what the picture
looks like. As more of the picture is downloaded, the resolution improves until
the whole picture is displayed.
- Metafile picture: A metafile picture usually contains vector picture information.
A metafile picture can contain any kind of picture data, such as a raster
picture.
- Palette: A palette is a list of the colors that are available to a
particular picture. Different picture file formats have a different maximum
number of colors. If your picture contains more colors than are available in a
specific picture format, the extra colors are replaced with colors in the color
palette. The colors in the resulting image may look distorted. This is known as
a "paletted effect."
- Pixel: A pixel is a fundamental unit of measurement in a raster-based
picture or on a monitor. Both raster pictures and monitors are defined by rows
of dots that can be individually assigned a color. These dots are called
pixels.
- Raster picture: A raster picture is a picture that is displayed by defining rows
of colored dots that are placed next to each other. Each dot is assigned an
individual color.
- Resolution: Resolution is the amount of picture data in a specific area of a
picture. Resolution is usually defined in pixels per inch. The higher the
resolution, the more precise and clear the picture is. However, when you
increase the resolution, the file size of a picture also increases.
- Transparency: Transparency is a method that allows areas of a picture to
appear transparent, therefore revealing the background. There are several
methods of transparency, including alpha channel transparency.
- Value: This property describes the lightness or darkness of a color.
For example, pink and baby blue have a similar value, although they have different hues.
- Vector picture: A vector picture is made up of areas that are defined by
coordinates and mathematical formulas. This file format is more versatile than
a raster picture format, because vector pictures can be scaled to any size. In
some cases, vector pictures can be ungrouped into smaller
components.
