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 this file format outweigh the advantages. For photographic quality images, a PNG, JPG, or TIF file is often more suitable. BMP files are suitable for wallpaper in Windows.Advantages
- 1-bit through 24-bit color depth
- Widely compatible with existing Windows programs, especially older programs
- No compression, which results in very large files
- 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 and MS-DOS-based programs, and are still compatible with many newer programs. PCX pictures support internal Run Length Encoded (RLE) compression.Advantages
- Standard format across many Windows and MS-DOS based programs
- Internal compression
- Not supported by Web browsers
PNG - Portable Network Graphic
PNG pictures store a single raster image at any color depth. PNG is a platform-independent format that supports a high level of lossless compression, alpha channel transparency, gamma correction, and interlacing. It is supported by more recent Web browsers.Advantages
- High-level lossless compression
- Alpha channel transparency
- Gamma correction
- Supported by more recent Web browsers
- Lack of support for PNG files in older browsers and programs
- As an Internet file format, PNG provides less compression than the lossy compression of JPG
- As an Internet file format, PNG offers no support for multi-image or animated files, which the GIF format supports
JPG - Joint Photographic Experts Group (JPEG)
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, sacrificing image quality 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, in 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 happens because the JPEG compression scheme compresses similar hues well, but does not work as well with sharp differences in brightness or solid areas of color.Advantages
- Superior compression for photographic or realistic artwork
- Variable compression allows file size control
- Interlacing (for Progressive JPEG files)
- Widely supported Internet standard
- Lossy compression degrades 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 or multiple raster image data in 8-bit, 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 semi-transparent 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
- Widely supported Internet standard
- Lossless compression and transparency supported
- Animated GIFs are prevalent and easy to create with a large number of GIF animation programs
- 256-color palette; detailed pictures and photo-realistic images lose color information and look paletted
- Lossless compression is inferior to the JPG or PNG formats in most cases
- Limited transparency; no semi-transparent or faded effects like those 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 graphic file format in the printing industry. It supports optional compression, and is not suitable for viewing in Web browsers.
The TIFF format is an extensible format, which means that a programmer can modify the original specification to add functionality or meet specific needs. This can lead to incompatibilities between different types of TIFF pictures.Advantages
- Widely supported, especially between Macintosh computers and Windows-based computers
- Optional compression
- Extensible format allows for many optional features
- 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 Autodesk's 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
- Limited support in Office 2000, which supports versions up through R12
- AutoCAD has a steep learning curve; however, other graphics programs are also capable of exporting DXF pictures
CGM - Computer Graphics Metafile
The CGM metafile can contain vector and bitmap information. It is an internationally standardized file format used by many organizations and government agencies, including the British Standards Institute (BSI), American National Standards Institute (ANSI), and the United States Department of Defense.Advantages
- International standard format
CDR - CorelDRAW!
The CorelDRAW! metafile can contain both vector and bitmap information. It is a widely used, artistic design file format. Advantages
- Widely used in the prepress and artistic design industries.
- Limited support in Office 2000, which supports version 6 and earlier
WMF - Windows Metafile
The Windows Metafile is a 16-bit metafile format that can contain both vector and bitmap information. It is optimized for the Windows operating system.Advantages
- Windows standard format that works well with Office 2000
EPSF - Encapsulated PostScript Format
The Encapsulated PostScript Format is a proprietary, printer description language that can describe both vector and bitmap information. Advantages
- Accurate representation on any PostScript printer
- Industry standard format
- 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, not necessarily looked at. 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 and bitmap information. It is an improvement over the Windows Metafile Format and contains extended features such as:
- 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 meet specific needs. This can lead to incompatibilities between different types of EMF pictures.Advantages
- Extensible file format
- Improved features compared to WMF
- 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. PICT files use Run Length Encoded (RLE) internal compression, which works reasonably well. PICT files support JPEG compression if QuickTime is installed (Macintosh only)
- Best file format for on-screen display on the Macintosh
- Best printing format from the Macintosh to a non-PostScript printer
- Fonts may be represented incorrectly when 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 proper 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, given your picture usage requirements.
On Screen Display
|Number of colors||Internet use||Non-Internet use|
|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)*||JPG at 72 ppi||JPG at 72 ppi|
|More than 256||JPG or PNG at 72 ppi||JPG, PNG, or TIF at 72 ppi|
Microsoft recommends a resolution of 72 pixels per inch, because most monitors have between 60 and 80 pixels per inch. Saving at a higher resolution does not result in a higher quality display, because your monitor can't display more pixels than physically exist in the monitor. You should calculate the points per inch according to finished size, not starting size. For example, if you are scanning an 8.5-by-2-inch letterhead for use on a Web page with a finished width of 2 inches, you would scan at 72 ppi for 2 inches, for a total of 144 pixels. The resulting file looks great when sized to 2 inches and displayed on a monitor. *Note
At 256 colors, JPG files offer a higher level of compression than GIF files do. However, JPG compression does not compress some simple files as well 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 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 JPG format, because it offers better compression. JPG compression works according to hue and works well with different hues of a similar value. JPG compression does not work as well with similar hues at different values.
How to create good printed output is a complex subject, because of the vast number of printers available and the capabilities of each to produce color and grayscale output. The primary factor in creating quality output is the number of lines per inch (LPI) that your printer is capable of.
To print in color or grayscale, a printer must print in halftones. Halftones are arrays of dots that are arranged in a grid and represent each image pixel as a shade of gray. For a dark gray, most of the dots in the grid are filled in, whereas for a light gray, only a few dots are filled in on the grid. The size of this grid is determined by the LPI setting for that printer. 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 at a different angle from the other, and slightly offset so that they 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 select the optimum scanning resolution in dots per inch (dpi).
|Printer type||Output dpi||Output LPI||Scanning ppi|
A good rule is to multiply the LPI for your printer by two, to calculate your target scanning resolution. To find out your printer's LPI, check your printer documentation.Note
You need to experiment when you apply this general rule. 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 could cause a program or Windows to stop responding. For more information about how to determine the size of bitmap pictures, click the article number below to view the article in the Microsoft Knowledge Base:
Importing bitmaps: Determining size and memory requirements
The only exception to this rule is with pure black and white, or "line art" images. These images use 1 bit to store color information. With these images you should scan at a 1-to-1 ratio. If you have a 600 dpi printer, you should scan at 600 ppi in Line Art mode.
If you want your picture to be in grayscale or to have fewer than 256 colors, then use either the TIFF or GIF format. The TIFF format is the printing industry standard for graphics, because it does not use a lossy compression scheme, which other formats such as JPEG do. It also supports multiple levels of transparency, which few other formats do.
If the picture has more than 256 colors, save it in the TIFF or PNG format. Microsoft recommends the PNG format if you need transparency; otherwise use the TIFF format.
You should still save your picture at printer resolution for the finished picture size. For example, assume that you have an 8.5-by-2-inch letterhead, and you want to print it 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.Note
If you are saving a picture for use in Microsoft Publisher 2000, and you want to separate areas of the picture into different spot colors, click the article number below to view the article in the Microsoft Knowledge Base:
How to assign and separate spot colors in EPS graphics in Publisher 2000
- Alpha Channel - An alpha channel describes an area of transparency in a picture, which allows a background to show through. An alpha channel allows over 64,000 levels of transparency, which makes semi-transparent and blended effects possible.
- Color Depth - 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 it 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 puts a priority on 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 puts a priority on producing a small picture file, even at the sake 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 dealing with picture files. It is the most common cause of problems when working with pictures in Microsoft Office. File size is determined by the following factors: picture size, resolution, file format, compression, and color depth.
- Gamma Correction - A method of correcting the lightness or darkness of pictures, so that they 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, after one sixty-fourth of it has been downloaded, you can see a general impression of what the picture looks like. As more of the image is downloaded, resolution improves until the entire picture is displayed.
- Metafile Picture - A metafile picture usually contains vector picture information but can contain any kind of picture information, such as a raster picture. In essence, a metafile is a container that can contain any kind of picture data.
- Palette - A palette is a list of the colors 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 any given 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 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. It is usually defined in pixels per inch. The higher the resolution, the more precision and clarity are in the picture. However, increasing the resolution also increases the file size of a picture.
- Transparency - Transparency is a method that allows areas of a picture to appear transparent, thus 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 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, and in some cases, ungrouped into smaller components.
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