Practical Advice on Net Image Creation
by Daniel G Helmick
With the explosive advance of multimedia computers into schools, offices and homes, the look of the information superhighway has changed as well. Internet users once accustomed to communicating through a march of words on their screen are now being met by a dazzling profusion of pictures, video and sound.
As you set out to create a Net home page, you may wish to add images of your own to enhance it. There are a number of considerations to keep in mind when preparing these graphics: the size of the image, its file size, the number of colors it contains, and the amount of detail which you need to retain.
A Bit About Graphics
Graphics are made up of rows of dots on the computer screen, each dot called a pixel. A standard VGA monitor will display 480 rows of 640 pixels each, or a total of 307,200 pixels for one full-screen image.
The most common graphics format found on personal computers is 8-bit color. In this display format, each pixel on the computer screen is represented by 1 byte (8 bits) of data, with the computer able to display up to 256 colors at once. A full 640x480 screen image would thus require a file more than 300 kilobytes in size.
Gaining swiftly in popularity is 24-bit color, available through the installation of special graphics cards. In this display format, the data for each pixel is stored in 3 bytes, providing a total onscreen palette of 16 million colors. This is a quantum jump in quality for the display of photographic images, offering fine gradations in color shading from one pixel to the next. It also means a considerable growth in file size, however, with a full screen image weighing in at nearly 1 megabyte.
A user on the Internet who wants a quick look at a Net page will generally not wish to sit around waiting for the time it takes to transfer a 1 megabyte graphic. Fortunately, a number of file formats use compression schemes which can often reduce files this size to a few dozen kilobytes.
Scanning and TIFF
Most scanners come packaged with programs which offer an immediate look at the image being scanned, as well as visual feedback on settings for contrast, brightness and sharpness. They will allow you to set the resolution of the image, as well as the amount of enlargement or reduction desired.
While most of these characteristics can later be tweaked by loading the file into an image processing program, it is best to scan a graphic at the pixel size you intend to use for your Net page. Increasing the size of a saved image will make it look blocky, while reducing it will blur details and destroy the integrity of lines and curves.
Simple scanning programs often fail miserably when converting scans to 256 colors, nor do they generally offer the optimal file format for storage of true-color images, JPEG. This is where an image processing program, such as Corel PhotoPaint or Aldus PhotoStyler, becomes invaluable for producing satisfactory graphics.
But first, you have to transfer the image from the scanning program to the image processing program.
One of the best ways to do this is to scan your image in 24-bit color mode, then save it as an uncompressed TIFF file. This will lead to large intermediate files, but once the images have been converted to other formats their file size will shrink dramatically. In the meantime, the TIFF format retains all the information needed for the image, whether it will ultimately be in 256 colors or 16 million colors.
Line Art and GIF
The most popular graphics format used on the Internet is GIF, originally created for the commercial online service CompuServe. This is an 8-bit format which offers some file compression, particularly when the image contains large areas of single colors.
GIF's primary limitation lies in its restriction to 256 colors. The types of images for which it is best suited are those with distinct edges and a limited number of colors, including:
- Line art and drawings
- Schematics
- Maps
- Text and Symbols
- Extremely small images (e.g., 20x20 pixels)
When called upon to deal with images which have more colors than its 8-bit palette contains, the GIF format will attempt to trick the eye into seeing these extra colors by a technique known as dithering. Thus, it may present a light green area of an image by alternating green pixels with yellow pixels. Unfortunately, this often has the dual effect of making the image appear blotchy, and undermining the file compression by breaking up large areas of color.
Photos and JPEG
A graphics format which is fast becoming more common on the Internet is JPEG (*.JPG). This file type offers 24-bit color and a dramatically high degree of compression, making it preferable for use in such images as:
- Photographs
- Paintings and subtly shaded artwork
- Photorealistic computer graphics
The drawback in using JPEG is that it is based on a lossy compression scheme, meaning the image compression is optimized for smooth color gradations from one pixel to the next. Thus, graphics containing sharp edges (such as line art) will be somewhat blurred by conversion to JPEG format, and will also be much larger than GIF files of the same image.
Programs which save in JPEG format also allow the user to set the amount of compression desired, in terms of the "Quality" of the image. A quality setting of 95 percent is generally indistinguishable from the original image, while a quality setting of 50 percent leads to a great loss of both detail and color from the image. Since this compression is fairly logarithmic in nature, the greatest reduction in file size will be found in the range of 85 to 95 percent.
GIF to JPEG
What if you have already saved a photograph in GIF format?
This is problematical. The image having been reduced to 8-bit color and dithered, there is no improvement in color quality to be gained by converting the image to JPEG format. However, some reduction in file size may still be possible in this conversion.
Converting the file directly from GIF to JPEG retains the color palette and dithering found in the original image. This will bring the file down to approximately half of its original size.
Somewhat better quality and compression may be obtained by first "smoothing" the GIF image to decrease the dithering it contains. This again requires the use of an image processing program.
First, load the GIF file into the program. If your program does not automatically operate in a 24-bit color mode, the next step will be to convert your image to 24-bit color in order to increase the efficiency of the image processing functions.
Next, use the "Smooth" or "Blur" image filters to smooth out the dithering in the image. These functions may permit you to set the amount of blurring achieved; experiment until you produce a result which looks good to you.
Finally, save the image as a JPEG file. Although the file compression and amount of detail obtained here will not approach that available from a 24-bit color scan, it will still be improved by some 20 percent over a direct GIF-to-JPEG conversion.
