Working with JPEG Images

JPEG images can display many more simultaneous colors than GIF images can. JPEG images can select colors from a palette of 16.7 million colors, but the total number of colors that can be simultaneously displayed depends on the capabilities of a user's graphics hardware and monitor.

As was stressed previously, the JPEG image format is best used for saving photographic images or other images that contain continuous color tones or blends. The exception is when an image contains text that needs to be sharply rendered, even if continuous color tones or blends are present; in that case, you should first try to save your image as a GIF image with an optimized color palette.

Optimizing JPEG Images

You optimize a JPEG image by increasing the amount of compression that is applied to the image as much as you can without overly compromising image quality or fidelity. To change the amount of compression applied to a JPEG image in Paint Shop Pro 8, click the Options button when saving the image and select a compression factor between 1 and 99, with 1 being the least amount and 99 being the most amount of compression. The optimum amount of compression that can be applied to a JPEG image can vary greatly from image to image, with some images not able to take a compression factor greater than 10, whereas other images may be able to take compression factors of 20, 30, 40, or greater. Many recent image editors include wizards or utilities that help automate the process of finding the optimum compression factor for a JPEG image.

When saving or optimizing a JPEG image, you can also specify which kind of chroma subsampling is done when saving the image. Chroma subsampling works by averaging the color information at a lower resolution than the luminance (or intensity) information for an image. Paint Shop Pro 8, for instance, defaults to averaging the color information for every 2-Ч-2 square of pixels within the image. You can turn off subsampling by specifying a 1-Ч-1 square of pixels, or you can increase it by specifying a 4-Ч-2 or 4-Ч-4 square of pixels, for instance.

Tip

For lower-resolution images, such as is commonly the case with images used for display on the Web, you will generally get better results (better image quality along with a smaller file size) by turning off chroma subsampling and significantly increasing the compression factor when saving JPEG images. Both Paint Shop Pro 8 and PhotoImpact 8 allow you to do this; Adobe Photoshop 7, however, does not.