Raster, or bitmap, images consist of tiny squares called pixels. These pixels contain bits of color and, when combined, build images. Smaller pixels in an image result in more detail and higher quality, whereas larger pixels result in less detail and lower quality. When you zoom in on an image, the pixels become more apparent while the details of the image become blurry.
Raster images are used in photography and digital applications. When you take a picture with a phone or a camera, the image is recorded as pixel data. And, when these images are uploaded online, the end result is a raster image.
Editing raster- bitmapped images
Bitmapped images may be edited in applications such as Adobe Photoshop. Scaling bitmapped images after they are imported into a page layout scales the pixels. So if you enlarge the image, you will see the individual pixels; if you reduce the image, you will sacrifice detail in the picture. Therefore, if you cannot initially capture the image at the correct size, it is best to scale it in an image-editing application such as Photoshop which has sophisticated ways to scale it and maintain quality.
If you plan to import your grayscale and color images into your page layout, be certain that the background of the picture box is filled with white, 0% black,or any color other than “None.” This will help to avoid jagged edges around the picture.
Importing graphic images
Avoid using large images that will later be cropped in the page layout software so much that it uses only a small portion of the whole image. Even though only part of the image will be visible after cropping, the rest that is hidden will still be processed by the RIP. This bad practice results in large file sizes and increases processing times. If it is more convenient to group images into one scan, use the cropping tool in your image editing program to convert the scan into separate images.
Preparing color images
You must take great care when preparing color images. If your scanner captures images in the RGB color space, it is essential that you convert the scans to CMYK before importing them into the page layout so that they will separate properly. Remember that images obtained from the web , digital cameras, or other methods may originally be in other color spaces such as Lab or RGB, and must be converted to CMYK. Note that screen captures may start as indexed color and must also be converted to CMYK.
If you plan to convert grayscale images into duotones, tritones, or quadtones, be sure that the spot colors are defined properly, and that your file type supports spot colors.
File Formats for Raster Images
Raster files are saved in various formats:
.tiff (Tagged Image File Format)
.psd (Adobe Photoshop Document)
.pdf (Portable Document Format)
.jpg (Joint Photographics Expert Group)
.png (Portable Network Graphic)
.gif (Graphics Interchange Format)
.bmp (Bitmap Image File)
Never use GIF, PNG or Bitmaps for print projects. Usually they are very low resolution.