Imagine a page of text describing a product offered for sale. Now imagine that same page with images of the product added. Even in your imagination there’s a difference – the image adds interest to the page and improves its appearance.
That’s the power of images, whether they are photographs, clip art, illustrations, charts, graphs or symbols. To attract attention and improve reader comprehension, nothing beats an image.
An image has maximum effectiveness when it satisfies these four criteria: the image is worthy of being printed; it is of good quality; it is relevant to the text; and it is consistent with the design and layout of the document. In this post we will focus on what makes a good quality digital image for print.
What is a digital image?
A digital image is an image stored as one of two types: vector or raster. Vector images are lines created from mathematical calculations while raster images (also called bitmap) are created from numerical values – ones and zeros – organized as a fixed number of rows and columns of picture elements or pixels.
Vector images are created by illustration or drawing programs such as Adobe Illustrator or Corel Draw. Raster images are created by digital cameras or scanners and are edited by image editors or paint programs such as Adobe Photoshop or Corel Paint Shop Pro.
Color in images
The simplest images, called binary images, contain only two colors. Each pixel is stored as a single bit (either 0 or 1). These images are sometimes referred to as black and white ormonochrome.
In a grayscale image, each pixel is a shade of gray that varies from black to white. Sometimes called a monochromaticimage, grayscale requires 8 bits of storage for each pixel where each bit represents 256 possible levels of gray.
For a full color image, each pixel has 24 bits of storage and can display 16 million colors, shades and hues. This explains why files containing color images are so large.
The two most common color models are RGB (red, green, blue) and CMYK (cyan, magenta, yellow and black). RGB color is the color model for computer monitors and the web, while CMYK is the color model for printing. If you are working with your images in the RGB model, you must convert them to CMYK before
placing them in your document.
Image resolution
Image resolution is the amount of detail an image holds, expressed as the number of pixels in the image. Resolution for digital cameras is often expressed as the number of pixel columns (width) by the number of pixel rows (height), such as 640 x 480. For printing, resolution is expressed as pixels per inch (ppi),
meaning the number of pixels in a linear inch. The more pixels per inch, the higher the resolution.
The most important thing to understand about resolution is the relationship between an image’s resolution (ppi) and an image’s print size (actual width and height). For a photograph to reproduce well in print, it must have a minimum resolution of 300 ppi in its print size. Such a file is termed high resolution (hi-res). In some cases we may be able to use a 200 ppi image, but we almost never can use a low resolution (lo-res) file (resolution is below 200 dpi) because there are not enough pixels to adequately represent the image.
Even if a file’s resolution is high enough, it still may not reproduce well if the image resolution doesn’t match the print size. If you ask us to enlarge the image to print in a bigger size than it was originally, the pixels that make up the image will move farther apart. This changes the number of pixels per inch, reducing the resolution. If the enlargement is significant, individual pixels may become visible, creating jagged edges in the image. This effect is called pixelation.
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