Monday, October 25, 2010

Picture This: Using Images in Documents





Photographs with action, emotion, real human drama grab attention.


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.
   
  it is consistent with the design and layout of the document.
In this article, 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 or monochrome. In a grayscale image, each pixel is a shade of gray that varies from black to white. Sometimes called a monochromatic image, 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 dots per inch (dpi), 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 (dpi) 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 dpi in its print size. Such a file is termed high resolution (hi-res). In some cases we may be able to use a 200 dpi 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.

File size
The size of a file is determined by whether it is a vector or raster file and whether it is binary, grayscale or color. File compression is a way of reducing file size without compromising image quality.

There are two compression methods: one (called lossless) keeps all the pixels of the original image but finds more efficient ways to represent recurring patterns of pixels in the file; and one (called lossey) eliminates pixels that aren’t needed to maintain quality.

File formats for printed images
The best file formats to use for images that will be printed are:
      For vector images: .eps (Encapsulated PostScript)
      For raster images: .tif (Tagged Image File Format)

Recall that vector images are constructed from mathematical formulas. This means that they are resolution-independent and can be scaled (resized) and manipulated (flipped, rotated, stretched, cropped, colorized, combined) with ease using programs like Adobe Illustrator and Corel Draw. This is an ideal format for initial design of logos and illustrations, and for clip art.  

Raster images are composed of rows and columns of pixels (sometimes called a bitmap). Because raster images are hard to resize and manipulate, they are best used at the size and orientation of the original. Raster images can be cropped, colorized, converted to grayscale or monochrome using image editing programs such as Adobe Photoshop or Corel Paint Shop Pro. All photographs are raster images.

The .tif file format usually produces the best quality image from a digital camera. The other choice is .jpg (Joint Photographic Experts Group), a file format that has been optimized for continuous-tone full color photographs by incorporating file compression. 

JPG compression looks at blocks of 8x8 pixels and selectively reduces the detail in each block. This maintains the physical size of the image, reduces the amount of space required to store it, but sacrifices the quality of the image. The extent of image degradation depends on the degree of compression (it is adjustable).

If you are editing or manipulating digital photographs, work in .tif format rather than .jpg since each new save in .jpg compresses the file. By contrast, a .tif file uses lossless compression, so there is no loss of pixels. 

The file formats .psd (Photoshop), .psp (Paint Shop Pro), .ai (Adobe Illustrator) and .cdw (Corel Draw) are proprietary. They are good to use while editing images in order to have access to all the editing tools in the program. However, after the image editing is complete, save raster images as a .tif file and vector images as an .eps file. 

Use images successfully
For maximum effectiveness, any image you use in a document must reproduce well. This requires attention to the file format and adhering to production standards. When you are deciding among possible images to include in your document, call us for assistance. We will be happy to help. 


No comments:

Post a Comment