digital images & files

photo © by Tom Gore

Digital images are now generally used throughout science and science publishing. There are two common types of digital images, vector images and raster images. Vector images are used for line art drawings that may also include fill tones and textures. These images are composed of mathematical descriptions of shapes and curves. They are made in programs such as Corel Draw or Adobe Illustrator. Raster or bitmapped images are produced by digital cameras and scanners driven by computers or are produced in software such as Adobe Photoshop or Corel Photopaint. These images are made of rows of pixels, each with a grey scale value. Colour images are really made of three or four black and white images that are called channels in most software.

Digital cameras with sufficient control and resolution for professional work are expensive ($3,000 and up) though many less expensive ones now have enough resolution for prints up to perhaps 8" by 10". Film scanners allow digital digital file creation from previously developed film (colour or black and white) and flat bed scanners allow digitization of prints and drawings.

There are a wide variety of file formats used for digital images including RAW, Tagged Information File Format (TIFF or TIF) and Joint Experts Photographic Group (JPEG or JPG). however generally only TIFF files are used for publication while JPEG files are used on the web. JPEG may soon be replaced by the newer JPEG 2000 standard which is based on wavelet compression technology. Another newer format called Portable Network Graphics (PNG) is gaining acceptance and may become standard in the future. An older format called the Graphics Interchange Format (GIF) is not suitable for most photographs but works well for line art and is often used to make small, fast loading web graphics.

Raster images are made up of pixels. These are little squares of greyscale or colour information. The resolution of an image is measured by the number of these pixels in each lineal inch (ppi or dpi). File size increases as the square of the increase in resolution, since the number of pixels increases both left to right and top to bottom. As a result there is a rapid increase in file size as resolution increases. In addition, a colour file is saved as three black and white channels representing red, green and blue (RGB) information or four channels representing cyan, yellow, magenta and black (CYMK).

an image fragment at 72 dpi. and 10 dpi.

To give you an idea of the resulting file sizes, a 6 by 6 inch photo saved as a TIF file will be:

Some software, such as Adobe Photoshop, allows TIF files to be saved using a compression system known as LZW compression. This only saves space if the image has large areas of flat tones of similar colour, such as a white wall or blue sky. Unfortunately the process of compression and decompression increase both save and load times. For example a 300 ppi colour file will save as

Note that the more complex image file is hardly smaller when compressed but takes a lot longer to either load or save. The TIF format usually gives better image quality than the JPG format which uses a lossy compression algorithm. Lossy compression discards information when the picture is saved. Saving a JPG file allows a choice of different levels of compression, great compression results in smaller but lower resolution files while lesser compression results in larger files with higher resolution. The following are file sizes for the same picture as before, this time saved as a 300 dpi JPG.

If you are doing analysis of a picture using software such as ImageJ or ImagePro you should save in the TIFF format. If you are repeatedly reworking a picture you should save in the TIFF format up till the last save since each opening and saving of a JPEG file will increase visible compression artifacts. Photographs are converted to halftones (which are specified in dots to the inch or dpi) for use on the printing press. While newspapers generally print photographs at 85 dpi, journals print at 133 to 150 dpi. For the best quality, digital images should be saved at 1.5 to 2 times the target publication's resolution. 300 tp 450 dpi is generally a good resolution to choose, giving reasonable quality without excessive file size. Some journals want line artwork to be saved at 1200 dpr. Do check the notes to authors of the particular journal to which you will be submitting first, noting the file format and resolution required.