DIGITAL IMAGE FORMATS:AN EXPLANATIONGRADE 8 - ICT

GRAPHICSGraphics - anything on the page that is not actual text, from simple line drawings to fully active images found on the World Wide Web.There are many different graphic file formats.The format you choose depends on several factors.

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FACTORS THAT AFFECT GRAPHIC FORMAT• Color depth• Compression• Portability• Transparency

COLOR DEPTHThe number of colors per pixel that can be contained in an image.Most graphics editing programs will allow you to set the color depth for your image.Different graphic formats contain different numbers of colors per pixel.Examples:1-bit Color 2 Colors Per Pixel

8-bit Color 256 Colors Per Pixel16-bit Color

65.5 Thousand Colors Per Pixel

24-bit Color

16.7 Million Colors Per Pixel

COMPRESSIONCompression refers to how an image is saved in

order to reduce the file size.The greater the compression, the lower the

qualityTwo types:Lossless

Reduces the file size without losing any pixel dataQuality is not compromised

Lossy Alters and/or eliminates some dataThe more the image is reduced, the more the quality

is degraded

After Compression

Notice that there is no difference, no data has been lost.

LOSSLESS COMPRESSION

Before Compression

Original . . . . . . Final compression

LOSSY COMPRESSIONNotice that each time the image is saved, the quality is degraded.In the third image, the image is very pixelated.

PORTABILITY

How easily you can open, modify, and view the files on computers using different operating systems, software and browsers.

TRANSPARENCYAllows the background color of an image to be “eliminated” or made transparent so that the background behind the image can be seen.It makes part of the image invisible, or “see-through.”

FACTORS TO CONSIDER WHEN CHOOSING A GRAPHIC FILE FORMATWhich file format is best for a particular application?

How important is the quality of the image when transferred, copied, or displayed?

How important is the speed at which the image transfers or loads?

How important is file size?

NUMBER OF COLORSImages have differing numbers of color within them.Black and white images take up the least amount of space with 1 bit of information.Current video cards are able to display 24 bits of color per pixel.This makes it possible for 16 million colors to be displayed (224 colors).This requires a significant amount of disk space.

THE FILE TYPES – TIFF (TAG IMAGE FILE FORMAT)TIFF can be lossless or lossy. The details of the image storage algorithm are included as part of the file. TIFF is used almost exclusively as a lossless image storage format that uses no compression at all. Most graphics programs that use TIFF do not use compression.File sizes are quite big.

THE FILE TYPES – PNG (PORTABLE NETWORK GRAPHICS

PNG is a lossless storage format. It looks for patterns in the image that it can use to compress file size. The compression is exactly reversible, so the image is recovered exactly.

THE FILE TYPES – GIF (GRAPHICS INTERCHANGE FORMAT)

GIF creates a table of up to 256 colors from a pool of 16 million. If the image has fewer than 256 colors, GIF can render the image exactly. GIF is "lossless" only for images with 256 colors or less. For a rich, true color image, GIF may "lose" 99.998% of the colors.

THE FILE TYPES – JPEG (JOINT PHOTOGRAPHIC EXPERTS GROUP)

JPG is optimized for photographs and similar continuous tone images that contain many, many colors. It can achieve astounding compression ratios even while maintaining very high image quality. It stores information as 24 bit color. The degree of compression of a JPG is adjustable.

THE FILE TYPES – BMP (BITMAP)BMP is an uncompressed proprietary format invented by Microsoft. There is really no reason to ever use this format.

THE FILE TYPES – PSD (PHOTOSHOP DOCUMENT)PSD is a proprietary format used by Photoshop. This is the preferred working format to edit images in the softwareThis package uses layers to build complex images, and layer information may be lost in the nonproprietary formats such as TIFF and JPG. However, it is best to save the product as a TIFF or JPG, so it can be viewed in the future when software changes.

THE FILE TYPES - SUMMARYTIFF files are generally large.PNG compression can be reversed if needed.GIF results in lose of color information.JPG are the best in terms of compressed files.BMP should be rarely, if ever used.PSD is exclusive to Photoshop.

TIFF – This is usually the best quality output from a digital camera. JPG – This is the format of choice for nearly all photographs on the web. GIF - For images with fewer than 256 colors and large areas of uniform color like cartoons.Do NOT use GIF for photographic images PNG - For images with large areas of exactly uniform color, but more than 256 colors Also, for display of photographs exactly without loss of quality on the web.

WHEN SHOULD YOU CHOOSE EACH?