Digital image file types explained

Digital Image File Types Explained
JPG, GIF, TIFF, PNG, BMP. What are they, and how do you choose? These and manyother file types are used to encode digital images. The choices are simpler than youmight think.
Part of the reason for the plethora of file types is the need for compression. Imagefiles can be quite large, and larger file types mean more disk usage and slowerdownloads. Compression is a term used to describe ways of cutting the size of thefile. Compression schemes can by lossy or lossless.
Another reason for the many file types is that images differ in the number of coloursthey contain. If an image has few colours, a file type can be designed to exploit thisas a way of reducing file size.
Lossy vs. Lossless compression
You will often hear the terms "lossy" and "lossless" compression. A losslesscompression algorithm discards no information. It looks for more efficient ways torepresent an image, while making no compromises in accuracy. In contrast, lossyalgorithms accept some degradation in the image in order to achieve smaller file size.
A lossless algorithm might, for example, look for a recurring pattern in the file, andreplace each occurrence with a short abbreviation, thereby cutting the file size. Incontrast, a lossy algorithm might store colour information at a lower resolution thanthe image itself, since the eye is not so sensitive to changes in colour of a smalldistance.
Number of colours
Images start with differing numbers of colours in them. The simplest images maycontain only two colours, such as black and white, and will need only 1 bit torepresent each pixel. Many early PC video cards would support only 16 fixed colours.
Later cards would display 256 simultaneously, any of which could be chosen from apool of 224, or 16 million colours. New cards devote 24 bits to each pixel, and aretherefore capable of displaying 224, or 16 million colours without restriction. A fewdisplay even more. Since the eye has trouble distinguishing between similar colours,24 bit or 16 million colours is often called TrueColor.
The file types
TIFF (Tagged Image File Format) is, in principle, a very flexible format that can be
lossless or lossy. The details of the image storage algorithm are included as part of
the file. In practice, TIFF is used almost exclusively as a lossless image storageformat that uses no compression at all. Most graphics programs that use TIFF do notcompression. Consequently, file sizes are quite big. (Sometimes a losslesscompression algorithm called LZW is used, but it is not universally supported.) PNG (Portable Network Graphics) is also a lossless storage format. However, in
contrast with common TIFF usage, 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
GIF (Graphics Interchange Format) creates a table of up to 256 colours from a pool of
16 million. If the image has fewer than 256 colours, GIF can render the image
exactly. When the image contains many colours, software that creates the GIF uses
any of several algorithms to approximate the colours in the image with the limited
palette of 256 colours available. Better algorithms search the image to find an
optimum set of 256 colours. Sometimes GIF uses the nearest colour to represent
each pixel, and sometimes it uses "error diffusion" to adjust the colour of nearby
pixels to correct for the error in each pixel.
GIF achieves compression in two ways. First, it reduces the number of colours of
colour-rich images, thereby reducing the number of bits needed per pixel, as just
described. Second, it replaces commonly occurring patterns (especially large areas of
uniform colour) with a short abbreviation: instead of storing "white, white, white,
white, white," it stores "5 white."
Thus, GIF is "lossless" only for images with 256 colours or less. For a rich, true colourimage, GIF may "lose" 99.998% of the colours.
JPG (or JPEG, Joint Photographic Experts Group) is optimized for photographs and
similar continuous tone images that contain many, many colours. It can achieve
astounding compression ratios even while maintaining very high image quality. GIF
compression is unkind to such images. JPG works by analyzing images and discarding
kinds of information that the eye is least likely to notice. It stores information as 24
bit colour. Important: the degree of compression of JPG is adjustable. At moderate
compression levels of photographic images, it is very difficult for the eye to discern
any difference from the original, even at extreme magnification. Compression factors
of more than 20 are often quite acceptable. Better graphics programs, such as Paint
Shop Pro and Photoshop, allow you to view the image quality and file size as a
function of compression level, so that you can conveniently choose the balance
between quality and file size.
RAW is an image output option available on some digital cameras. Though lossless, it
is a factor of three of four smaller than TIFF files of the same image. The
disadvantage is that there is a different RAW format for each manufacturer, and so
you may have to use the manufacturer's software to view the images. (Some
graphics applications can read some manufacturer's RAW formats.)
BMP (BitMaP) is an uncompressed proprietary format invented by Microsoft. There is
really no reason to ever use this format.
PSD (PhotoShop Document) and PSP (PaintShop Pro) are proprietary formats used
by graphics programs. Photoshop's files have the PSD extension, while Paint Shop Pro
files use PSP. These are the preferred working formats as you edit images in the
software, because only the proprietary formats retain all the editing power of the
programs. These packages use layers, for example, to build complex images, and
layer information may be lost in the nonproprietary formats such as TIFF and JPG.
However, be sure to save your end result as a standard TIFF or JPG, or you may not
be able to view it in a few years when your software has changed.
Currently, GIF and JPG are the formats used for nearly all web images. PNG issupported by most of the latest generation browsers. TIFF is not widely supported byweb browsers, and should be avoided for web use. PNG does everything GIF does,and better, so expect to see PNG replace GIF in the future. PNG will not replace JPG,since JPG is capable of much greater compression of photographic images, even whenset for quite minimal loss of quality.
When should you use each?
TIFF - This is usually the best quality output from a digital camera. Digital cameras
often offer around three JPG quality settings plus TIFF. Since JPG always means at
least some loss of quality, TIFF means better quality. However, the file size is huge
compared to even the best JPG setting, and the advantages may not be noticeable.
A more important use of TIFF is as the working storage format as you edit andmanipulate digital images. You do not want to go through several load, edit, savecycles with JPG storage, as the degradation accumulates with each new save. One ortwo JPG saves at high quality may not be noticeable, but the tenth certainly will be.
TIFF is lossless, so there is no degradation associated with saving a TIFF file.
Do NOT use TIFF for web images. They produce big files, and more importantly, mostweb browsers will not display TIFFs.
JPG - This is the format of choice for nearly all photographs on the web. You can
achieve excellent quality even at rather high compression settings. I also use JPG as
the ultimate format for all my digital photographs. If I edit a photo, I will use my
software's proprietary format until finished, and then save the result as a JPG.
Digital cameras save in a JPG format by default. Switching to TIFF or RAW improvesquality in principle, but the difference is difficult to see. Shooting in TIFF has twodisadvantages compared to JPG: fewer photos per memory card, and a longer waitbetween photographs as the image transfers to the card. I rarely shoot in TIFF mode.
Never use JPG for line art. On images such as these with areas of uniform colour withsharp edges, JPG does a poor job. These are tasks for which GIF and PNG are wellsuited. See JPG vs. GIF for web images.
GIF - If your image has fewer than 256 colours and contains large areas of uniform
colour, GIF is your choice. The files will be small yet perfect. Here is an example of
an image well-suited for GIF:
Do NOT use GIF for photographic images, since it can contain only 256 colours perimage.
PNG - PNG is of principal value in two applications:
1. If you have an image with large areas of exactly uniform colour, but contains morethan 256 colours, PNG is your choice. Its strategy is similar to that of GIF, but itsupports 16 million colours, not just 256.
2. If you want to display a photograph exactly without loss on the web, PNG is yourchoice. Later generation web browsers support PNG, and PNG is the only losslessformat that web browsers support.
PNG is superior to GIF. It produces smaller files and allows more colours. PNG alsosupports partial transparency . Partial transparency can be used for many usefulpurposes, such as fades and antialiasing of text. Unfortunately, Microsoft's InternetExplorer does not properly support PNG transparency, so for now web authors mustavoid using transparency in PNG images.
Other formats
When using graphics software such as Photoshop or Paint Shop Pro, working filesshould be in the proprietary format of the software. Save final results in TIFF, PNG,or JPG.
Use RAW only for in-camera storage, and copy or convert to TIFF, PNG, or JPG assoon as you transfer to your PC. You do not want your image archives to be in aproprietary format. Although several graphics programs can now read the RAWformat for many digital cameras, it is unwise to rely on any proprietary format forlong term storage. Will you be able to read a RAW file in five years? In twenty? JPG isthe format most likely to be readable in 50 years.Thus, it is appropriate to use RAW tostore images in the camera and perhaps for temporary lossless storage on your PC,but be sure to create a TIFF, or better still a PNG or JPG, for archival storage., Rick Matthews, Department of Physics, Wake Forest University


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