The Basics...There are many different file types used to encode digital images. JPG, GIF, TIFF, PNG, BMP, PDF and PSD are just a few. But what are they, what do they stand for and what are the differences? Part of the reason for the variety of file types is the need for compression. Image files can be quite large, and the larger the file the more disk space it uses and the slower it is to download. Compression is a term referring to the ways of reducing the size of a file. Another reason for the variety of file types is that images differ in the number of colours they contain. Therefore if an image is made up of only a few colours, a file type can be designed to use this as a way to reduce the file size.
Compression schemes can be either lossy or lossless.
A lossless compression does not discard any information. It attempts to represent an image in a more efficient way, while making no compromises in accuracy. In contrast, lossy compression allow some reduction in the image quality in order to achieve a smaller file size. Here I discuss the basic features of a 7 common file types.
PDF is a formal open standard known as ISO 32000. Maintained by the International Organization for Standardization, ISO 32000 will continue to be developed with the objective of protecting the integrity and longevity of PDF, providing an open standard for the more than one billion PDF files in existence today.
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JPGJPG (Joint Photographic Experts Group) is optimized for photographs and similar continuous tone images that contain many, many colours. For this reason all digital cameras save in a JPG format by default. It can achieve excellent compression ratios while still maintaining very high image quality. 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. JPG is the most popular format for nearly all photographs uploaded to the internet.
Never use JPG for line art. In these type of images where there are areas of uniform colour with sharp edges, JPG does not perform well. For these type of images GIF and PNG are better suited.
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PSDPSD (PhotoShop Document) is the format used by the graphics program Photoshop. This is the preferred working format as you edit images in the software, because only this format retains all the editing power of the program. Photoshop uses layers, filters and effects to build complex images, and layer information may be lost in other formats such as GIF and JPG. PSD files need to be much larger than other file formats in order to store all this information. For this same reason, however, be sure to save your end result as a standard GIF or JPG, or you may not be able to view it when your software has changed but moreover you cannot use PSD files for web images or in animation programs.
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TIFFTIFF (Tagged Image File Format) is a very flexible format that can be lossless or lossy. Although normally 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 compress and as a result file sizes can be quite big.
Some digital cameras often offer TIFF as am output option as well as the default JPG. TIFF is usually the best quality output from a digital camera as JPG always results in at least some loss of quality. However, the file size of TIFF is huge. A more important use of TIFF is as a working storage format as you edit and manipulate digital images in a photo editing programme such as Photoshop. TIFF can save the different image layers that you have been editing while JPG can only save the file as a single flat image. Also when you go through several load, edit, save cycles with JPG storage, the image quality degrades each time. TIFF is lossless, so there is no degradation associated with saving a TIFF file.
On the downside TIFF files should not be used for web images as the file size is so large that they take a long time to load and some browsers will not even render them
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PNGPNG (Portable Network Graphics) is a lossless storage format. In contrast with common TIFF usage, however, it looks for patterns in the image that can be used to compress the file size without the loss of any quality. The compression is exactly reversible, so the image is recovered exactly.
PNG is becoming increasingly popular as it is superior to GIF and JPG depending on the situation. PNG produces smaller files and allows more colors than GIF and PNG also supports partial transparency. Partial transparency can be used for many useful purposes, such as fades and antialiasing of text.
In terms of superiority to JPG if you want to display a photograph exactly without loss on the web, PNG is your choice. All modern web browsers support PNG, and PNG is the only lossless format that all web browsers support.
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GIFGIF (Graphics Interchange Format) images create a table of up to 256 colours from a selection of 16 million. If the original image has fewer than 256 colours, GIF can render the image exactly but when the image contains more than 256 colours, software that creates the GIF uses any of several compression algorithms to approximate the colours in the image with the limited palette of 256 colours available. For this reason do not use GIF for photographic images, since the restriction of 256 colours per image is much too low for a decent quality photograph.
GIF achieves compression in two ways. Firstly, it reduces the number of colours in colour-rich images, therefore reducing the number of bits needed per pixel. Secondly, it replaces reoccurring patterns 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 colour image, with more than 256 colours GIF could lose up to 99% of the colours.
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BMPBMP (Bitmap), also known as Device Independent Bitmap (DIB) file format or simply a bitmap, is a raster graphics image file format used to store bitmap digital images, independently of the display device such as a graphics adapter, especially on Microsoft Windows and OS/2 operating systems. The BMP file format is capable of storing 2D digital images of any width, height, and resolution, both monochrome and colour, in various colour depths, and optionally with data compression, alpha channels, and colour profiles.
The simplicity of the BMP file format, and its widespread familiarity in Windows and elsewhere, makes it a very common format that image processing programs from many operating systems can read and write. Many older graphical user interfaces used bitmaps in their built-in graphics sub-systems.
While most BMP files have a relatively large file size due to lack of any compression, many BMP files can be considerably compressed with lossless data compression algorithms such as ZIP, where in extreme cases of non-photographic data they can be reduced to just 0.1% of original size, because they contain redundant data. Some formats, such as RAR, even include routines specifically targeted at efficient compression of such data.
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NOTE:This list only documents the most common file formats and is not an extensive file format list. I do however intend to add to it in the near future