Graphics File Types

see also:    http://dx.sheridan.com/advisor/pcx_file.html 

Bit-Mapped (Raster) Images

BMP (Bit-Mapped )  -  Microsoft created the bmp standard.  The compression used is called "RLE" (Run Length Encoding) and has a Color Depth of 1- to 24-bits. These are usually fairly large files. BMP files are not supported by Web browsers !!

DIB (device-independent bitmap) - same as BMP  -  the bit-mapped graphics format used by Windows. Graphics stored in
DIB format generally end with a .bmp extension, but can hace a dib extension. It's called device-independent because colors are represented in a format independent of the final output device. When a DIB image is output (to a monitor or printer), the device driver translates the DIB colors into actual colors that the output device can display.  DIB files are not supported by Web browsers !!

GIF (Graphics Interface Format) - a lossless but compressed graphics file format used chiefly for websites, since a GIF file offers animation and transparency.  Unfortunately, GIF's have a maximum color table of 256 colors - but this also allows highly compressed files. The compression scheme works like this - if the algorithm comes across several parts of the image that are the same, say a sequence of 15 digits like this, 12345, 12345, 12345, it makes the number 1 stand for the sequence 12345 so that you signify the same 15-digit sequence with just 9 digits (1=12345 is stored in the hash table and 111 is stored in the image sequence), obviously saving a lot of space. It stores the key to this (1 = 1 2 3 4 5) in a hash table, which is attached to the image so that the decoding program can unscramble it.

JPEG (Joint Photographic Experts Group - files end with ".jpg") - pronounced jay-peg. JPEG is a lossy compression technique for color images.  By "lossy", this means that a certain amount of detail can be left out purposefully, or "lost", so that the size of the file can be reduced.  This allows jpg files to compress the image and saves space.  The image can be saved at half the size of a BMP file, and still look sharp and clear since the only detail that is lost is in the very fine tiny areas.  JPG files are typically larger in file size than GIF's but they can represent millions of colors, and are the choice for photographic pictures.  Image editors such as Adobe Photoshop allow you to control the "quality" which also controls the file size and the loss.  The lower the quality, the smaller the file size, and the more loss in clarity you will have (i.e. the picture look worse).

TGA (TarGA) - Truevision (Targa) - rarely used anymore - TGA files are bitmap file format for storage of 24- and 32-bit truecolor images. TGA supports colomaps, alpha channel, gamma value, postage stamp image, textual information, and developer-definable data.  These are similar to bmp files, tga file are quite rare, and cannot be opened by the majority of applications.  TGA files should be converted to bmp or jpg for portability.  

TIFF (tagged image file format - files end with ".tif") -  a very powerful image format - it is the most widely used file format in studio desktop publishing today. It supports all forms of color including RGB, Indexed, CMYK, and Lab. TIFF files can be compressed by using an LZW lossless compression approach or JPEG lossy compression. For high-end print production, it is the best practice to use either LZW compression or a very small amount of JPEG compression. The JPEG compression approach is a lossy compression that will degrade image quality when used in large amounts.  These are very complex files !!  The old, original compression is called LZW compression.  Newer TIF files support zip compression.  TIF files come in 2 flavors – IBM and Mac.  They also allow “layer” compression in either RLE or Zip  In addition, TIF files can be saved using multiple resolutions – called an “image pyramid.  The entire image, layers and all, is saved multiple times,  For example, once at 72 pixels per inch, once at 144 pixels per inch, and once at 300 pixels per inch.  Again, few image viewers support image pyramid TIF’s.

NOTE:  beware of creating complex TIF's – few image readers can handle zipped TIF, or layered TIF, or Image Pyramid TIF files !!!

TIFFs (also called .TIF) are very large files used for saving processed images.

TIFFs are used only in tethered studio applications, if ever, for camera capture. Tiffs don't have any of the post-processing advantages of RAW and have enormous file sizes that will completely clog up any workflow if you are shooting many images on cards.

TIFF (or PSD, the format native to Photoshop) is perfectly fine for archiving files after you've played around with them or for sending to a client on CD. Just don't set your camera to this format for recording on cards since it's so cumbersome.

 

PSD (PhotoShop Document) - Adobe Photoshop uses the TIFF format - so a PSD is basically the same as a TIFF !!

PCX  -  a compressed image file format, very popular with older games such as Doom and Heretic because players could create their own PCX images (the file format is very simple), but rarely used anymore (Thumbs Plus uses PCX).  Zsoft Corporation originally developed the PCX file format for their commercial paint program, PC Paintbrush, used by Windows.  Using a PCX file for use in documents in a print production workflow is unacceptable because of the lack of support for the CMYK color space. Additionally, using a PCX file in a Web-based application will not happen unless the image file is converted to a GIF or JPEG image.  PCX files are not supported by Web browsers !!

PNG (Portable Network Graphic) - developed in 1994 after Unisys warned they would charge royalties for GIF's - the PNG format is royalty-free.  Unfortunately, its technologies haven't been supported by many browsers until recently, so PNG has been a relatively un-used format.  That is all changing as developers are beginning to see the advantages PNG formats offer.  To test your browser for png support - go to:  http://www.fnordware.com/superpng/samples.html  Netscape 4.x will show the images but does not show the transparencies.

PNG Editors - atandard image editors (Photoshop, MS Paint, etc.) do not handle png files.  Macromedia's "Fireworks MX" does, as does PhotoLine 32.  A cheap JPEG-to-PNG converter called "2-PNG" is also available at http://batch.fcodersoft.com/2png/index.htm although it is DOS-based and can only do very limited edits (cropping, rotation, etc).

One area where PNG is superior to GIF is color. Since PNG is true color format, it supports color up to 64-bit color (although no images require more than 24 or 32-bit).  This allows for much more color combinations, making it able to support photo quality images.  Additionally, PNG uses a lossless method of compression with filters, which allows files to be up to 30% smaller than GIF files.  Another area where PNG is superior to GIF is with interlacing.  PNG uses a two dimensional method of interlacing instead of one, so the general image appears faster and more clearly than a GIF image, as the graphic below demonstrates.  PNG load images both horizontally and vertically, instead of just vertically.  

Another feature of PNG is the use of alpha channels for different levels of transparency.  While a GIF offers full transparency or no transparency, PNG allows users the ability to set the level of transparency up to 256 different ways.  For instance, an image of a car could have semitransparent windows.  Alpha channels set images appear seamlessly over any background.  Therefore, images can fade into one another, shadows can be cast with images, and many other techniques can be applied spruce up any image. 

Gamma correction, or cross-platform control of image brightness, is another bonus for PNG.  GIFs can appear differently of different systems, but PNG lets images created on Macs look the same on Windows machines, and vice versa. 

GIF's remain the only image format to support animation !!!

NOTE:  some IE browsers have not been updated to view PNG files properly.  If you have problems with PNG in Internet Explorer - add this key to your registry or edit your existing key :

[HKEY_CLASSES_ROOT\MIME\Database\Content Type\image/png]

"CLSID"="{25336920-03F9-11cf-8FD0-00AA00686F13}"

Vector Images

WMF (Windows MetaFile) - this is the most common vector graphic file type, and was developed by Microsoft.  Very few applications can work with metafiles, but Microsoft Word displays them beautifully.  WMF files import into Word sharp and clear - much better than bit-mapped images.

EMF (Enhanced MetaFile) - this is just an improved version of WMF, and is also from Microsoft.

PS (Post Script) - A page description language (PDL) developed by Adobe Systems. PostScript is primarily a language for printing documents on laser printers, which include both images and text. PostScript is the standard for desktop publishing because it is supported by imagesetters, the very high-resolution printers used by service bureaus to produce camera-ready copy.  You will never see an image file in Post Script, but it is mentioned here because it is capable of including vector graphics within the PS document.   NOTE:  Adobe Acrobat PDF files are also capable of this.

EPS (Enhanced Post Script) - this format is an image-only version of Post Script.

AI (Adobe Illustrator) - Illustrator's version of metafiles.  The ai format cannot be opened by any other program except Adobe Adobe Illustrator and Photoshop.

NVM (Netviz Metafile) - Netviz' version of metafiles.  The NVM format cannot be opened by any other program except Netviz.