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Images: Image File Extensions

Image File Formats

Raster vs Vector

Raster images, or bitmaps, are composed of a fixed set of tiny dots or pixels. These kind of images are produced with scanning or photographing an object. Since raster images are pixel based, they are resolution dependent. The number of pixels that make up an image as well as how many of those pixels are displayed per inch, both determine the quality of an image. As you may have guessed, the more pixels in the image and the higher the resolution is, the higher quality the image will be. Some of the basic file types include JPG, GIF, PNG and TIFF.

Vector images are composed of mathematical equations where lines and curves (or paths) make up graphic shapes, images, and text in XML format. One of the greatest things about vector images is that you can re-size them infinitely larger or smaller, and they will still print out just as clearly, with no increase (or decrease) in file. Thus, they are resolution independent.  SVG and EPS are vector file types.

Raster (or bitmap) images are generally what you think about when thinking of images. These are the types of images that are produced when scanning or photographing an object. Raster images are compiled using pixels, or tiny dots, containing unique color and tonal information that come together to create the image. 

Since raster images are pixel based, they are resolution dependent. The number of pixels that make up an image as well as how many of those pixels are displayed per inch, both determine the quality of an image. As you may have guessed, the more pixels in the image and the higher the resolution is, the higher quality the image will be.

For example, if we scale a raster image to enlarge it, without changing resolution, it will lose quality and look blurry or pixilated. This is because we are stretching the pixels over a larger area, thus making them look less sharp. This is a common problem but can be remedied by using raster image editing programs such as Photoshop to change resolution and properly scale images.

 

Image File Types

Bitmap (.bmp)

BMP or Bitmap Image File is a format developed by Microsoft for Windows. There is no compression or information loss with BMP files which allow images to have very high quality, but also very large file sizes. Due to BMP being a proprietary format, it is generally recommended to use TIFF files.

Compression: None
Best For: High quality scans, archival copies
Learn more about BMP file types

From University of Michagan Libraries: All About Images

EPS (.eps)

An EPS or Encapsulated PostScript file is a common vector file type. EPS files can be opened in many illustration applications such as Adobe Illustrator or CorelDRAW.

Compression: None - uses vector information
Best For: Vector artwork, illustrations
Special Attributes: Saves vector information

Learn more about EPS file types

 

From University of Michagan Libraries: All About Images

GIF (.gif)

GIF or Graphics Interchange Format files are widely used for web graphics, because they are limited to only 256 colors, can allow for transparency, and can be animated. GIF files are typically small is size and are very portable. 

Compression: Lossless - compression without loss of quality
Best For: Web Images
Special Attributes: Can be Animated, Can Save Transparency

Learn more about GIF file types

From University of Michagan Libraries: All About Images

JPEG (.jpg, .jpeg)

JPEG, which stands for Joint Photographic Experts Groups is a “lossy” format meaning that the image is compressed to make a smaller file. The compression does create a loss in quality but this loss is generally not noticeable. JPEG files are very common on the Internet and JPEG is a popular format for digital cameras - making it ideal for web use and non-professional prints.

Compression: Lossy - some file information is compressed or lost
Best For: Web Images, Non-Professional Printing, E-Mail, Powerpoint
Special Attributes: Can choose amount of compression when saving in image editing programs like Adobe Photoshop or GIMP.

From University of Michagan Libraries: All About Images

PNG (.png)

PNG or Portable Network Graphics files are a lossless image format originally designed to improve upon and replace the gif format. PNG files are able to handle up to 16 million colors, unlike the 256 colors supported by GIF.

Compression:  Lossless - compression without loss of quality
Best For: Web Images
Special Attributes: Save Transparency

Learn more about PNG file types  

From University of Michagan Libraries: All About Images

RAW Image Files (.raw, .cr2, .nef, .orf, .sr2, and more)

RAW images are images that are unprocessed that have been created by a camera or scanner. Many digital SLR cameras can shoot in RAW, whether it be a .raw, .cr2, or .nef. These RAW images are the equivalent of a digital negative, meaning that they hold a lot of image information, but still need to be processed in an editor such as Adobe Photoshop or Lightroom. 

Compression: None 
Best For: Photography
Special Attributes: Saves metadata, unprocessed, lots of information

Learn more about RAW file types

From University of Michagan Libraries: All About Images

SVG (.svg)

SVG stands for Scalable Vector Graphics, and is a vector graphic format—based on XML and is used to display a variety of graphics on the Web and other environments. Under the hood, SVG documents are nothing more than simple plain text files that describe lines, curves, shapes, colors, and text. SVG has a flexiblity and versatility that can’t ever be matched by traditional PNG, GIF or JPGs.

Compression: None - uses vector information.
Best For:  Illustrations like logos, icons, and graphs
Special Attributes:  Works well with current web development demands of scalability, responsiveness, interactivity, programmability, performance, and accessibility.

Learn more about SVG file types

 

From LifeWire | What is an SVG File?

From SitePoint | SVG 101

From 5 Advantages of Using SVG files

TIFF (.tif, .tiff)

TIFF or Tagged Image File Format are lossless images files meaning that they do not need to compress or lose any image quality or information (although there are options for compression), allowing for very high-quality images but also larger file sizes.

Compression: Lossless - no compression. Very high-quality images.
Best For: High quality prints, professional publications, archival copies
Special Attributes: Can save transparencies

Learn more about TIFF file types

From University of Michagan Libraries: All About Images

Image Transparency

What does it mean to save an image with transparency?


Notice the two images below:

Block M with and without white backgrounds

  • The image on the left was saved using a file format that did not support transparency and displays with an unwanted white background.
  • The image on the right was saved using a transparency supported format and looks very crisp and clean on this colored background.

File types that support transparency: TIFF, PNG, GIF, BMP. Most vector file formats implicitly support transparency

How to Save an Image with a transparent background