Friday, March 19, 2010

Understanding image file formats

What are the different image file formats? Following on from post on Wednesday (Image file formats for artist bloggers) I've created a table for the different file formats used by artist bloggers

I knew some of this - but not all of it before I started - but I do understand it all an awful lot better for having written it all out!

CommentsCharacteristics
NO COMPRESSION
BMP (bmp)

This is probably the very first image file format I ever came across. I now regard it as an old format for old Windows programs.

  • simple
  • uncompressed
  • good image quality
  • produces large files
  • accepted by Windows programs

LOSSY COMPRESSION
JPEG (jpg, jpeg)best used to achieve good quality small files

JPEGstands for "Joint Photographic Experts Group" and was developed to deal with photographs. Nearly every digital camera can save images as jpeg files.


It's the format of choice for nearly all photographs on the web. Frequently used by artists / photographers / bloggers to reduce the size of digital image files for use on the Internet


Use of lossycompression enables a sliding scale of compression which in turn permits choice over the level of compression. However, it also means that frequent editing of the same file will make it degrade over time. Data is lost every time the file is opened and closed using this file format

  • optimised for photographs and other images having complex images with large numbers of colours
  • digital cameras can create jpeg files
  • support full colour images (16.8 million colours)
  • lossy format compression - enables choice over level of compression
  • can create small files
  • file extensions vary according to operating system used
  • files degrade every time they are opened and closed. NOT the best format to store images you want to keep over time
  • unsuitable for line art

LOSSLESS COMPRESSION

GIF format (gif)

compatible with simple web graphics using flat colours

gif (pronounced “jiff”) stands for Graphics Interchange Format. It was introduced by CompuServe (I was an early CompuServe user!) and was then subsequently adopted for widespread use on the web


Its use of the lossless data compressiontechnique enables a reduction in file size without any degradation in visual quality

Its main constraint is its limited use of colours. It's suitable for use in graphics which have few colours and use simple shapes (eg diagrams, shapes, logos and cartoon style images). Also good for simple web animations.


It's unsuitable for detailed images and/or those using complex colours and gradients.

  • very limited colours (8 bit palette = 256 colours)
  • frequently used for web graphics
  • uses lossless compression format (ie no degradation in visual quality)
  • creates small files
  • suitable for simple graphics using large areas with simple colours
  • suitable for simple web animation
  • unsuitable for colour complexity and detailed images
  • unsuitable for use with photographs

PNG (png)excellent for image editing; good for web
The PNG (Portable Network Graphics) pronounced 'ping' file format was created as the free, open-source successor to the GIF. (Some say that png stands for "PNGs Not GIFs")

It is a raster format and was designed specifically for image editing and use on the web.

PNG has advantages over GIF:
  • alpha channels (variable transparency),
  • gamma correction (cross-platform control of image brightness), and
  • two-dimensional interlacing (a method of progressive display).
  • PNG also compresses better than GIF in almost every case.

Adoption of the png format said to be slow due to inconsistent treatment by web browsers.
  • designed specifically for image editing and the web
  • supports up to 48-bit truecolor / 16-bit grayscale (16 million colours)
  • offers a variety of transparency options
  • uses two stage compression (DEFLATE)
  • large files - because uses lossless compression
  • files can be shared on web
  • can hold short description of content for use on web
  • useful format for intermediate edit stages due to lossless format
  • suitable for photographs and other complex colour images
  • works well with large areas of homogeneous colour
  • unsuitable for animation

TIFF (tif, tiff)

high quality file - excellent for printing and photography

The TIFF (Tagged Image File Format) format is used for storing images. Control over the format is now owned by Adobe

Originally created with a view to having a standard format for scanners. TIFF became the standard storage format for facsimiles.

Lossless compression is an option. How it saves files depends on the Photoshop version used.

TIFF is more complex than PNG.

The format has not had a major update since 1992.

  • popular format for high colour-depth images (ie giving a broader range of distinct colours)
  • usually uses lossless compression
  • flexible format allows for wide range of options re. compression and colour spaces
  • retains info in layers depending on how it is saved
  • flexibility can create compatibility problems
  • creates very large files
  • unsuitable for web images
  • useful archive file format


5 comments:

janabouc said...

This is so helpful. Thanks for putting it all together. At some point you might want to update with info about RAW files too. I just got a new camera so I can shoot in RAW.

Katherine Tyrrell said...

I did think about that - however this is a post which is oriented towards applications (ie it's about web/print outputs rather than camera inputs)

Plus
- most people don't shoot in 'raw'.
- we don't use 'raw' files to save images in to use on blogs and websites and for printing.

My understanding is that 'raw' files need to be processed before they can be used for printing or bitmap processing.

But the big reason is I don't understand 'raw'!

Katherine Tyrrell said...

for those who do want to know more about raw - this is a good place to start http://en.wikipedia.org/wiki/Raw_image_format

Jenna White said...

I have over 26,000 pictures saved on my computer, which I am about to back up onto an external hard drive very soon. So I should save them on the external as TIF files to be sure that they keep their original quality?

Katherine Tyrrell said...

Gosh - are they backed up already?

It's the opening and closing which does the damage. i'm not too sure about just straight copying without opening them

I guess it depends on how much it matters to you that they are all in pristine condition - and probably how much time you;ve got. Believe me converting 26k of files to TIFF is not going to be quick. There's no way you can do it as a batch job - several batch jobs possibly depending on the software you use.

Bear in Mind TIFF are huge files. My TIFFs files multiply size wise by a fact of about 10.

I just make sure I transfer the ones I use for prints as TIFF Files.

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