Time Capsules for Image Storage

File formats are the time capsules you'll use to retain your images for the future.

Which format should I use? Should I compress? What are the differences? Is JPEG good for text?

What is lossy?

Does JPEG really degrade each time you save an image?

File Formats for a Photo Database Organizer.

Image Storage

• What are suitable file formats to use?
• Should you compress images?
• What is the TIF file format?
• What is PNG file format?
• What is JPG file format?
• What is GIF file format?

• Back to the Digital Imaging FAQ Index

What are suitable file formats to use?

The four most common file formats used are:

• TIF - Tagged Image File Format, uncompressed and compressed formats
• PNG - Portable Network Graphics, standardized compression
• JPG - Joint Photographic Experts Group, compressed format
• GIF - Graphics Interchange Format, compressed format dating back to CompuServe in the 1980's

Compression comes in two forms: lossless and lossy. Lossless compression loses none of the image information during compression and decompression. Lossy compression as its name says removes some of the original photographic detail. Lossy compression can reduce the size of image files to a far greater extent than lossless compression. 

JPG is a lossy technique that is designed to compress color and grayscale continuous-tone images. The information discarded at lower levels of JPG compression is designed to be information that the human eye cannot detect. Please note that lossy formats such as JPG may result in the loss of additional detail each time you open and save a file. This can cause severe color shifts and distortion (example). If the image format from your digital camera is lossy then your master copy should be the original image ex the camera or an immediate conversion to a lossless format. 

The following table shows characteristics of each of these image formats:

The next table shows the suitability of each format to particular applications.

Note: TIF LZW has limited support as Unisys has a patent on the LZW compression algorithm used by both GIF and TIF with LZW compression. Not all companies choose to support it for this reason. It is a small loss as LZW is a relatively inefficient algorithm when applied to real world images, see the LZW compression results in the next section.

The images below illustrate the imperfections of JPG on text and edges.

PNG original 26Kb	JPG 6Kb. Note the crazing like imperfections around the text.	GIF 12Kb

Should you compress images?

The following table gives the sizes of various image formats for an original image of 1830 x 1240 x 16 million colors.

Graphics images are often stored in a compressed format to save space on your hard drive and to maximize the number of images that will fit on removable storage such as CD-R. 

See examples of image size and quality degradation with increased compression.

The question was should you compress images? The answer is dependent on your needs.

• While still manipulating images use a lossless compression format.
• If you want to retain EXIF information the only alternatives are TIF and JPG.
• When archiving (e.g. to CD-R) you should create at least two copies. One copy could be a lossy format for viewing and the other a lossless master.
• If you don't need to retain a master copy (i.e. you are never going to change the image again) then you could consider doing both your archives in JPG format.

What is TIF file format?

TIF or TIFF is short for "Tagged Image File Format". It was designed primarily for raster data interchange. Its main strengths are a highly flexible and platform-independent format which is supported by numerous image processing applications.

TIFF can contain multiple kind of picture formats, like uncompressed, LZW and Fax 3 & 4 formats. The header section of TIFF-file contains the information about the image, like the color depth, color encoding and compression type. Because of the flexibility, very different levels of image quality can be produced with TIFF. The current version is TIFF 6.0. 

What is JPG file format?

JPEG (pronounced "jay-peg") is a standardized image compression mechanism. JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard.

JPEG compression is lossy, meaning that the decompressed image isn't quite the same as the one you started with.  JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness.  Thus, JPEG is intended for compressing images that will be looked at by humans.  If you plan to machine-analyze your images, the small errors introduced by JPEG may be a problem for you, even if they are invisible to the eye.

JPEG compression is designed for compressing either 16 million color (24 bit) or 256 gray scale images of natural, real-world scenes.  It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings.

A useful property of JPEG is that the degree of lossiness can be varied by adjusting compression parameters.  This means that you can trade off file size against output image quality. Most JPEG compressors let you pick a file size vs. image quality tradeoff by selecting a quality setting.  The quality scale is purely arbitrary; its not a percentage of anything.

For full-color images, the uncompressed data is normally 24 bits/pixel.  The best known lossless compression methods can compress such data about 2:1 on average.  JPEG can typically achieve 10:1 to 20:1 compression without visible loss, bringing the effective storage requirement down to 1 to 2 bits/pixel.  30:1 to 50:1 compression is possible with small to moderate defects, while for very-low-quality purposes such as previews or archive indexes, 100:1 compression is quite feasible.  An image compressed 100:1 with JPEG takes up the same space as a full-color one-tenth-scale thumbnail image, yet it retains much more detail than such a thumbnail.

JPEG compression is poor on:

• large areas of pixels that are all exactly the same color

• icons that use only a few colors

• very sharp edges: a row of pure-black pixels adjacent to a row of pure-white pixels, for example.  Sharp edges tend to come out blurred unless you use a very high quality setting.  

• small text that's only a few pixels high

• plain black-and-white (two level) images

If your image contains sharp colored edges, you may notice slight fuzziness or jaggedness around such edges no matter how high you make the quality setting.

It would be nice if, having compressed an image with JPEG, you could decompress it, manipulate it (crop off a border, say), and recompress it without any further image degradation beyond what you lost initially. 

Unfortunately THIS IS NOT THE CASE.  In general, recompressing an altered image loses more information.  Hence it's important to minimize the number of generations of JPEG compression between initial and final versions of an image. See an example of how bad this can be.

There are a few specialized operations that can be done on a JPEG file without decompressing it, and thus without incurring the generational loss that you'd normally get from loading and re-saving the image in a regular image editor.  In particular it is possible to do 90-degree rotations and flips losslessly, if the image dimensions are a multiple of the file's block size (typically 16x16, 16x8, or 8x8 pixels for color JPEGs).  This is of importance as many users of digital cameras would like to be able to rotate their images from landscape to portrait format without incurring loss --- and practically all digicams that produce JPEG files produce images of the right dimensions for these operations to work.  So software that can do lossless JPEG transforms has started to pop up.  But you do need special software; rotating the image in a regular image editor won't be lossless.

It turns out that if you decompress and recompress an image at the same quality setting first used, relatively little further degradation occurs. This means that you can make local modifications to a JPEG image without material degradation of other areas of the image.  (The areas you change will still degrade, however.)  Counterintuitively, this works better the lower the quality setting.  But you must use *exactly* the same setting, or all bets are off.  Also, the decompressed image must be saved in a full-color format; if you dosomethinglikeJPEG=>GIF=>JPEG, the color quantization step loses lots of information. [Authors note:  Our tests used the same quality setting and still suffered extreme degradation.]

Unfortunately, cropping doesn't count as a local change!  JPEG processes the image in small blocks, and cropping usually moves the block boundaries, so that the image looks completely different to JPEG.  You can take advantage of the low-degradation behavior if you are careful to crop the top and left margins only by a multiple of the block size (typically 16 pixels), so that the remaining blocks start in the same places.  (True lossless cropping is possible under the same restrictions about where to crop, but again this requires specialized software.) 

The bottom line is that JPEG is a useful format for compact storage and transmission of images, but you don't want to use it as an intermediate format for sequences of image manipulation steps.  Use a lossless 24-bit format (PNG, TIFF, etc) while working on the image, then JPEG it when you are ready to file it away or send it out on the net.  If you expect to edit your image again in the future, keep a lossless master copy to work from.  The JPEG you put up on your Web site should be a derived copy, not your editing master.

The information here on the JPG format has been summarized from: http://www.faqs.org/faqs/jpeg-faq/ 

What is GIF file format?

GIF, 'Graphics Interchange Format' was introduced in the 1980s on CompuServe to allow high-quality, high-resolution graphics to be displayed on a variety of graphics hardware and was intended as an exchange and display mechanism for graphics images. The rise of the Internet and in particular the web saw GIF usage explode. 

Saving a photographic image as GIF firstly achieves a 3:1 compression by a reduction in color depth from 16 million colors to 256. A GIF graphic is stored as a sequence of pixels with 256 color values from a image specific color palette.  Dithering reduces the visual impact of the reduction in number of colors. This raster data is then compressed according to the LZW algorithm (Unisys patented).

GIF is potentially useful on those areas where JPEG compression is poor and where you are looking for a file size that is significantly less than PNG, that is for images having:

• large areas of pixels that are all exactly the same color

• icons that use only a few colors

• very sharp edges: a row of pure-black pixels adjacent to a row of pure-white pixels, for example.

• small text that's only a few pixels high

• plain black-and-white (two level) images

GIF does significantly better than JPEG on images with only a few distinct colors, such as line drawings. Not only is GIF lossless for such images, but it often compresses them more than JPEG can. For example, large areas of pixels that are all exactly the same color are compressed very efficiently indeed by GIF. JPEG can't squeeze such data as much as GIF does without introducing visible defects.