Choosing An Appropriate Raster Image Format

Background

Any image that is to be archived for future use requires specific storage considerations. However, the choice of file format is diverse, offering advantages and disadvantages that make them better suited to a specific environment. When digitising images a standards-based and best practice approach should be taken, using images that are appropriate to the medium they are used within. For disseminating the work to others, a multi-tier approach is necessary, to enable the storing of a preservation and dissemination copy. This document will discuss the formats available, highlighting the different compression types, advantages and limitations of raster images.

Factors to Consider when Choosing Image Formats

When creating raster-based images for distribution file size is the primary consideration. As a general rule, the storage requirements increase in proportion to the improvement in image quality. A side effect of this process is that network delivery speed is halved, limiting the amount that can be delivered to the user. For Internet delivery it is advised that designers provide a small image (30-100k) that can be accessed quickly for mainstream users, and provide a higher quality copy as a link or available on a CD for professional usage.

When digitising the designer must consider three factors:

• File format
• Bit-depth
• Compression type

Distribution Methods

The distribution method will have a significant influence upon the file format, encoding type and compression used in the project.

• Photograph archival - For photographs, the 24-bit lossless TIFF format is recommended to allow the image to be reproduced accurately. The side-effect is that file sizes will begin at 10Mb for simpler images and increase dramatically. This is intended for storage only, not distribution.
• Photograph distribution - For photographs intended for Internet distribution, the lossy JPEG format is recommended. This uses compression to reduce file size dramatically. However, image quality will decrease.
• Simpler images - Simpler images, such as cartoons, buttons, maps or thumbnails, which do not require 16.8 million colours should be stored in an 8-bit format, such as GIF or PNG-8. Though 256 colours images can be stored correctly in a 24-bit format, a side effect of this process is the 8-bit file size is often equal or higher than 24-bit images.

To summarise, Table 1 shows the appropriateness of different file formats for streaming or progressive recording.

	Maximum no. of colours	Compression Type	Suited for:	Issues
BMP	16,777,216	None	General usage. Common on Windows platforms	A Windows format rather than an Internet format. Unsupported by most browsers.
GIF87a	256	Lossless	High quality images that do not require photographic details	File sizes can be quite large, even with compression
GIF89a	256	Lossless	Same as GIF87a, animation facilities are also popular	See above
JPEG	16,777,216	Lossy	High quality photographs delivered in limited bandwidth environment.	Degrades image quality and produces wave-like artefacts on image.
PNG-8	256	Lossless	Developed to replace GIF. Produces 10-30% smaller than GIF files.	File sizes can be large, even with compression.
PNG-24	16,777,216	Lossless	Preserves photograph information	File sizes larger than JPG.
TIFF	16,777,216	Lossless	Used by professionals. Redundant file information provides space for specialist uses (e.g. colorimetry calibration). Suitable for archival material.	Unsuitable for Internet-delivery

Table 1: Comparison table of image file formats

Once chosen, the file format will, to a limited extent, dictate the possible file size, bit depth and compression method available to the user.

Compression Type

Compression type is a third important consideration for image delivery. As the name suggests, compression reduces file size by using specific algorithms. Two compression types exist:

• Lossless compression - Lossless compression stores colour information and the location of the pixel with which the colour is associated. The major advantage of this compression method is the image can be restored to its original state without loss of information (hence lossless). However, the compression ratios are not as high as lossy formats, typically reducing file sizes by half. File formats that use this compression type include PNG and GIF.
• Lossy compression - Offers significantly improved compression ratio, at the expense of image quality. Lossy compression removes superfluous image information that cannot be regained. The degree of quality loss will depend upon the amount of compression applied to the image (e.g., JPEG uses a percentage system to determine the amount of compression). Therefore it is possible to create an image that is 1:100 the size of the original file.

As an archival format, lossy compression is unsuitable for long-term preservation. However, its small file size is used in many archives as a method of displaying lower resolution images for Internet users.

Bit-depth

Bit-depth refers to the maximum number of colours that can be displayed in an image. The number of colours available will rise when the bit depth is increased. Table 2 describes the relationship between the bit depth and number of colours.

Bit depth	1	4	8	8	16	24	32
Maximum No. of colours	2	16	256	256	65,536	16,777,216	16,777,216

Table 2: A conversion table showing the relationship between bit-depth and maximum number of colours

The reduction of bit-depth will have a significant effect upon image quality. Figure 3 demonstrates the quality loss that will be encountered when saving at a low bit-depth.

24-bit	8-bit	4-bit	1-bit
Original image	Some loss of colour around edges. Suitable for thumbnail images	Major reduction in colours. Petals consist almost solely of a single yellow colour.	Only basic layout data remains.

Figure 3: Visual comparison of different bit modes

Image Conversion Between Different Formats

Image conversion is possible using a range of applications (Photoshop, Paint Shop Pro, etc.). Lossless-to-lossless conversion (e.g. PNG-8 to GIF89a) can be performed without quality loss. However, lossless-to-lossy (PNG-8 to JPEG) or lossy-to-lossy conversion will result in a quality loss, dependent upon the degree of compression used. For dissemination of high-quality images, a lossy format is recommended to reduce file size. Smaller images can be stored in a lossless format.

Further Information

• Gimp-Savvy,
  <http://gimp-savvy.com/cgi-bin/img.cgi?ufwsEFfOvPeIueo740>
• Raster Image Files,
  <http://scv.bu.edu/Tutorials/ImageFiles/image101.html>