Visualization Publishing
For the purposes of this paper, the term publishing refers to the creation of a visualization that can be delivered to an end-user and does not require that user to have the software that created the visual to view it. With this definition, we expand publishing to include formats used by word processing software, web publishing, animation and color printing. In today's world, even when the final use for a graphic is color printing (e.g. magazines, brochures, etc.), the image is usually delivered to the printer in an electronic format.
Raster images are the foundational format for visualization publishing. Raster images are the most common method of outputting visualization results. Animations are constructed from a series of raster images and printing of digital graphics nearly always involves rasterization (conversion to a raster image) at some stage of the process.
Raster images can be written in one of many different file formats. Common formats include Windows Bitmap (.bmp), TARGA (.tga), TIFF (.tif), JPEG (.jpg), and GIF (.gif). The critical issues to consider are color depth and whether the format uses lossy or lossless compression. For optimal quality, 24-bit true color should be used. The size of uncompressed true color raster images can be estimated in bytes as three times the product of the pixel resolution (width and height) plus ~30. For example, a 640x480 24-bit uncompressed image will be just under 922 Kbytes.
Windows Bitmap (.bmp) files do not use compression and support several color depths from 1 bit monochrome (black and white only) to 24-bit true color. File sizes tend to be large because of the lack of compression, but bmp files generally compress very well in archives (e.g. gzip, pkzip, tar, etc.). An additional advantage of Windows Bitmap files is that all Windows computers include software that can read .bmp files.
Portable Network Graphics (PNG), TARGA and TIFF formats both use lossless compression algorithms and support a wide range of color depths including 24-bit. Lossless compression does not compromise image quality. In contrast, lossy compression allows for modifying pixel colors in order to maximize compression. This usually results in the appearance of image noise. It is most pronounced near regions of high contrast such as the pixels adjacent to lines and edges. Lossless compression and uncompressed formats can be saved and converted any number of times without any degradation of image quality. PNG files also are quite small as they use a very appropriate compression for environmental data.
JPEG and GIF formats are both commonly used for graphics on the Internet. Their advantages are small file sizes and compatibility with virtually all computers and operating systems (provided they have web browsers or other image viewing software installed). Unfortunately, these are largely inappropriate for environmental data as they create compression artifacts which are highly undesirable.