Licensing options for this format begin with FME Desktop Professional Edition.
The GIF Rasterizer Writer allows FME to be used to translate vector data on-the-fly for display in web browsers.
Client-side image maps may also be produced to provide clickable, intelligent images in the web browser. It also enables FME to be used as a powerful gateway between vector geographic data and raster-based Geographical Information Systems (GIS).
The writer can be used to generate both GIF and Portable Network Graphics (PNG) images.
PNG replaces the use of GIF in many situations. See the chapter PNG Rasterizer Writer for information on PNG support. In general, information about the GIF writer also applies to the PNG writer, since they are both the same module.
The FME GIF writer does not use the patented Lempel-Ziv Welch (LZW) compression algorithm; therefore, the GIF images it produces are roughly 10 times larger than the corresponding PNG image.
Note: This writer supports only vector input data. To write GIF and PNG files from raster input data, see the GIF Reader/Writer and PNG Reader/Writer chapters.
Overview
GIF files are compressed raster image files. CompuServe designed the format in 1987 as an efficient means of transporting images across low speed networks. The format is one of several standard image formats supported by most world wide web browsers.
GIF Images
GIF images consist of 8-bit pixels. The value of each pixel is an index to a color table, having eight bits each of red, green and blue. Therefore, a single GIF image may have at most 256 different colors in it, although these colors are chosen from a range of 2^24 different colors.
Color
GIF images support the notion of a transparent color. A special code is appended to the GIF image stating that a certain color index is to be considered transparent. Applications understanding this code – not all do – allow the background to appear through the transparent pixels. This is often used to create attractive world wide web displays, since most web browsers understand the transparent color option. The FME GIF writer module allows a transparent color index to be specified and it honors the transparent pixel when GIFs are used as point symbols, line styles or tiling fill patterns for polygons.
A GIF image background may be a solid color or it may be a tiled background pattern. The GIF writer supports either choice; however, if a background pattern is to be used, the pattern must come from another GIF file.
Interlaced and Non-interlaced Output
GIF images may be interlaced or non-interlaced. Interlaced GIF images download into many web browsers faster because the coarse image is displayed first, followed by more detail. The GIF writer supports both interlaced and non-interlaced output.
Image Maps
The image maps produced by the GIF writer consist of HTML code suitable for embedding into a web page displaying the created GIF image. As the mouse is moved over the GIF, each polygonal feature in the image will have a URL associated with it. This URL may bring up a dialog box with the feature’s attributes, or link to another web page.
For additional information on creating clickable GIF images, see gif_href in GIF Rasterizer Feature Representation
GIF Rasterizer Writer Parameters
Writer Overview
The writer creates a single image and draws points, lines, and polygons into it. The file name, window, size, background, color table, transparency, and interlacing of the output image are all determined in the workspace. Each feature written to the image contains special attributes used to determine the appearance of the feature.
Feature types are color entries. The color table entries are defined in the Feature Type dialog:
All features with the specified color name are drawn in that color.