![]() The process is just as before, but this time only the palette file will be stored into $GEOSERVER_DATA_DIR/palettes. This file type can be generated by applications such as Paint Shop Pro and IrfanView, but also can be generated manually in a text editor. The palette file must be in JASC-PAL format, and have a. GeoServer will load the palette from the file and use it. If the file is named mypalette.gif or mypalette.png, the user will be able to refer it appending palette=mypalette to the GetMap request. The sample file can be either in GIF or PNG format. In this case, the user will generate an 256 color images using an external program (such as Photoshop), and then will save it into the $GEOSERVER_DATA_DIR/palettes directory. Use the internet safe palette, a standard palette built in into GeoServer, by appending palette=safe to the GetMap request. Optimal palette computation is anyways a repetitive work that can be done up front: a user can compute the optimal palette once, and tell GeoServer to use it. As you’ll see, this is an expensive process (CPU bound), but as you’ll see, depending on the speed of the network connecting the server and the client, the extra cost can be ignored (especially if the bottleneck can be found in the network instead of the server CPU). These output formats, if no other parameters are provided, do compute the optimal palette on the fly. The following zoom of an image shows antialiasing in action: Antialiasing will smooth the borders of the line giving a softer, better looking shape, and it will do so by adding pixels with an intermediate color, thus increasing the number of colors that are needed to fully display the image. In fact, this is true only if no antialiasing is enabled. One may think this layer needs only 2 colors: the background one (eventually transparent) and gray. Let’s take a road layer, where each road is depicted by a solid gray line, 2 pixels thick. So, as it turns out, paletted images can be used with profit on vector data sets, either using the PNG8 or GIF formats.Īntialiasing plays a role too. Does not support translucency, but allows for fully transparent pixels. GIF: a non lossy format with a 256 color palette, best suited for vector layers. The full color version is sometimes referred as PNG24, the paletted version as PNG8. This format is best suited to vector layers, especially in the paletted version. In full color images each pixel is encoded as a 24bits integer with full transparency information (so PNG images can be translucent), in paletted mode each pixel is an 8 bit index into a 256 color table (the palette). PNG: a non lossy format allowing for both full color and paletted. On the contrary, it’s not suited to most vector layers, because even slight compression generates visible artifacts on uniform color areas. JPEG is best suited for imagery layers, where the pixel color varies continuously from one pixel to the next one, and allows for the best compressed outputs. JPEG: a lossy format with tunable compression. Internet standards offer a variety of image formats, all having different strong and weak points. ![]() ![]() In the latter case, the smaller footprint of paletted images is usually a big gain in both performance and costs, because more data can be served with the same internet connection, and the clients will obtain responses faster. For many maps, one can easily find 256 representative colors. Depending of the actual map, this may be a very stringent limitation, visibly degrading the image quality, or it may be that the output cannot be differentiated from a full color image. This allows for images that are 3-4 times smaller than the standard images, with the limitation that only 256 different colors can appear on the image itself. Basically, instead of representing each pixel with its full color triplet, which takes 24bits (plus eventual 8 more for transparency), they use a 8 bit index that represent the position inside the palette, and thus the color. Some image formats, such as GIF or PNG, can use a palette, which is a table of (usually) 256 colors to allow for better compression.
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