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PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

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Figure 6.6: Band area.<br />

controlled by the application and choices made by users on these options have no effect either on<br />

Multi Band or Single Band£ button.<br />

6.2.2 Color Legend<br />

<strong>The</strong> color legend is used to indicate the significance <strong>of</strong> color by numbers. It consists <strong>of</strong> a title, a<br />

color bar and a common unit <strong>for</strong> numbers and tick notations. In the example below, the red color<br />

area expresses high temperature and the blue color area expresses low temperature.<br />

Figure 6.7: <strong>The</strong> temperature color legend.<br />

As each temporal raster is a matrix <strong>of</strong> values <strong>of</strong> points in space, hence it is referred to as a map.<br />

Each map possesses normally a minimum value and a maximum value. If Time travel is an application<br />

that shows the evolution <strong>of</strong> a set <strong>of</strong> maps, then the color legend representing all these maps needs to<br />

be computed from this set. It does not make sense when the color legend <strong>of</strong> a specific map is used<br />

<strong>for</strong> a set <strong>of</strong> maps. This also means that the minimum value (respectively maximum) at the bottom<br />

(respectively top) <strong>of</strong> the color legend is the minimum value <strong>of</strong> a set <strong>of</strong> maps.<br />

<strong>The</strong> color legend is available through another window than Time travel window. <strong>The</strong> Output color -<br />

palette file field let user enter the desired name <strong>for</strong> the created color palette file. In the Color palette<br />

region, the left compose box contains all <strong>of</strong> the available color palettes that help users to freely choose<br />

a satisfied one. Behind this box, the chosen color palette is shown.<br />

<strong>The</strong> Apply button aims to apply the chosen palette to the current map <strong>for</strong> users to visualize, along<br />

with creating a corresponding color palette file with the name given by the Output color palette file<br />

field. Users can repeat this step as many times as the number <strong>of</strong> existing palettes. Once a color palette<br />

file is created, its name is registered in the Created color palette(s) compose box. On the one<br />

hand, these color palette files serve as style files <strong>for</strong> maps and are used to create corresponding color<br />

legends on the other hand. Finally, the Create button creates the color legend <strong>of</strong> the current color<br />

palette file. <strong>The</strong> result <strong>of</strong> the color legend is simply a PNG image file.<br />

6.3 Implementation<br />

<strong>The</strong> application is developed as a plugin [12] in QGIS using PyQt, the bridge between Python and<br />

Qt4 graphics library.<br />

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