There are applications in which the user may wish to have an individual icon (e.g. a photo) for each feature of the layer. Another example is SVG image, the individual elements of which visually represent the values of some attributes of the object. The obvious way to meet this requirement is to create a service that provides access to the database that stores these images and to include into the ExternalGraphic element of SLD a URL that references this service. The disadvantage of this solution is that for each image the service will have to perform a separate query to the database, which is usually significantly slower than the execution of a single query that retrieves all the images at once. It would be convenient to store the images together with other attributes of the object, to obtain a single query to the data source and use in SLD the same way as are other attributes that affect the visual representation of the object. It is clear that retrieving from a database a lot of images at the same time, we can face the problem of transferring large amounts of data over the network. However, the developer should have the opportunity of choosing the way of access to images, depending on the nature of the data and characteristics of the network. Currently, however, it seems that the developer using geotools, has no alternative, because current implementation of SLD has no means for acquiring images from the attributes of the feature. However, in order to obtain images from the attributes we can use the inlineContent parameter of the ExternalGraphic element. We only need to implement the possibility to use expressions as values for this parameter. Applying the expression PropertyName we received exactly what we need. I have already implemented this approach for raster images in my fork of geotools: https://github.com/geotools/geotools/compare/master...igorvolk:inlineContent_expresssion The necessary tests have been prepared. I have also a solution for SVG images. After I will prepare a comprehensive test that includes both raster and SVG images, I'm going to propose a pull request.
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