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syntax.rst

syntax.rst 2.8 KB
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  1. .. _sld-extensions_composite-blend_syntax:
    2. 

  2. 

  3. Specifying compositing and blending in SLD
    4. 

  4. ==========================================
    5. 

  5. 

  6. Composites
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  7. ----------
    8. 

  8. 

  9. Both compositing and blending can be specified in SLD by adding the following VendorOption to
    10. 

  10. either the end of a Symbolizer or FeatureTypeStyle:
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  11. 

  12. .. code-block:: xml
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  13. 

  14.    <VendorOption name="composite">multiply</VendorOption>        
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  15. 

  16. In case a custom opacity is desired, it can be added after the operation name separated with a comma:
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  17. 

  18. .. code-block:: xml
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  19. 

  20.    <VendorOption name="composite">multiply, 0.5</VendorOption>
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  21. 

  22. .. note:: See the :ref:`full list of available modes <sld-extensions_composite-blend_modes>`.
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  23. 

  24. .. warning::
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  25. 

  26.    Blending against symbolizers causes exceptions inside the JDK when using OpenJDK. The `issue is known <https://bugs.openjdk.java.net/browse/JDK-8048782>`_ and has been reportedly fixed, but only in OpenJDK 9.
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  27. 

  28.    One way to get around this issue with OpenJDK 7/8 is to install the `Marlin renderer <https://github.com/bourgesl/marlin-renderer>`_. This replaces the OpenJDK core renderer and does not suffer from the same issue.
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  29. 

  30.    Oracle JDK 7 or 8 does not show this issue. 
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  31. 

  32. Composite bases
    33. 

  33. ---------------
    34. 

  34. 

  35. For FeatureTypeStyles an additional vendor option can be added to control compositing groups:
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  36. 

  37. .. code-block:: xml
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  38. 

  39.    <VendorOption name="composite-base">true</VendorOption>
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  40. 

  41. This will tell the rendering engine to use that FeatureTypeStyle as the destination, and will compose all subsequent FeatureTypeStyle/Layers on top of it, until another base is found. Once the full set of layers against a base is composed, then the base itself will be composed against the next set of composed layers, using its own compositing operator, if present.
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  42. 

  43. Without this setting, the destination will be the full stack of all previous FeatureTypeStyles and layers drawn before the current one. This can be limiting for two reasons:
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  44. 

  45. * It limits the usefulness of alpha-composite masking operations
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  46. * It breaks the WMS model, where the client can decide freely how to stack layers (the desired 
    47. 

  47.   compositing effects will be achieved only when a certain stack of layers is used)
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  48.   
    49. 

  49. Consider the following example:
    50. 

  50. 

  51. .. figure:: images/stacking.png
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  52. 

  53. In this example, the first two layers are drawn on top of each other, forming "Merge1".
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  54. 

  55. The third layer is a composite base, as such it won't be merged on top of the already drawn map immediately, but it will be drawn to an off-screen buffer, and layer 4 and 5 will be drawn/composited on top of it. Once that happens, "Merge2" is ready, and gets drawn on top of "Merge1",
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  56. 

  57. The next layer is another base, so "Base2" will be again drawn to an off-screen buffer, and layer 7 and 8 will be drawn/composited on top of it, forming Merge3. Once Merge3 is ready, it will be drawn/composited on top of the already fused Merge1/Merge2, generating the final map.
    58.