The {@link com.bc.beam.usage.ndvi} package contains example code for a simple MERIS ndvi processor. This file will give a detailed step by step instruction on how to implement a scientific processor using the processor framework supplied by the MERIS/AATSR Toolbox.

The example processor is invoked with a single commad line argument - the path to a request file. See also Request File Definition. An example processing request file NdviProcReq.xml is located next to the source code files. You must modify the input product and output product paths according to your computers setup.

Packages used in this example are:

  • {@link org.esa.beam.framework.dataio Data I/O}
  • {@link org.esa.beam.framework.processor Processor framework}
  • {@link org.esa.beam.util Utility classes}

    • The ndvi processor main class - @link org.esa.beam.examples.ndvi_processor.NdviProcessorMain NdviProcessorMain

    This class implements the main() method of the processor to be able to invoke the processor from the command line. It is a general convention that every executable within the Toolbox project has a xxxMain.java file which contains the main() method. This makes it easier to navigate the code.

    This class implements just one method, the main() method. Within main, the ndvi processor is created and the command line and the processor are passed to a ProcessorRunner. Additionally there is some code for exception handling, but basically this is everything that's happening in main.

    The {@link ProcessorRunner} takes the further control of what happens to the processor. It performs a lot of actions in the background which otherwise have to be programmed over and over for each processor. The runner performs the following steps:

    A special note should be take to the last paragraph: the processor runner can handle request files containing any number of requests and lets the processor perform all calculations in batch mode.

    The processor class - {@link NdviProcessor}

    This class contains all of the ndvi processor specific functionality. It inherits from the abstract {@link org.esa.beam.framework.processor.Processor} class which is embedded in the processor framework.

    The only method that must be implemented by NdviProcessor is the process method. This is the "worker-method". It is invoked by the framework each time a request has to be processed.

    A first step in the process method is checking the request type. See the checkRequestType method. It can be seen how the base class functionality of the processor class is used to retrieve the request processed, which is at this point automatically read from the request file, parsed and (optionally) verified.

    The second thing to do is opening the input product denoted in the request. This happens in the loadInputProduct method. Here, the functionality of the ProductIO framework is used to read the input product skeleton (not the product data but metadata, product size, band names ...). The class ProductIO automatically creates an object of type Product that holds all this information. Note that ProductIO automatically decides which product type the requested product is and which ProductReader to use. Once the input product is read, the two bands needed for the ndvi calculation are requested from the product.

    Now the output product must be created. This happens in the createOutputProduct method. This method reads the product width and height from the input product. Then an output product is created which at this point is just an empty hull with a product type, product name and same dimensions as the input product. Then the resulting ndvi band is added. The product now neither contains data nor has a disk representation, it's pure virtual. Again, the ProductIO class supplies the product with an appropriate disk writer, the default one in this example. And finally, the disk represenation is written out.

    The request file definition

    All processors using the processor framework are "request-driven". A request is a xml-file defining one or more "processing requests". An example request file can be found in this examples directory.

    The request file must contain a few basic lines. These are:

    All tags supported in a request file and understood by the request loader are defined in {@link org.esa.beam.framework.processor.RequestTags}. You can define any number of named parameters to customize processor requests.

    For future expansion, it is mandatory that every processor defines a request type, in this case the type is defined by "NDVI_EXAMPLE".

    Then this simple example request just defines two more entrie, the input product and the output product.

    @see org.esa.beam.framework.dataio @see org.esa.beam.framework.dataop @see org.esa.beam.framework.processor @see org.esa.beam.util