Martin Desruisseaux created SIS-498:
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             Summary: Replace PassthroughTransform by CompoundTransform
                 Key: SIS-498
                 URL: https://issues.apache.org/jira/browse/SIS-498
             Project: Spatial Information Systems
          Issue Type: Task
          Components: Referencing
    Affects Versions: 1.0
            Reporter: Martin Desruisseaux


{{PassthroughTransform}} could be replaced by a new implementation, {{CompoundTransform}}, which juxtapose an arbitrary amount of {{MathTransform}} components (in a way similar to {{CompoundCRS}} which juxtapose an arbitrary amount of {{SingleCRS}} elements). {{PassthroughTransform}} can be understood as a special case of {{CompoundTransform}} where leading or trailing ordinates are "transformed" by identity transforms.

The use of {{CompoundTransform}} may simplify internal mechanic and improve performances compared to {{PassthroughTransform}}. It may be necessary to decompose {{CompoundTransform}} into synthetic concatenation of {{PassthroughTransform}} at WKT time if we want to format an OGC 01-009 compliant WKT string, but we are already doing this kind of exercise for map projections. Decomposition would look like:

{noformat}
CONCAT_MT[PASSTHROUGH_MT[…], PASSTHROUGH_MT[…]]
{noformat}

h2. Existing implementation
A first draft of {{CompoundTransform}} implementation is available in the {{org.apache.sis.internal.coverage}} package. We should complete this implementation by resolving all "TODO" comments, remove the restriction to {{MathTransform1D}} (while keeping the {{MathTransform1D}}-based implementation as a specialization), add a {{MathTransform2D}} implementation (can happen only with a compound of 2 {{MathTransform1D}}), make the implementations serializable, move them as package-private classes in {{org.apache.sis.referencing.operation.transform}} package, and defines {{PassthroughTransform}} as a subclass for above-cited special case.




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