GeorectifyCoverage and SLD

Question

I'm building a small webapp to adjust georeference on Geotiff files using geoserver wps and SLD.

The idea comes from this post. Geoserver: Georectification via SLD fails

I have test the process using the WPS request builder and the process ends with success. Here is the xml file:

<?xml version="1.0" encoding="UTF-8"?><wps:Execute version="1.0.0" service="WPS" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.opengis.net/wps/1.0.0" xmlns:wfs="http://www.opengis.net/wfs" xmlns:wps="http://www.opengis.net/wps/1.0.0" xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:gml="http://www.opengis.net/gml" xmlns:ogc="http://www.opengis.net/ogc" xmlns:wcs="http://www.opengis.net/wcs/1.1.1" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="http://www.opengis.net/wps/1.0.0 http://schemas.opengis.net/wps/1.0.0/wpsAll.xsd">
  <ows:Identifier>gs:GeorectifyCoverage</ows:Identifier>
  <wps:DataInputs>
    <wps:Input>
      <ows:Identifier>data</ows:Identifier>
      <wps:Reference mimeType="image/tiff" xlink:href="http://geoserver/wcs" method="POST">
        <wps:Body>
          <wcs:GetCoverage service="WCS" version="1.1.1">
            <ows:Identifier>massimo:66</ows:Identifier>
            <wcs:DomainSubset>
              <ows:BoundingBox crs="http://www.opengis.net/gml/srs/epsg.xml#4326">
                <ows:LowerCorner>1.50163651697 41.3801721851</ows:LowerCorner>
                <ows:UpperCorner>1.50245631346 41.380726914</ows:UpperCorner>
              </ows:BoundingBox>
            </wcs:DomainSubset>
            <wcs:Output format="image/tiff"/>
          </wcs:GetCoverage>
        </wps:Body>
      </wps:Reference>
    </wps:Input>
    <wps:Input>
      <ows:Identifier>gcp</ows:Identifier>
      <wps:Data>
        <wps:LiteralData>[14, 133] [1.5015810519509158, 41.38069860394026] [125, 83] [1.5017486900143464,  41.38074388634669] [243, 70] [1.501939126854404,  41.38078011224911]</wps:LiteralData>
      </wps:Data>
    </wps:Input>
    <wps:Input>
      <ows:Identifier>targetCRS</ows:Identifier>
      <wps:Data>
        <wps:LiteralData>EPSG:4326</wps:LiteralData>
      </wps:Data>
    </wps:Input>
  </wps:DataInputs>
  <wps:ResponseForm>
    <wps:ResponseDocument>
      <wps:Output mimeType="image/tiff">
        <ows:Identifier>result</ows:Identifier>
      </wps:Output>
    </wps:ResponseDocument>
  </wps:ResponseForm>
</wps:Execute>

and the picture I apply the transformation to :

I have made the following style :

 <?xml version="1.0" encoding="UTF-8"?>
    <StyledLayerDescriptor xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/sld
    http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd" version="1.0.0">
      <NamedLayer>
            <Name>GeorectifyCoverage</Name>
            <UserStyle>
                <Title>Georectify Coverage</Title>
                <Abstract>Warps image to specified GCP</Abstract>
                <FeatureTypeStyle>
                    <Transformation>
                        <ogc:Function name="gs:GeorectifyCoverage">
                            <ogc:Function name="parameter">
                                <ogc:Literal>data</ogc:Literal>
                            </ogc:Function>
                            <ogc:Function name="parameter">
                                <ogc:Literal>gcp</ogc:Literal>
                                        <ogc:Literal>[14, 133] [1.5015810519509158, 41.38069860394026] [125, 83] [1.5017486900143464, 
41.38074388634669] [243, 70] [1.501939126854404, 
41.38078011224911]</ogc:Literal>
                            </ogc:Function>
                            <ogc:Function name="parameter">
                                <ogc:Literal>targetCRS</ogc:Literal>

                                        <ogc:Literal>EPSG:4326</ogc:Literal>

                            </ogc:Function>   
                        </ogc:Function>
                    </Transformation>
                    <Rule>
                        <RasterSymbolizer>
                            <Opacity>1.0</Opacity>
                        </RasterSymbolizer>
                    </Rule>
                </FeatureTypeStyle>
            </UserStyle>
        </NamedLayer>
    </StyledLayerDescriptor>

I've tried both: - pass the params using "Variable substitution in SLD" in javascript
- use gs:GeorectifyCoverage from a style with hard coded params(like in the code snippet).

The getMap request always return the exception: "Error rendering coverage on the fast path java.lang.NullPointerException".

What can be wrong? Does anyone have working example ?

I tested this on geoServer 2.10.1 on docker container and geoServer 2.9.2 on Ubuntu 16.10.