Geotools interpolation - kotlin

Question

I have some spatial data with a value. lat, lon and z value. And a polygon where this data is from. Now i'm trying to create a geotiff so i can serve this through geoserver. I want a smooth graph, so i need to do some interpolation dit missing values in my polygon. Values outside my polygon should nu cropped or marked as no data.
I'm trying to do this through geotools, but i'm having trouble with thé interpolation. My geotiff before and after are exactly the same. Ant idea that is wrong? This is my code:
class GeoToolsExperiment {
fun execute(pointWithDatas: List<PointWithData>, polygon: Polygon) {
val crs: CoordinateReferenceSystem = CRS.decode("EPSG:3857")
val bounds = ReferencedEnvelope(polygon.envelopeInternal, crs)
val factory: GridCoverageFactory = CoverageFactoryFinder.getGridCoverageFactory(null)
val width = (bounds.width * 100000).toInt()
val height = (bounds.height * 100000).toInt()
val writableRaster: WritableRaster = createRaster(width, height)
val gc: GridCoverage2D = createGridCoverage(factory, writableRaster, bounds)
fillRaster(pointWithDatas, gc, writableRaster)
val ci = interpolate(gc)
writeToTif(ci)
}
private fun writeToTif(ci: GridCoverage?) {
val outFile = "test.tif"
val out = File(outFile)
val format: AbstractGridFormat = GeoTiffFormat()
val writer = format.getWriter(out)
try {
writer.write(ci)
writer.dispose()
} catch (e: IllegalArgumentException) {
// TODO Auto-generated catch block
e.printStackTrace()
} catch (e: IOException) {
e.printStackTrace()
}
}
private fun interpolate(gc: GridCoverage2D): GridCoverage {
val interp = Interpolation.getInstance(Interpolation.INTERP_BILINEAR)
val interpolate = Operations.DEFAULT.interpolate(gc, interp)
return interpolate as GridCoverage2D
}
private fun createGridCoverage(
factory: GridCoverageFactory,
writableRaster: WritableRaster,
bounds: ReferencedEnvelope
): GridCoverage2D {
val gc: GridCoverage2D = factory.create("band1", writableRaster, bounds)
return gc
}
private fun createRaster(width: Int, height: Int): WritableRaster {
val writableRaster: WritableRaster = RasterFactory.createBandedRaster(
DataBuffer.TYPE_DOUBLE, width,
height, 1, null
)
return writableRaster
}
private fun fillRaster(
pointWithDatas: List<PointWithData>,
gc: GridCoverage2D,
writableRaster: WritableRaster
) {
pointWithDatas.forEach { pwd ->
val directPosition = DirectPosition2D(pwd.point.x, pwd.point.y)
val transformPointToGridCoverage = gc.gridGeometry.worldToGrid(directPosition)
val data = DoubleArray(1)
data[0] = pwd.data
writableRaster.setDataElements(transformPointToGridCoverage.x, transformPointToGridCoverage.y, data)
}
}
}
Polygon
Vector Data
Result from the code
Result i'm looking for (created manual with qgis)