Problem solved.
RasterRenderingInfo rasterRenderingInfo;
mdlRaster_renderingInfoGet(&rasterRenderingInfo, raster);
rasterRenderingInfo.imageTransparency = transp;
rasterRenderingInfo.transparency = TRUE;
mdlRaster_renderingInfoSet(&rasterRenderingInfo, raster, EVENT_UPDATE);mdlView_updateSingle(0);
Peter