Rendering issue when shading near boundary/threshold

[mgrunbauer]

We're trying to render a parameter with values near thresholds. Adaguc renders an image with strange "blocky" artifacts:

We're using the following layer configuration:

  <Layer type="database">
    <Name>wstar_ustar_ratio_atm</Name>
    <Title>UWCW_HA43(NL) W*/U* at 0 </Title>
    <Variable>wstar-ustar-ratio-atm</Variable>
    <FilePath maxquerylimit="2048" retentionperiod="{ADAGUCENV_RETENTIONPERIOD}" retentiontype="datatime" filter="(uwcw_ha43_nl_2km_wstar-ustar-ratio-atm_geoweb_nc_[0-9T]{11}\.nc)">/data/adaguc-data/arcusuwcw/</FilePath>
    <Styles>wstar_ustar_ratio</Styles>
    <Dimension name="time" units="ISO8601" interval="PT1H" default="min">time</Dimension>
    <Dimension name="wstar_ustar_ratio_at_atm" units=" " default="min" hidden="true">atmosphere</Dimension>
    <Dimension name="forecast_reference_time" units="ISO8601">reference_time</Dimension>
  </Layer>

And the following styling:

  <Style name="wstar_ustar_ratio">
    <ShadeInterval min="0" max="2" label="0-2" fillcolor="#FFFF00"/>
    <ShadeInterval min="2" max="4" label="2-4" fillcolor="#00FF00"/>
    <ShadeInterval min="4" max="6" label="4-6" fillcolor="#009B28"/>
    <ShadeInterval min="6" max="7" label="6-7" fillcolor="#00FFFF"/>
    <ShadeInterval min="7" max="8" label="7-8" fillcolor="#7F8EFF"/>
    <ShadeInterval min="8" max="9" label="8-9" fillcolor="#5D00FF"/>
    <ShadeInterval min="9" max="10" label="9-10" fillcolor="#FF00FF"/>
    <ShadeInterval min="10" max="12" label="10-12" fillcolor="#FF7A00"/>
    <ContourLine width="1" linecolor="#444444" textcolor="#444444" textformatting="%2.0f" classes="2,4,6,7,8,9,10,12"/>
    <RenderMethod>shaded,shadedcontour</RenderMethod>
    <NameMapping name="shaded" title="Filled contours" abstract="Filled background"/>
    <NameMapping name="shadedcontour" title="Filled contours and lines" abstract="Contour lines with filled background"/>
    <SmoothingFilter>0</SmoothingFilter>
    <StandardNames standard_name="wstar_ustar_ratio"/>
  </Style>

We've tried to use a different RenderSetting:

  <RenderSettings renderhint="discreteclasses" settings="fast" />

But the result was quite "grainy" and not what we want.