[gdal-dev] gdal2tiles - transparent area on base image becomes opaque in tiles

[easyl]

open base_mercator.vrt 
it should looks like

<VRTDataset>
  ...
  <VRTRasterBand dataType="Byte" band="1" subClass="VRTWarpedRasterBand"/>
  <VRTRasterBand dataType="Byte" band="2" subClass="VRTWarpedRasterBand"/>
  <VRTRasterBand dataType="Byte" band="3" subClass="VRTWarpedRasterBand"/>
  <VRTRasterBand dataType="Byte" band="4" subClass="VRTWarpedRasterBand"/>
   ...
    <BandList>
      <BandMapping src="1" dst="1"/>
      <BandMapping src="2" dst="2"/>
      <BandMapping src="3" dst="3"/>
    </BandList>
    <SrcAlphaBand>4</SrcAlphaBand>
  </GDALWarpOptions>
</VRTDataset>

Try to manually modify base_mercator.vrt 
<VRTDataset>
  ...
  <VRTRasterBand dataType="Byte" band="1" subClass="VRTWarpedRasterBand">
    <Metadata/>
    <ColorInterp>Red</ColorInterp>
  </VRTRasterBand>
  <VRTRasterBand dataType="Byte" band="2" subClass="VRTWarpedRasterBand">
    <Metadata/>
    <ColorInterp>Green</ColorInterp>
  </VRTRasterBand>
  <VRTRasterBand dataType="Byte" band="3" subClass="VRTWarpedRasterBand">
    <Metadata/>
    <ColorInterp>Blue</ColorInterp>
  </VRTRasterBand>
  <VRTRasterBand dataType="Byte" band="4" subClass="VRTWarpedRasterBand">
    <Metadata/>
    <ColorInterp>Alpha</ColorInterp>
  </VRTRasterBand>
  ...
  <GDALWarpOptions>
      ...
    <BandList>
      <BandMapping src="1" dst="1"/>
      <BandMapping src="2" dst="2"/>
      <BandMapping src="3" dst="3"/>
      <BandMapping src="4" dst="4"/>
    </BandList>
  </GDALWarpOptions>
</VRTDataset>

and run "gdal2tiles.py --zoom=0-9 --no-kml --webviewer=openlayers
base_mercator.vrt" again

Which options should I pass into gdalwarp to generate correct band list?

btw, MapTiler generates tiles with gray boundary as well...
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