Transparency not being rendered in tiles and preview

[solarissmoke]

Hi there! Thanks so much for this library. I am seeing some peculiar behaviour with handling of the alpha band in a tiff - where the tiler seems to handle transparency correctly for some pixels, but not for others. Here is the /info for the tiff:

{
   ...
  "crs": "http://www.opengis.net/def/crs/EPSG/0/3857",
  "band_metadata": [
    ["b1", {}],
    ["b2", {}],
    ["b3", {}],
    ["b4", {}]
  ],
  "band_descriptions": [
    ["b1", ""],
    ["b2", ""],
    ["b3", ""],
    ["b4", ""]
  ],
  "dtype": "uint8",
  "nodata_type": "Alpha",
  "colorinterp": [
    "red",
    "green",
    "blue",
    "alpha"
  ],
  "scales": [1.0, 1.0, 1.0, 1.0],
  "offsets": [0.0, 0.0, 0.0, 0.0],
  "driver": "GTiff",
  "count": 4,
  "width": 12288,
  "height": 12288,
  "overviews": [2, 4, 8, 16, 32, 64]
}

And here is the preview generated by titiler:

You can see a whole load of black areas, which are transparent when viewed in GQIS. After poking about I found that the areas that are being made transparent have value (0, 0, 0, 0), and those that are black have some small alpha value (e.g., 0, 0, 0, 1). It seem that this small difference alters the output when the expected behaviour is for both (0, 0, 0, 0) and (0, 0, 0, 1) is essentially the same - both should be pretty much transparent.

That is, an alpha value of 1 is resulting in a black pixel, instead of a transparent one.

I found #643 which seems to be a similar issue, but I am not sure that nodata values are the problem here.

Is there an issue with the COG / what can I do to fix it?

Thank you very much.

[vincentsarago]

rio-tiler doesn't not support partial alpha values. Alpha band should either be 0 or 255.

[solarissmoke]

Thanks a lot for your prompt response - much appreciated.