Data Not Visible At Certain Zoom Level When Applying Custom Algorithm

[dimplejaiin]

I've written a custom algorithm in Titiler to apply offset correction for Sentinel-2 datasets fetched from the Planetary STAC catalog. While the algorithm works, I'm encountering an issue where the data becomes invisible at certain zoom level. Sharing a recording of the issue in attachments and brief overview of the code and the steps to reproduce the error.

Steps to Reproduce:

1. Implement the following custom OffsetCorrection algorithm:

   import re
   from datetime import datetime
   from typing import ClassVar
   
   import numpy as np
   from rio_tiler.models import ImageData
   from titiler.core.algorithm.base import BaseAlgorithm
   
   class OffsetCorrection(BaseAlgorithm):
       """Offset Correction Algorithm
       Reference: https://planetarycomputer.microsoft.com/dataset/sentinel-2-l2a#Baseline-Change
       """
   
       valid_bands: ClassVar[list[str]] = [
           "B01", "B02", "B03", "B04", "B05",
           "B06", "B07", "B08", "B8A", "B09",
           "B10", "B11", "B12",
       ]
       cutoff_date: ClassVar[datetime] = datetime(2022, 1, 25)
   
       def is_valid_asset(self, asset_url: str) -> bool:
           if "sentinel2" not in asset_url:
               return False
   
           date_match = re.search(r"/(\d{4}/\d{2}/\d{2})/", asset_url)
           if not date_match:
               return False
   
           date_str = date_match.group(1)
           image_date = datetime.strptime(date_str, "%Y/%m/%d")
   
           return image_date > self.cutoff_date
   
       def apply_offset_correction(
           self, asset_url: str, band_array: np.ndarray, band_name: str
       ) -> np.ndarray:
           if not self.is_valid_asset(asset_url):
               return band_array
   
           if band_name in self.valid_bands:
               print(f"Applying offset correction to band {band_name}")
               out_img_subtracted = band_array.astype(np.int32) - 1000
               corrected_array = np.clip(out_img_subtracted, 0, None).astype(np.uint16)
           else:
               print(f"Band name {band_name} is not in the list of valid bands")
               corrected_array = band_array
   
           return corrected_array
   
       def __call__(self, img: ImageData) -> ImageData:
           band_names = [
               img.metadata[band]["eo:bands"][0]["name"] for band in img.metadata
           ]
           corrected_arrays = [
               self.apply_offset_correction(asset, band_array, band_name)
               for asset, band_array, band_name in zip(img.assets, img.array, band_names)
           ]
   
           corrected_array_stack = np.stack(corrected_arrays, axis=0)
   
           return ImageData(
               corrected_array_stack,
               assets=img.assets,
               crs=img.crs,
               bounds=img.bounds,
               band_names=img.band_names,
               metadata=img.metadata,
               cutline_mask=img.cutline_mask,
           )

2. Use the following tile URL to request tiles:

   https://api/stac/stac/tiles/WebMercatorQuad/x/y/z.webp?url=https://planetarycomputer.microsoft.com/api/stac/v1/collections/sentinel-2-l2a/items/S2B_MSIL2A_20190919T184039_R070_T11SKT_20201004T150757&resampling_method=nearest&rescale=0%2C5000&nodata=0&algorithm=offset-correction&assets=B04&assets=B03&assets=B02
   

Observed Issue:

• The tiles render correctly at certain zoom levels.
• The data becomes invisible after zoom out from certain level.

Attachments:

• Video of the Issue

Request for Assistance:
Any insights or recommendations would be greatly appreciated!

[dimplejaiin]

This issue resolved with masking the array:

data = np.ma.MaskedArray(corrected_array_stack)
data.mask = ~img.mask