[Experiment] sketch algorithm support

[vincentsarago]

This PR sketchs out support for complex algorithm (advanced expression)

TL&DR

we introduce the concept of the Algorithm base class

class BaseAlgorithm():
    """Algorithm baseclass."""

    input_nbands: int

    output_nbands: int
    output_dtype: str
    output_min: Optional[Sequence]
    output_max: Optional[Sequence]

    @abc.abstractmethod
    def apply(self, img: ImageData) -> ImageData:
        """Apply"""
        ...

The base class has some attribute used for metadata (what inputs It needs and what outputs it will produce).

which is then used to create custom algorithm with an apply method that takes an ImageData object as input and return another ImageData object (

rio-viz/rio_viz/app.py

Lines 306 to 307 in 68a6b6b

	if postprocess_params.image_process:
	img = postprocess_params.image_process.apply(img)

)

class HillShade(BaseAlgorithm):
    """Hillshade."""

    # User defined options
    azimuth: int = 90
    angle_altitude: float = 90

    # Algo metadata 
    input_nbands: int = 1
    output_nbands: int = 1
    output_dtype: str = "uint8"

    def apply(self, img: ImageData) -> ImageData:
        """Create hillshade from DEM dataset."""
        data = img.data[0]
        mask = img.mask

        x, y = numpy.gradient(data)

        slope = numpy.pi / 2.0 - numpy.arctan(numpy.sqrt(x * x + y * y))
        aspect = numpy.arctan2(-x, y)
        azimuthrad = self.azimuth * numpy.pi / 180.0
        altituderad = self.angle_altitude * numpy.pi / 180.0
        shaded = numpy.sin(altituderad) * numpy.sin(slope) + numpy.cos(
            altituderad
        ) * numpy.cos(slope) * numpy.cos(azimuthrad - aspect)
        hillshade_array = 255 * (shaded + 1) / 2

        # ImageData only accept image in form of (count, height, width)
        arr = numpy.expand_dims(hillshade_array, axis=0).astype(dtype=numpy.uint8)

        return ImageData(
            arr,
            mask,
            assets=img.assets,
            crs=img.crs,
            bounds=img.bounds,
        )

http://127.0.0.1:8080/preview?algo=contours&algo_params={"minz":1600,"maxz":2100,"increment":10}

http://127.0.0.1:8080/preview?algo=hillshade&algo_params={"angle_altitude":20}

[vincentsarago]

Those are metadata about the input/outputs of the algorithm

[vincentsarago]

additional parameters for the algorithm must be provided as JSON encoded algo_params query parameter

[vincentsarago]

TODO: add frontend support

[vincentsarago]

This was integrated directly in TiTiler

[jo-chemla]

TODO: add frontend support

Frontend support for algorithms as well as custom expression added in PR to titiler /cog/viewer developmentseed/titiler#1119 for anyone interested