## vrt.derive()


Derive bands from rasters or VRTs with a GDAL pixel function.


Usage

``` python
vrt.derive(
    sources,
    function,
    *,
    dtype="float32",
    nodata=math.nan,
    description="",
    band=None
)
```


## Parameters


`sources: str | Path | UPath | VRT | Sequence[str | Path | UPath | VRT]`  
One input, or several on an identical grid. A [VRT](vrt.VRT.md#freezebase.vrt.VRT) is nested whole, so the function sees its mosaicked pixels.

`function: PixelFunction`  
Function applied to the sources. `propagateNoData` is set to `"true"` unless its arguments already set it, so a nodata input gives a nodata output.

`dtype: str = ``"float32"`  
NumPy dtype name of the derived bands.

`nodata: float | None = math.nan`  
NODATA of the derived bands.

`description: str = ``""`  
Description of every derived band.

`band: int | None = None`  
1-based band read from each input. With several inputs it defaults to 1; with one input, it derives only that band instead of every band.


## Returns


`VRT`  
With one input, a derived band per input band; with several, one derived band. Each source carries its input's NODATA, and `transfer_type` defaults to `dtype`.


## Raises


`ValueError`  
If `nodata` does not fit in `dtype`, [sources](vrt.VRT.md#freezebase.vrt.VRT.sources) is empty, the grids differ (use [warp](vrt.warp.md#freezebase.vrt.warp) first), `band` is out of range, or an input band is complex or has a scale or offset.