Convert spectral values between declared units

The only supported ways to change value semantics. Raw counts become reflectance through an explicit dark/white calibration, and reflectance becomes (pseudo-)absorbance as -log10(reflectance). Anything else, or a conversion without the calibration it needs, is refused with a classified error instead of being approximated.

Usage

at_convert_values(
  x,
  from = NULL,
  to = c("reflectance", "absorbance"),
  calibration = NULL,
  call = rlang::caller_env()
)

Arguments

x

A numeric vector, matrix or [y, x, band] array of values.

from

The unit of x: one of "raw", "reflectance", "radiance", "intensity", "absorbance". Taken from attr(x, "value_unit") when NULL.

to

Target unit: "reflectance" or "absorbance".

calibration

For raw -> reflectance, a list with numeric white and dark references (scalars, or one value per band for arrays).

call

The calling environment, for error reporting.

Value

x converted, with attribute value_unit set to to and conversion recording the operation. Reflectance values <= 0 give NA absorbance; a zero white-minus-dark denominator gives NA.

See also

Other hsi: at_band_operations(), at_band_view(), at_cubert_export_envi(), at_hsi_meta(), at_read_stats(), at_tivita_profile()

Examples

at_convert_values(c(0.5, 0.1), from = "reflectance", to = "absorbance")
#> [1] 0.30103 1.00000
#> attr(,"value_unit")
#> [1] "absorbance"
#> attr(,"conversion")
#> [1] "reflectance->absorbance"
at_convert_values(c(100, 550, 1000), from = "raw", to = "reflectance",
                  calibration = list(white = 1000, dark = 100))
#> [1] 0.0 0.5 1.0
#> attr(,"value_unit")
#> [1] "reflectance"
#> attr(,"conversion")
#> [1] "raw->reflectance"