DESCRIPTION

i.hyper.rgb creates RGB or CMYK composites from hyperspectral imagery imported as 3D raster maps (raster_3d) by i.hyper.import.

The module reads wavelength metadata from hyperspectral 3D raster bands and automatically selects the bands closest to specified target wavelengths for each color channel. Users can choose between RGB (red-green-blue) and CMYK (cyan-magenta-yellow-key) color spaces.

i.hyper.rgb is part of the i.hyper module family designed for hyperspectral data import, processing, and analysis in GRASS. It complements i.hyper.composite by providing color space conversions with advanced statistical band selection and accessibility features.

The module offers multiple statistical methods for band selection when multiple bands fall within the target wavelength range:

• mean -- Average of candidate bands
• median -- Median value of candidate bands
• mode -- Most frequent value of candidate bands
• min -- Minimum value of candidate bands
• max -- Maximum value of candidate bands
• sd1_pos, sd2_pos, sd3_pos -- Mean plus 1, 2, or 3 standard deviations
• sd1_neg, sd2_neg, sd3_neg -- Mean minus 1, 2, or 3 standard deviations

Color blind safe palette adjustments are available for protanopia (red-blind), deuteranopia (green-blind), and tritanopia (blue-blind) vision deficiencies, improving accessibility of hyperspectral visualizations.

The resulting output consists of individual raster maps for each color channel (e.g., output_red, output_green, output_blue) organized into an image group (output_rgb or output_cmyk) for convenient display and further processing.

NOTES

Band extraction uses Rast3d_extract_z_slice() from libgrass_raster3d, called via ctypes. The 3D map is opened with RASTER3D_NO_CACHE, which forces Rast3d_get_block() into the tile-bulk path: all tiles at the target depth level are read in a single pass, each exactly once, instead of spawning r3.to.rast and manipulating the computational region for every channel.

The module expects input data to be a 3D raster map created by i.hyper.import or any 3D raster with wavelength metadata stored in band-level metadata following the i.hyper standard format: wavelength, FWHM, valid, and unit.

When wavelength metadata is not found, the module falls back to using band indices as wavelength values, which may produce unexpected results. It is recommended to use properly imported hyperspectral data.

The -n flag normalizes output bands to the 0-255 range, suitable for 8-bit display formats. Without normalization, output bands retain their original reflectance or radiance values.

Default wavelengths are chosen to match typical RGB color perception:

• Red: 650 nm
• Green: 550 nm
• Blue: 450 nm

For CMYK composites:

• Cyan: 490 nm
• Magenta: 580 nm
• Yellow: 570 nm
• Key (black): 800 nm (near-infrared for contrast)

These defaults can be customized using the wavelength options to match specific sensor characteristics or visualization needs.

Color blind adjustments apply transformation matrices to the output channels, simulating how the composite would appear to individuals with different types of color vision deficiency. This feature helps ensure that visualizations convey information effectively to all viewers.

EXAMPLES

::: code

# Create a standard RGB composite from PRISMA data
i.hyper.rgb input=prisma \
            output=prisma_composite \
            colorspace=rgb \
            statistic=mean

# Result: prisma_composite_red, prisma_composite_green, prisma_composite_blue
# Image group: prisma_composite_rgb

:::

::: code

# Create a normalized RGB composite suitable for display
i.hyper.rgb input=enmap \
            output=enmap_display \
            colorspace=rgb \
            statistic=mean \
            -n

# Display the composite
d.rgb red=enmap_display_red \
      green=enmap_display_green \
      blue=enmap_display_blue

:::

::: code

# Create a deuteranopia-adjusted RGB composite
# Useful for ensuring visualization accessibility
i.hyper.rgb input=tanager \
            output=tanager_colorblind \
            colorspace=rgb \
            statistic=median \
            colorblind=deuteranopia \
            -n

:::

::: code

# Create CMYK composite with custom wavelengths
i.hyper.rgb input=prisma \
            output=prisma_cmyk \
            colorspace=cmyk \
            cyan_wavelength=480 \
            magenta_wavelength=590 \
            yellow_wavelength=560 \
            key_wavelength=850 \
            statistic=mean

:::

::: code

# Use standard deviation for enhanced contrast
i.hyper.rgb input=enmap \
            output=enmap_enhanced \
            colorspace=rgb \
            statistic=sd2_pos \
            red_wavelength=660 \
            green_wavelength=560 \
            blue_wavelength=470 \
            -n

:::

::: code

# Create multiple versions for comparison
# Standard RGB
i.hyper.rgb input=hyperspectral output=standard colorspace=rgb -n

# Protanopia-adjusted
i.hyper.rgb input=hyperspectral output=protanopia colorspace=rgb \
            colorblind=protanopia -n

# Tritanopia-adjusted
i.hyper.rgb input=hyperspectral output=tritanopia colorspace=rgb \
            colorblind=tritanopia -n

:::

SEE ALSO

i.hyper.import, i.hyper.composite, i.hyper.preproc, i.hyper.explore, i.hyper.export, d.rgb, i.group, r3.support, r.rescale

REFERENCES

• Viénot, F., Brettel, H., & Mollon, J. D. (1999). Digital video colourmaps for checking the legibility of displays by dichromats. Color Research & Application, 24(4), 243-252.
• Machado, G. M., Oliveira, M. M., & Fernandes, L. A. (2009). A physiologically-based model for simulation of color vision deficiency. IEEE Transactions on Visualization and Computer Graphics, 15(6), 1291-1298.

AUTHORS

Created for the i.hyper module family

Based on work by Alen Mangafić and Tomaž Žagar, Geodetic Institute of Slovenia