DESCRIPTION

i.rgb.hsl converts three input images red, green and blue, being components in the RGB color space, into hue, saturation and lightness dimensions of the HSL color space.

Each output image is given a linear gray scale color table. The current geographic region and mask settings are respected.

EXAMPLES

8-bit

The digital numbers of the 8-bit Landsat 7 bands red, green and blue, range between 0 and 255:

for BAND in 10 20 30 ;do echo `echo -e "Band ${BAND}: " && r.info -r lsat7_2002_$BAND` ;done

Band 10: min=42 max=255
Band 20: min=28 max=255
Band 30: min=1 max=255

Coverting these bands to hue, saturation and lightness:

i.rgb.hsl r=lsat7_2002_30 g=lsat7_2002_20 bl=lsat7_2002_10 hue=h lightness=l saturation=s

The output images range within:

for DIMENSION in h s l; do echo `echo "${DIMENSION}:" && r.info -r ${DIMENSION}` ;done

h: min=0 max=359.434
s: min=0 max=1
l: min=0.08431373 max=1

Note, the bits option is set to 8 by default. Since the input images, in this example, are 8-bit, there is no need to modify it.

Converting back to the original RGB color space using i.hsl.rgb:

i.hsl.rgb hue=h saturation=s lightness=l r=red g=green bl=blue

Confirming that the range of the values of the output images red, green and blue, is identical to the range of the input Landsat 7 bands:

for IMAGE in red green blue ;do echo `echo -e "${IMAGE}: " && r.info -r $IMAGE` ;done

red: min=0 max=255
green: min=28 max=255
blue: min=42 max=255

11-bit

For 11-bit sensors, like QuickBird2 [0], set bits=11. For example:

i.rgb.hsl r=Red g=Green bl=Blue h=hue s=saturation l=lightness bits=11

16-bit

The Landsat 8 instruments OLI and TIRS are capable of 12-bit. Products are, however, delivered as 16-bit images (scaled to 55,000 grey levels). [0]

Therefore, bitness needs to be set to 16, like:

i.rgb.hsl red=B4 green=B3 blue=B2 h=hue s=saturation l=lightness bits=16

Arbitrary lower bit depths

The module allows for data of less than 8-bit. For example, data from the DMSP Operational Linescan System (OLS) nighttime lights time series, which feature 6-bit sensors [2].

For example:

i.rgb.his r=F152007 g=F152005 bl=F152004 h=h s=s l=l bits=6

References

[0]

[1] http://landsat.usgs.gov/landsat8.php

[2] http://ngdc.noaa.gov/eog/sensors/ols.html

AUTHOR

Nikos Alexandris