The raster option allows user to specify a raster map name from which to copy the color map.
The raster_3d option allows user to specify a 3D raster map name from which to copy the color map.
The -e flag equalizes the original raster's color table. It can preclude the need for grey.eq rule, when used as -e color=grey. Note however, that this will not yield a color table identical to color=grey.eq, because grey.eq scales the fraction by 256 to get a grey level, while -e uses it to interpolate the original color table. If the original color table is a 0-255 grey scale, -e is effectively scaling the fraction by 255. Different algorithms are used. -e is designed to work with any color table, both the floating point and the integer raster maps.
The -g flag divides the raster's grey value range into 100 logarithmically equal steps (where "step" is a rule with the same grey level for the start and end points). It can preclude the need for grey.log rule, when used as -g color=grey. Note however, that this will not yield a color table identical to color=grey.log. Different algorithms are used. Unlike color=grey.log, -g is designed to work with both floating point and integer rasters, without performance issues with large datasets, of any original color table. Logarithmic scaling doesn't work on negative values. In the case when the value range includes zero, there's no realistic solution.
The -e and -g flags are not mutually exclusive.
offset and scale modify color rules to match the values of a raster map using the formula new_value = (old_value + offset) x scale. For example, if the units of a raster map are Kelvin x 50, the color rules celsius can be applied with offset=273.15 and scale=50.
If the user specifies the -w flag, the current color table file for the input map will not be overwritten. This means that the color table is created only if the map does not already have a color table. If this option is not specified, the color table will be created if one does not exist, or modified if it does.
Color table types aspect, grey, grey.eq (histogram-equalized grey scale), byg (blue-yellow-green), byr (blue-yellow-red), gyr (green-yellow-red), rainbow, ramp, ryg (red-yellow-green), random, and wave are pre-defined color tables that r.colors knows how to create without any further input.
In case several input raster maps are provided the range (min, max) of all maps will be used for color table creation. Hence the created color table will span from the smallest minimum to the largest maximum value of all input raster maps and will be applied to all input raster maps.
In general, tables which associate colors with percentages (aspect, bcyr, byg, byr, elevation, grey, gyr, rainbow, ramp, ryb, ryg and wave) can be applied to any data, while those which use absolute values (aspectcolr, curvature, etopo2, evi, ndvi, population, slope, srtm, and terrain) only make sense for data with certain ranges. One can get a rough idea of the applicability of a colour table by reading the corresponding rules file ($GISBASE/etc/colors/<name>). For example the slope rule is defined as:
0 255:255:255 2 255:255:0 5 0:255:0 10 0:255 255 15 0:0:255 30 255:0:255 50 255:0:0 90 0:0:0
This is designed for the slope map generated by r.slope.aspect, where the value is a slope angle between 0 and 90 degrees.
Similarly, the aspectcolr rule:
0 white 1 yellow 90 green 180 cyan 270 red 360 yellow
is designed for the aspect maps produced by r.slope.aspect, where the value is a heading between 0 and 360 degrees.
The rules color table type will cause r.colors to read color table specifications from standard input (stdin) and will build the color table accordingly.
Using color table type rules, there are two ways to build a color table: by category values and by "percent" values.
To build a color table by category values' indices, the user should determine the range of category values in the raster map with which the color table will be used. Specific category values will then be associated with specific colors. Note that a color does not have to be assigned for every valid category value because r.colors will interpolate a color ramp to fill in where color specification rules have been left out. The format of such a specification is as follows:
category_value color_name category_value color_name .. .. .. .. category_value color_name end
Each category value must be valid for the raster map, category values must be in ascending order and only use standard GRASS color names (aqua, black, blue, brown, cyan, gray, green, grey, indigo, magenta, orange, purple, red, violet, white, yellow).
Colors can also be specified by color numbers each in the range 0-255. The format of a category value color table specification using color numbers instead of color names is as follows:
category_value red_number:green_number:blue_number category_value red_number:green_number:blue_number .. .. .. .. .. .. .. .. category_value red_number:green_number:blue_number end
Specifying a color table by "percent" values allows one to treat a color table as if it were numbered from 0 to 100. The format of a "percent" value color table specification is the same as for a category value color specification, except that the category values are replaced by "percent" values, each from 0-100, in ascending order. The format is as follows:
percent_value% color_name percent_value% color_name .. .. .. .. percent_value% color_name end
Using "percent" value color table specification rules, colors can also be specified by color numbers each in the range 0-255. The format of a percent value color table specification using color numbers instead of color names is as follows:
percent_value% red_number:green_number:blue_number percent_value% red_number:green_number:blue_number .. .. .. .. .. .. .. .. percent_value% red_number:green_number:blue_number end
Note that you can also mix these two methods of color table specification; for example:
0 black 10% yellow 78 blue 100% 0:255:230 end
To set the NULL (no data) color, use the "nv" (null values) parameter:
0 black 10% yellow nv white end
To set the color to used for undefined values (beyond the range of the color rules) use the "default" parameter:
0 red 1 blue default grey end
The color table assigned to a raster map is stored in $GISDBASE/location/mapset/colr/.
1 red 2 green 3 blue end
The color table can then by assigned to map threecats by the following GRASS commands (two ways are available):
# read input from stdin cat rules.file | r.colors map=threecats rules=- # read directly from file r.colors map=threecats rules=rules.file
To create a natural looking lookup table (LUT) for true map layer elevation, use the following rules specification file. It will assign light green shades to the lower elevations (first 20% of the LUT), and then darker greens (next 15%, and next 20%) and light browns (next 20%) for middle elevations, and darker browns (next 15%) for higher elevations, and finally yellow for the highest peaks (last 10% of LUT).
0% 0:230:0 20% 0:160:0 35% 50:130:0 55% 120:100:30 75% 120:130:40 90% 170:160:50 100% 255:255:100
To invert the current rules:
r.colors map=current_raster -n rast=current_raster
See also wiki page Color tables (from GRASS User Wiki)
ColorBrewer is an online tool designed to help people select good color schemes for maps and other graphics.
Available at: r.colors source code (history)
Latest change: Monday Feb 07 22:06:13 2022 in commit: 150a0e9242b9f01b7bf0fcfa46dc753fb75ed02e
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