ras_palette - format descriptions for palette files for NCAR View
NCAR View supports a binary as well as a textual format for color palettes.
The binary format is compatible with NCSA HDF and must have a file extension of ".pal". The palette has a red, a green, and a blue table, each with 256 single unsigned byte entries. A 0 represents no intensity for a given color, and a 255 full intensity. In the file, the palette is stored as:
R0 R1 ... R255 G0 G1 ... G255 B0 B1 ... B255
for a total of 768 bytes.
The textual format can be created by a program or manually with a text editor. It provides for 256 RGB entries but also allows for a sparse color table. In the latter case, missing values are calculated from those provided using linear interpolation. If unspecified, the 0 entry defaults to black and the 255 entry to white. Each color palette entry has the following format:
index_number red green blue
where index_number, red, green, and blue are all integers in the range 0 to 255.
All of the examples below deal with textual color palettes.
- Set color index 0 to black.
0 0 0 0
- Set color index 255 to white
255 255 255 255
- Let's suppose you have a file named "my.txt" and it has just two
lines in it: the black entry and the white entry from above. This will
produce a grayscale color palette, where the table starts at black and
ramps up to white; since only entries 0 and 255 are specified, the
remaining indicies are interpolated.
0 0 0 0 255 255 255 255
- (Note: An empty palette file (e.g. "empty.txt") is legal and, since the 0 index defaults to black and the 255 index to white, produces a grayscale ramp just as in the above example.)
- Let's suppose you want entry 0 to be black, and the remaining indices to
start at 1 with a fully-saturated blue and shift linearly to 255 with a
fully-saturated red. Only three entries are required in your file,
0 0 0 0 1 0 0 255 255 255 0 0
Color palettes are only useful with indexed-color imagery.
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