Pamlookup User Manual(0) Pamlookup User Manual(0)
pamlookup - map an image to a new image by using it as indices into a
pamlookup -lookupfile=lookupfile -missingcolor=color [-fit] indexfile
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one. You may separate an option name
and its value with white space instead of an equals sign.
This program is part of Netpbm(1).
pamlookup takes a two dimensional array of indices and a lookup table
as input. For each position in the index array, it looks up the index
in the lookup table and places the result of the lookup in the output
image. The output thus has the same width and height as the index
image, and tuple types determined by the lookup table.
An index is either a whole number or an ordered pair of whole numbers.
If the index image has a depth of one, each index in it is a whole num-
ber: the value of the one sample. If the index image has a depth
greater than one, each index in it is an ordered pair of the first and
second samples in the relevant tuple.
The lookup table is a PAM or PNM image. If the index image contains
whole number indices, the lookup image is a single row and the index is
a column number. The lookup result is the value of the tuple or pixel
at the indicated column in the one row in the lookup table. If the
index image contains ordered pair indices, the first element of the
ordered pair is a row number and the second element of the ordered pair
is a column number. The lookup result is the value of the tuple or
pixel at the indicated row and column in the lookup table.
For example: Consider an index image consisting of a 3x2x1 PAM as fol-
0 1 0
2 2 2
and a lookup table consisting of a 3x1 PPM image as follows:
red yellow beige
The lookup table above says Index 0 corresponds to the color red, Index
1 corresponds to yellow, and Index 2 corresponds to beige. The output
of pamlookup is the following PPM image:
red yellow red
beige beige beige
Now let’s look at an example of the more complex case where the indices
are ordered pairs of whole numbers instead of whole numbers. Our index
image will be this 3x2x2 PAM image:
(0,0) (0,1) (0,0)
(1,1) (1,0) (0,0)
Our lookup table for the example will be this two dimensional PPM:
This lookup table says Index (0,0) corresponds to the color red, Index
(0,1) corresponds to yellow, Index (1,0) corresponds to green, and
Index (1,1) corresponds to black. The output of pamlookup is the fol-
lowing PPM image:
red yellow red
black green red
If an index specifies a row or column that exceeds the dimensions of
the lookup table image, pamlookup uses the value from the top left cor-
ner of the lookup image, or the value you specify with the -missing-
The indexfile argument identifies the file containing the index PAM or
PNM image. - means Standard Input. The mandatory -lookupfile option
identifies the file containing the lookup table image. Again, - means
Standard Input. It won’t work if both the index image file and lookup
table file are Standard Input. The output image goes to Standard Out-
You can use ppmmake and pnmcat to create a lookup table file.
If you want to use two separate 1-plane images as indices (so that your
output reflects the combination of both inputs), use pamstack to com-
bine the two into one two-plane image (and use a 2-dimensional lookup
lookupfile names the file that contains the PAM or PNM image
that is the lookup table. This option is mandatory.
This option is meaningful only if the lookup image (and there-
fore the output) is a PNM image. color specifies the color that
is to go in the output wherever the index from the input is not
present in the lookup table (not present means the index exceeds
the dimensions of the lookup image -- e.g. index is 100 but the
lookup image is a 50 x 1 PPM).
If you don’t specify this option of -fit, pamlookup uses the
value from the top left corner of the lookup image whenever an
index exceeds the dimensions of the lookup image.
Specify the color (color) as described for the argument of the
ppm_parsecolor() library routine .
Another way to deal with a too-small lookup image is to use the
-fit This option says to shrink or expand the lookup image as neces-
sary to fit the indices present in the index image, per the
index image’s maxval. For example, if your index image has a
single plane and a maxval of 255 and your lookup image is 1 row
of 10 columns, pamlookup stretches your lookup image to 255
columns before doing the lookups. pamlookup does the stretching
(or shrinking) with the pamscale(1) program.
When you use -fit, pamlookup never fails or warns you due to
invalid lookup image dimensions, and the -missingcolor option
has no effect.
Example: rainfall map
Say you have a set of rainfall data in a single plane PAM image. The
rows and columns of the PAM indicate lattitude and longitude. The sam-
ple values are the annual rainfall in (whole) centimeters. The highest
rainfall value in the image is 199 centimeters. The image is in the
You want to produce a PPM rainfall map with green for the wettest
places, red for the driest, and other colors in between.
First, compose a lookup table image, probably with a graphical editor
and the image blown way up so you can work with individual pixels. The
image must have a single row and 200 columns. Make the leftmost pixel
red and the rightmost pixel green and choose appropriate colors in
between. Call it colorkey.ppm.
pamlookup rainfall.ppm -lookupfile=colorkey.ppm >rainfallmap.ppm
Now lets say you’re too lazy to type in 200 color values and nobody
really cares about the places that have more than 99 centimeters of
annual rainfall. In that case, just make colorkey.ppm 100 columns wide
and do this:
pamlookup rainfall.ppm -lookupfile=colorkey.ppm -missingcolor=black >rainfallmap.ppm
Now if there are areas that get more than 100 centimeters of rainfall,
they will just show up black in the output.
Example: graphical diff
Say you want to compare two PBM (black and white) images visually.
Each consists of black foreground pixels on a white background. You
want to create an image that contains background where both images con-
tain background and foreground where both images contain foreground.
But where Image 1 has a foreground pixel and Image 2 does not, you want
red in the output; where Image 2 has a foreground pixel and Image 1
does not, you want green.
First, we create a single image that contains the information from both
pamstack image1.pbm image2.pbm >bothimages.pam
Note that this image has 1 of 4 possible tuple values at each location:
(0,0), (0,1), (1,0), or (1,1).
Now, we create a lookup table that we can index with those 4 values:
ppmmake white 1 1 >white.ppm
ppmmake black 1 1 >black.ppm
ppmmake red 1 1 >red.ppm
ppmmake green 1 1 >green.ppm
pnmcat -leftright black.ppm red.ppm >blackred.ppm
pnmcat -leftright green.ppm white.ppm >greenwhite.ppm
pnmcat -topbottom blackred.ppm greenwhite.ppm >lookup.ppm
Finally, we look up the indices from our index in our lookup table and
produce the output:
pamlookup bothimages.ppm -lookupfile=lookup.ppm >imagediff.ppm
pnmremap(1), ppmmake(1), pnmcat(1), pamstack(1), pnm(1), pam(1)
pamlookup was new in Netpbm 10.13 (December 2002).
netpbm documentation 10 November 2002 Pamlookup User Manual(0)
Man(1) output converted with