1PAM format specification(5) File Formats Manual PAM format specification(5)
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6 pam - Netpbm common 2-dimensional bitmap format
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10 The PAM image format is a lowest common denominator 2 dimensional map
11 format.
12 It is designed to be used for any of myriad kinds of graphics, but can
14 theoretically be used for any kind of data that is arranged as a two
15 dimensional rectangular array. Actually, from another perspective it
16 can be seen as a format for data arranged as a three dimensional array.
17 The name "PAM" is an acronym derived from "Portable Arbitrary Map."
19 This derivation makes more sense if you consider it in the context of
20 the other Netpbm format names: PBM, PGM, and PPM.
21 This format does not define the meaning of the data at any particular
23 point in the array. It could be red, green, and blue light intensities
24 such that the array represents a visual image, or it could be the same
25 red, green, and blue components plus a transparency component, or it
26 could contain annual rainfalls for places on the surface of the Earth.
27 Any process that uses the PAM format must further define the format to
28 specify the meanings of the data.
29 A PAM image describes a two dimensional grid of tuples. The tuples are
31 arranged in rows and columns. The width of the image is the number of
32 columns. The height of the image is the number of rows. All rows are
33 the same width and all columns are the same height. The tuples may
34 have any degree, but all tuples have the same degree. The degree of
35 the tuples is called the depth of the image. Each member of a tuple is
36 called a sample. A sample is an unsigned integer which represents a
37 locus along a scale which starts at zero and ends at a certain maximum
38 value called the maxval. The maxval is the same for every sample in
39 the image. The two dimensional array of all the Nth samples of each
40 tuple is called the Nth plane or Nth channel of the image.
41 Though the basic format does not assign any meaning to the tuple val‐
43 ues, it does include an optional string that describes that meaning.
44 The contents of this string, called the tuple type, are arbitrary from
45 the point of view of the basic PAM format, but users of the format may
46 assign meaning to it by convention so they can identify their particu‐
47 lar implementations of the PAM format. Some tuple types are defined as
48 official subformats of PAM. See Defined Tuple Types ⟨#tupletype⟩ .
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52 It is easy to get confused about the relationship between the PAM for‐
53 mat and PBM, PGM, PPM, and PNM. Here is a little enlightenment:
54 "PNM" is not really a format. It is a shorthand for the PBM, PGM, and
56 PPM formats collectively. It is also the name of a group of library
57 functions that can each handle all three of those formats.
58 "PAM" is in fact a fourth format. But it is so general that you can
60 represent the same information in a PAM image as you can in a PBM, PGM,
61 or PPM image. And in fact a program that is designed to read PBM, PGM,
62 or PPM and does so with a recent version of the Netpbm library will
63 read an equivalent PAM image just fine and the program will never know
64 the difference.
65 To confuse things more, there is a collection of library routines
67 called the "pam" functions that read and write the PAM format, but also
68 read and write the PBM, PGM, and PPM formats. They do this because the
69 latter formats are much older and more popular, so even a new program
70 must work with them. Having the library handle all the formats makes
71 it convenient to write programs that use the newer PAM format as well.
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75 A convenient way to read and write the PAM format accurately is via the
76 libnetpbm(1) C subroutine library.
77 A PAM file consists of a sequence of one or more PAM images. There are
79 no data, delimiters, or padding before, after, or between images.
80 Each PAM image consists of a header followed immediately by a raster.
82 Here is an example header:
84 P7
87 WIDTH 227
88 HEIGHT 149
89 DEPTH 3
90 MAXVAL 255
91 TUPLTYPE RGB
92 ENDHDR
93 The header begins with the ASCII characters "P7" followed by newline.
97 This is the magic number.
98 Note: xv thumbnail images also start with the "P7" magic number. (This
100 and PAM were independent extensions to the Netpbm formats). The rest
101 of the format makes it easy to distinguish PAM from that format,
102 though).
103 The header continues with an arbitrary number of lines of ASCII text.
105 Each line ends with and is delimited by a newline character.
106 Each header line consists of zero or more whitespace-delimited tokens
108 or begins with "#". If it begins with "#" it is a comment and the rest
109 of this specification does not apply to it.
110 A header line which has zero tokens is valid but has no meaning.
112 The type of header line is identified by its first token, which is 8
114 characters or less:
115 ENDHDR This is the last line in the header. The header must contain
119 exactly one of these header lines.
120 HEIGHT The second token is a decimal number representing the height of
123 the image (number of rows). The header must contain exactly one
124 of these header lines.
125 WIDTH The second token is a decimal number representing the width of
128 the image (number of columns). The header must contain exactly
129 one of these header lines.
130 DEPTH The second token is a decimal number representing the depth of
133 the image (number of planes or channels). The header must con‐
134 tain exactly one of these header lines.
135 MAXVAL The second token is a decimal number representing the maxval of
138 the image. The header must contain exactly one of these header
139 lines.
140 TUPLTYPE
143 The header may contain any number of these header lines, includ‐
144 ing zero. The rest of the line is part of the tuple type. The
145 rest of the line is not tokenized, but the tuple type does not
146 include any white space immediately following TUPLTYPE or at
147 the very end of the line. It does not include a newline. There
148 must be something other than white space after the TUPLTYPE to‐
149 ken.
150 If there are multiple TUPLTYPE header lines, the tuple type is
152 the concatenation of the values from each of them, separated by
153 a single blank, in the order in which they appear in the header.
154 If there are no TUPLTYPE header lines the tuple type is the null
155 string.
156 The raster consists of each row of the image, in order from top to bot‐
160 tom, consecutive with no delimiter of any kind between, before, or af‐
161 ter, rows.
162 Each row consists of every tuple in the row, in order from left to
164 right, consecutive with no delimiter of any kind between, before, or
165 after, tuples.
166 Each tuple consists of every sample in the tuple, in order, consecutive
168 with no delimiter of any kind between, before, or after, samples.
169 Each sample consists of an unsigned integer in pure binary format, with
171 the most significant byte first. The number of bytes is the minimum
172 number of bytes required to represent the maxval of the image.
173 The character referred to as "newline" herein is the character known in
175 ASCII as Line Feed or LF.
176
179 Height, width, depth, and maxval are at least 1.
180 Height, width, and depth have no defined maximum, but processors and
182 generators of images usually have their own limitations.
183 The maxval of an image is never greater than 65535. (The reason it is
185 limited is to make it easier to build an image processor, in which in‐
186 termediate arithmetic values often have to fit within 31 or 32 bits).
187 There was no specified limitation before October, 2005, but essentially
188 all implementations have always observed it.
189
192 Some tuple types are defined in this specification to specify official
193 subformats of PAM for especially popular applications of the format.
194 Users of the format may also define their own tuple types, and thus
195 their own subformats.
196 Tuple type affects only the meanings of the samples (which are unsigned
198 integers) in the tuples of the image. It does not affect how the sam‐
199 ples or tuples are encoded. Tuple type may affect the meaning of a tu‐
200 ple's position in the array (e.g. it may indicate in a visual image
201 that a tuple in Row 1 is one at the top of the image rather than the
202 bottom).
203 Tuple type never determines how many samples are in a tuple (that is
205 instead determined by the DEPTH header line). Tuple type could be said
206 to imply a depth (number of samples per tuple) because certain tuple
207 types are valid only in combination with certain DEPTH values, but it
208 is good programming practice to use DEPTH for the depth when decoding
209 the raster and separately validate that the depth is consistent with
210 the tuple type. Also, it is good practice to accept a depth that is
211 too great and just ignore the higher numbered planes.
212 PAM Used For Visual Images
215 A common use of PAM images is to represent visual images such as are
216 typically represented by images in the older and more concrete PBM,
217 PGM, and PPM formats.
218 Black And White
220 A black and white image, such as would alternatively be represented by
222 a PBM image, has a tuple type of "BLACKANDWHITE". Such a PAM image has
223 a depth of 1 and maxval 1 where the one sample in each tuple is 0 to
224 represent a black pixel and 1 to represent a white one. The maxval,
225 height, width, and order of tuples in the raster bear the obvious rela‐
226 tionship to those of the equivalent PGM image.
227 Note that in the PBM format, a sample value of zero means white, but in
229 PAM, zero means black.
230 Grayscale
232 A grayscale image, such as would alternatively be represented by a PGM
234 image, has a tuple type of "GRAYSCALE". Such a PAM image has a depth
235 of 1. The maxval, height, width, and raster bear the obvious relation‐
236 ship to those of the equivalent PGM image.
237 Color
239 A color image, such as would alternatively be represented by a PPM im‐
241 age, has a tuple type of "RGB". Such a PAM image has a depth of 3.
242 The maxval, height, width, and raster bear the obvious relationship to
243 those of the PPM image. The first plane represents red, the second
244 green, and the third blue.
245 Transparent
247 Each of the visual image formats mentioned above has a variation that
249 contains transparency information. In that variation, the tuple type
250 has "_ALPHA" added to it (e.g. "RGB_ALPHA") and one more plane. The
251 highest numbered plane is the opacity plane (sometimes called an alpha
252 plane or transparency plane).
253 In this kind of image, the color represented by a pixel is actually a
255 combination of an explicitly specified foreground color and a back‐
256 ground color to be identified later.
257 The planes other than the opacity plane describe the foreground color.
259 A sample in the opacity plane tells how opaque the pixel is, by telling
260 what fraction of the pixel's light comes from the foreground color.
261 The rest of the pixel's light comes from the (unspecified) background
262 color.
263 For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a
265 gray tone 60% of white and Plane 1 indicates opacity 25%. The fore‐
266 ground color is the 60% gray, and 25% of that contributes to the ulti‐
267 mate color of the pixel. The other 75% comes from some background
268 color. So let's assume further that the background color of the pixel
269 is full white. Then the color of the pixel is 90% of white: 25% of
270 the foreground 60%, plus 75% of the background 100%.
271 The sample value is the opacity fraction just described, as a fraction
273 of the maxval. Note that it is not gamma-adjusted like the foreground
274 color samples.
275
279 No Internet Media Type (aka MIME type, content type) for PBM has been
280 registered with IANA, but the unofficial value image/x-portable-arbi‐
281 trarymap is assigned by this specification, to be consistent with con‐
282 ventional values for the older Netpbm formats.
283
286 The conventional suffix for the name of a PAM file is ".pam". But this
287 is not required.
288
292 Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)
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295 This manual page was generated by the Netpbm tool 'makeman' from HTML
296 source. The master documentation is at
297 http://netpbm.sourceforge.net/doc/pam.html
299netpbm documentation 27 November 2013 PAM format specification(5)