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)Csubroutinelibrary.
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
86 WIDTH 227
87 HEIGHT 149
88 DEPTH 3
89 MAXVAL 255
90 TUPLTYPE RGB
91 ENDHDR
92 The header begins with the ASCII characters 'P7' followed by newline.
94 This is the magic number.
95 Note: xv thumbnail images also start with the "P7" magic number. (This
97 and PAM were independent extensions to the Netpbm formats). The rest
98 of the format makes it easy to distinguish PAM from that format,
99 though).
100 The header continues with an arbitrary number of lines of ASCII text.
102 Each line ends with and is delimited by a newline character.
103 Each header line consists of zero or more whitespace-delimited tokens
105 or begins with '#'. If it begins with '#' it is a comment and the rest
106 of this specification does not apply to it.
107 A header line which has zero tokens is valid but has no meaning.
109 The type of header line is identified by its first token, which is 8
111 characters or less:
112 ENDHDR This is the last line in the header. The header must contain
116 exactly one of these header lines.
117 HEIGHT The second token is a decimal number representing the height of
120 the image (number of rows). The header must contain exactly one
121 of these header lines.
122 WIDTH The second token is a decimal number representing the width of
125 the image (number of columns). The header must contain exactly
126 one of these header lines.
127 DEPTH The second token is a decimal number representing the depth of
130 the image (number of planes or channels). The header must con‐
131 tain exactly one of these header lines.
132 MAXVAL The second token is a decimal number representing the maxval of
135 the image. The header must contain exactly one of these header
136 lines.
137 TUPLTYPE
140 The header may contain any number of these header lines, includ‐
141 ing zero. The rest of the line is part of the tuple type. The
142 rest of the line is not tokenized, but the tuple type does not
143 include any white space immediately following TUPLTYPE or at
144 the very end of the line. It does not include a newline. There
145 must be something other than white space after the TUPLTYPE
146 token.
147 If there are multiple TUPLTYPE header lines, the tuple type is
149 the concatenation of the values from each of them, separated by
150 a single blank, in the order in which they appear in the header.
151 If there are no TUPLTYPE header lines the tuple type is the null
152 string.
153 The raster consists of each row of the image, in order from top to bot‐
157 tom, consecutive with no delimiter of any kind between, before, or
158 after, rows.
159 Each row consists of every tuple in the row, in order from left to
161 right, consecutive with no delimiter of any kind between, before, or
162 after, tuples.
163 Each tuple consists of every sample in the tuple, in order, consecutive
165 with no delimiter of any kind between, before, or after, samples.
166 Each sample consists of an unsigned integer in pure binary format, with
168 the most significant byte first. The number of bytes is the minimum
169 number of bytes required to represent the maxval of the image.
170 The character referred to as 'newline' herein is the character known in
172 ASCII as Line Feed or LF.
173
176 Height, width, depth, and maxval are at least 1.
177 Height, width, and depth have no defined maximum, but processors and
179 generators of images usually have their own limitations.
180 The maxval of an image is never greater than 65535. (The reason it is
182 limited is to make it easier to build an image processor, in which
183 intermediate arithmetic values often have to fit within 31 or 32 bits).
184 There was no specified limitation before October, 2005, but essentially
185 all implementations have always observed it.
186
189 Some tuple types are defined in this specification to specify official
190 subformats of PAM for especially popular applications of the format.
191 Users of the format may also define their own tuple types, and thus
192 their own subformats.
193 PAM Used For Visual Images
196 A common use of PAM images is to represent visual images such as are
197 typically represented by images in the older and more concrete PBM,
198 PGM, and PPM formats.
199 Black And White (PBM)
201 A black and white image, such as would be represented by a PBM image,
203 has a tuple type of "BLACKANDWHITE". Such a PAM image has a depth of 1
204 and maxval 1 where the one sample in each tuple is 0 to represent a
205 black pixel and 1 to represent a white one. The height, width, and
206 raster bear the obvious relationship to those of the equivalent PBM
207 image.
208 Note that in the PBM format, a zero value means white, but in PAM, zero
210 means black.
211 Grayscale (PGM)
213 A grayscale image, such as would be represented by a PGM image, has a
215 tuple type of "GRAYSCALE". Such a PAM image has a depth of 1. The
216 maxval, height, width, and raster bear the obvious relationship to
217 those of the equivalent PGM image.
218 Color (PPM)
220 A color image, such as would be represented by a PPM image, has a typle
222 type of "RGB". Such a PAM image has a depth of 3. The maxval, height,
223 width, and raster bear the obvious relationship to those of the PPM
224 image. The first plane represents red, the second green, and the third
225 blue.
226 Transparent
228 Each of the visual image formats mentioned above has a variation that
230 contains transparency information. In that variation, the tuple type
231 has '_ALPHA' added to it (e.g. 'RGB_ALPHA') and one more plane. The
232 highest numbered plane is the opacity plane (sometimes called an alpha
233 plane or transparency plane).
234 In this kind of image, the color represented by a pixel is actually a
236 combination of an explicitly specified foreground color and a back‐
237 ground color to be identified later.
238 The planes other than the opacity plane describe the foreground color.
240 A sample in the opacity plane tells how opaque the pixel is, by telling
241 what fraction of the pixel's light comes from the foreground color.
242 The rest of the pixel's light comes from the (unspecified) background
243 color.
244 For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a
246 gray tone 60% of white and Plane 1 indicates opacity 25%. The fore‐
247 ground color is the 60% gray, and 25% of that contributes to the ulti‐
248 mate color of the pixel. The other 75% comes from some background
249 color. So let's assume further that the background color of the pixel
250 is full white. Then the color of the pixel is 90% of white: 25% of
251 the foreground 60%, plus 75% of the background 100%.
252 The sample value is the opacity fraction just described, as a fraction
254 of the maxval. Note that it is not gamma-adjusted like the foreground
255 color samples.
256
260 Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)
261netpbm documentation 09 October 2005 PAM format specification(5)