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 greater than zero called the maxval. The maxval is the same for
39 every sample in the image. The two dimensional array of all the Nth
40 samples of each tuple is called the Nth plane or Nth channel of the
41 image.
42 Though the basic format does not assign any meaning to the tuple val‐
44 ues, it does include an optional string that describes that meaning.
45 The contents of this string, called the tuple type, are arbitrary from
46 the point of view of the basic PAM format, but users of the format may
47 assign meaning to it by convention so they can identify their particu‐
48 lar implementations of the PAM format. Some tuple types are defined as
49 official subformats of PAM. See Defined Tuple Types ⟨#tupletype⟩ .
50 <H2 id="format_universe">The Confusing Universe of Netpbm Formats</H2>
52 It is easy to get confused about the relationship between the PAM for‐
54 mat and PBM, PGM, PPM, and PNM. Here is a little enlightenment:
55 "PNM" is not really a format. It is a shorthand for the PBM, PGM, and
57 PPM formats collectively. It is also the name of a group of library
58 functions that can each handle all three of those formats.
59 'PAM' is in fact a fourth format. But it is so general that you can
61 represent the same information in a PAM image as you can in a PBM, PGM,
62 or PPM image. And in fact a program that is designed to read PBM, PGM,
63 or PPM and does so with a recent version of the Netpbm library, will
64 read an equivalent PAM image just fine and the program will never know
65 the difference.
66 To confuse things more, there is a collection of library routines
68 called the 'pam' functions that read and write the PAM format, but also
69 read and write the PBM, PGM, and PPM formats. They do this because the
70 latter formats are much older and more popular, so even a new program
71 must work with them. Having the library handle all the formats makes
72 it convenient to write programs that use the newer PAM format as well.
73
76 A convenient way to read and write the PAM format accurately is via the
77 libnetpbm(1)Csubroutinelibrary.
78 A PAM file consists of a sequence of one or more PAM images. There are
80 no data, delimiters, or padding before, after, or between images.
81 Each PAM image consists of a header followed immediately by a raster.
83 Here is an example header:
85 P7 WIDTH 227 HEIGHT 149 DEPTH 3 MAXVAL 255 TUPLTYPE RGB ENDHDR
87 The header begins with the ASCII characters 'P7' followed by newline.
89 This is the magic number.
90 Note: xv thumbnail images also start with the "P7" magic number. (This
92 and PAM were independent extensions to the Netpbm formats). The rest
93 of the format makes it easy to distinguish PAM from that format,
94 though).
95 The header continues with an arbitrary number of lines of ASCII text.
97 Each line ends with and is delimited by a newline character.
98 Each header line consists of zero or more whitespace-delimited tokens
100 or begins with '#'. If it begins with '#' it is a comment and the rest
101 of this specification does not apply to it.
102 A header line which has zero tokens is valid but has no meaning.
104 The type of header line is identified by its first token, which is 8
106 characters or less:
107 ENDHDR This is the last line in the header. The header must contain
111 exactly one of these header lines.
112 HEIGHT The second token is a decimal number representing the height of
115 the image (number of rows). The header must contain exactly one
116 of these header lines.
117 WIDTH The second token is a decimal number representing the width of
120 the image (number of columns). The header must contain exactly
121 one of these header lines.
122 DEPTH The second token is a decimal number representing the depth of
125 the image (number of planes or channels). The header must con‐
126 tain exactly one of these header lines.
127 MAXVAL The second token is a decimal number representing the maxval of
130 the image. The header must contain exactly one of these header
131 lines.
132 TUPLTYPE
135 The header may contain any number of these header lines, includ‐
136 ing zero. The rest of the line is part of the tuple type. The
137 rest of the line is not tokenized, but the tuple type does not
138 include any white space immediately following TUPLTYPE or at
139 the very end of the line. It does not include a newline. There
140 must be something other than white space after the TUPLETYPE
141 token.
142 If there are multiple TUPLTYPE header lines, the tuple type is
144 the concatenation of the values from each of them, separated by
145 a single blank, in the order in which they appear in the header.
146 If there are no TUPLTYPE header lines the tuple type is the null
147 string.
148 The raster consists of each row of the image, in order from top to bot‐
152 tom, consecutive with no delimiter of any kind between, before, or
153 after, rows.
154 Each row consists of every tuple in the row, in order from left to
156 right, consecutive with no delimiter of any kind between, before, or
157 after, tuples.
158 Each tuple consists of every sample in the tuple, in order, consecutive
160 with no delimiter of any kind between, before, or after, samples.
161 Each sample consists of an unsigned integer in pure binary format, with
163 the most significant byte first. The number of bytes is the minimum
164 number of bytes required to represent the maxval of the image.
165 The character referred to as 'newline' herein is the character known in
167 ASCII as Line Feed or LF.
168
171 The maxval of an image is never greater than 65535. (The reason it is
172 limited is to make it easier to build an image processor, in which
173 intermediate arithmetic values often have to fit within 31 or 32 bits).
174 There was no specified limitation before October, 2005, but essentially
175 all implementations have always observed it.
176 Height and width are at least 1.
178 Height and width have no defined maximum, but processors and generators
180 of images usually have their own limitations.
181
184 Some tuple types are defined in this specification to specify official
185 subformats of PAM for especially popular applications of the format.
186 Users of the format may also define their own tuple types, and thus
187 their own subformats.
188 PAM Used For Visual Images
191 A common use of PAM images is to represent visual images such as are
192 typically represented by images in the older and more concrete PBM,
193 PGM, and PPM formats.
194 Black And White (PBM)
196 A black and white image, such as would be represented by a PBM image,
198 has a tuple type of "BLACKANDWHITE". Such a PAM image has a depth of 1
199 and maxval 1 where the one sample in each tuple is 0 to represent a
200 black pixel and 1 to represent a white one. The height, width, and
201 raster bear the obvious relationship to those of the equivalent PBM
202 image.
203 Note that in the PBM format, a zero value means white, but in PAM, zero
205 means black.
206 Grayscale (PGM)
208 A grayscale image, such as would be represented by a PGM image, has a
210 tuple type of "GRAYSCALE". Such a PAM image has a depth of 1. The
211 maxval, height, width, and raster bear the obvious relationship to
212 those of the equivalent PGM image.
213 Color (PPM)
215 A color image, such as would be represented by a PPM image, has a typle
217 type of "RGB". Such a PAM image has a depth of 3. The maxval, height,
218 width, and raster bear the obvious relationship to those of the PPM
219 image. The first plane represents red, the second blue, and the third
220 green.
221 Transparent
223 Each of the visual image formats mentioned above has a variation that
225 contains transparency information. In that variation, the tuple type
226 has '_ALPHA' added to it (e.g. 'RGB_ALPHA') and one more plane. The
227 highest numbered plane is the opacity plane (sometimes called an alpha
228 plane or transparency plane).
229 In this kind of image, the color represented by a pixel is actually a
231 combination of an explicitly specified foreground color and a back‐
232 ground color to be identified later.
233 The planes other than the opacity plane describe the foreground color.
235 A sample in the opacity plane tells how opaque the pixel is, by telling
236 what fraction of the pixel's light comes from the foreground color.
237 The rest of the pixel's light comes from the (unspecified) background
238 color.
239 For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a
241 gray tone 60% of white and Plane 1 indicates opacity 25%. The fore‐
242 ground color is the 60% gray, and 25% of that contributes to the ulti‐
243 mate color of the pixel. The other 75% comes from some background
244 color. So let's assume further that the background color of the pixel
245 is full white. Then the color of the pixel is 90% of white: 25% of
246 the foreground 60%, plus 75% of the background 100%.
247 The sample value is the opacity fraction just described, as a fraction
249 of the maxval. Note that it is not gamma-adjusted like the foreground
250 color samples.
251
255 Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)
256netpbm documentation 09 October 2005 PAM format specification(5)