1Pamaltsat User Manual(0) Pamaltsat User Manual(0)
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6 pamaltsat - increase or decrease the saturation of an image using one
7 of several alternative methods.
8
11 pamaltsat [-method name] [-strength number] [-linear] [infile]
12
15 This program is part of Netpbm(1).
16 pamaltsat decreases or increases the saturation of a Netpbm image by
18 one of various non-standard (alternative) methods.
19 The input is a Netpbm image from Standard Input or a file named by the
21 arguments. The output is a Netpbm image in the same format written to
22 Standard Output.
23 The most conventional way to change the saturation of an image is what
25 pambrighten does.
26
30 To increase saturation by a factor of 2.1 using the logarithmic method:
31 pamaltsat -method log -strength 2.1 test.ppm
33 To convert a color image to grayscale:
36 pamaltsat -strength 0 test.ppm
38
43 The following saturation methods are available.
44 Logarithmic Method
47 This saturation model is inspired by the concept of color integrity(1),
48 which works with color in terms of intensity ratios, where intensity is
49 a value of the luminosity function
50 ⟨https://en.wikipedia.org/wiki/Luminosity_function⟩ , rather than
51 brightness, or the numerical value of the sample in the image file.
52 From this viewpoint, it is natural to define the saturation of a color
53 as the ratio of maximum and minimum intensities of its primary compo‐
54 nents. In order, however, to make saturation an additive value and to
55 endow the -strength parameter with the semantics of a multiplier, it is
56 convenient to operate on the logarithm of that ratio. The addition of
57 such saturations acquires physical sense, and multiplication corre‐
58 sponds to the raising of intensity to the power of the multiplier.
59 With this method, pamaltsat raises the intensity of each component to
61 the power of the strength value. Since the total intensity of the re‐
62 sulting color will differ from that of the original, it is necessary to
63 restore the intensity by multiplying the component intensities of the
64 saturated color by the ratio of the intensity of the original color to
65 that of the saturated color.
66 Although it is always possible to decrease saturation by any given fac‐
68 tor, there are two cases where it cannot be increased. When the total
69 intensity (or brightness) of a color is too high for the desired satu‐
70 ration, pamaltsat applies the maximum possible saturation that keeps
71 the original intensity. For example, any color with at least one com‐
72 ponent at the maxiumum is already fully saturated. When one of the
73 primary components is zero, the definition of saturation given above no
74 longer works because of a zero in the denominator. pamaltsat offers no
75 special treatment of this situation because it does not create discon‐
76 tinuities and therefore produces no visible defects at reasonable
77 strength levels. When, however, strength approaches infinity, each
78 color tends to its primary component with the highest intensity.
79 This method was invented by Anton Shepelev.
81 Spectral Method
85 This is the default method. It treats color as a spectrum with three
86 bands: one for the intensity of each primary component. Since neutral
87 gray has a uniform (constant) spectrum, saturation can be measured as
88 the difference of the spectrum of the given color from the uniform
89 spectrum of the same total intensity. The spectral method uses one of
90 the simplest measures of such a difference: the difference between the
91 highest and lowest component intensities, which is an additive value
92 and therefore amenable to multiplication with good physical sense. Al‐
93 though a complete hue-saturation model can be dervied from this ap‐
94 proach, pamaltsat need not concern itself with it because it always
95 preserves both hue and total intensity.
96 In order to change saturation, pamaltsat first multiplies the intensity
98 of each component by the desired strength. The saturation of the re‐
99 sult is the strength times the saturation of the original, and likewise
100 the total intensity, but it is then restored by subtraction of the neu‐
101 tral gray with a suitable intensity.
102 The effect of this method on each component intensity may be expressed
104 in the following equation:
105 sat = orig * strength - Iorig * (strength - 1)
107 where sat is the saturated sample, orig the original sample, and Iorig
110 the total intensity of the original color.
111 The method is also related to color integrity because both its opera‐
113 tions are part of that concept: multiplication of component intensities
114 by the same quotient is an important operation because changes bright‐
115 ness but keeps color balance, and subtraction of a constant from all
116 component intensities is described by the inventor of color integrity
117 as 'subtraction of white.'
118 This procedure may produce both negative and over-unity component in‐
120 tensities. For such samples, pamaltsat decreases the strength to the
121 highest value that keeps the resulting color in range.
122 This method was invented by Anton Shepelev.
124
128 In addition to the options common to all programs based on libnetpbm
129 (most notably -quiet, see
130 Common Options ⟨index.html#commonoptions⟩ ), pamaltsat recognizes the
131 following command line options:
132 -method method
136 specifies the saturation method to use:
137 The default is spectrum
140 -strength strength
143 This specifies a real nonnegative factor whereby to modify satu‐
144 ration. A value greater than unity will increase saturation,
145 whereas a value less than unity will decrease it. Unity will
146 leave the image unchanged, and zero will produce greyscale out‐
147 put according to Rec 709.
148 pamaltsat preserves the total intensity of each pixel and never
150 affects neutral gray pixels.
151 This option is mandatory.
153 -linear
156 This tells pamaltsat that the input is the intensity-linear
157 variation of a Netpbm image forat, in which the samples are pro‐
158 portional to light intensity rather than to brightness, as they
159 are in true-or gamma-adjusted- Netpbm image formats.
160
164 Since pamaltsat does not affect neutral colors, you should adjust the
165 color balance before saturation. You can do this with pamlevels.
166
170 pamaltsat is written with an eye to extending it with new saturation
171 methods, which programmers are welcome to contribute. The only re‐
172 quirement is that every new method depend on a single strength parame‐
173 ter with the semantics described under the -strength command-line op‐
174 tion.
175
179 pambrighten(1), ppmflash(1),
180
184 This program was first submitted by Anton Shepelev (an‐
185 ton.txt@gmail.com).
186
189 pamaltsat was new in Netpbm 10.84 (September 2018).
190
194 •
195 SYNOPSIS ⟨#synopsis⟩
197 •
199 DESCRIPTION ⟨#description⟩
201 •
203 EXAMPLES ⟨#examples⟩
205 •
207 SATURATION METHODS ⟨#saturation_methods⟩
209 •
211 OPTIONS ⟨#options⟩
213 •
215 USAGE NOTES ⟨#usage_notes⟩
217 •
219 EXTENSIBILITY ⟨#extensibility⟩
221 •
223 SEE ALSO ⟨#seealso⟩
225 •
227 AUTHOR ⟨#author⟩
229 •
231 HISTORY ⟨#history⟩
233
235 This manual page was generated by the Netpbm tool 'makeman' from HTML
236 source. The master documentation is at
237 http://netpbm.sourceforge.net/doc/pamaltsat.html
239netpbm documentation 14 September 2018 Pamaltsat User Manual(0)