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:
35 pamaltsat -strength 0 test.ppm
37
41 The following saturation methods are available.
42 Logarithmic Method
45 This saturation model is inspired by the concept of color integrity(1),
46 which works with color in terms of intensity ratios, where intensity is
47 a value of the luminosity function
48 ⟨https://en.wikipedia.org/wiki/Luminosity_function⟩ , rather than
49 brightness, or the numerical value of the sample in the image file.
50 From this viewpoint, it is natural to define the saturation of a color
51 as the ratio of maximum and minimum intensities of its primary compo‐
52 nents. In order, however, to make saturation an additive value and to
53 endow the -strength parameter with the semantics of a multiplier, it is
54 convenient to operate on the logarithm of that ratio. The addition of
55 such saturations acquires physical sense, and multiplication corre‐
56 sponds to the raising of intensity to the power of the multiplier.
57 With this method, pamaltsat raises the intensity of each component to
59 the power of the strength value. Since the total intensity of the
60 resulting color will differ from that of the original, it is necessary
61 to restore the intensity by multiplying the component intensities of
62 the saturated color by the ratio of the intensity of the original color
63 to that of the saturated color.
64 Although it is always possible to decrease saturation by any given fac‐
66 tor, there are two cases where it cannot be increased. When the total
67 intensity (or brightness) of a color is too high for the desired satu‐
68 ration, pamaltsat applies the maximum possible saturation that keeps
69 the original intensity. For example, any color with at least one com‐
70 ponent at the maxiumum is already fully saturated. When one of the
71 primary components is zero, the definition of saturation given above no
72 longer works because of a zero in the denominator. pamaltsat offers no
73 special treatment of this situation because it does not create discon‐
74 tinuities and therefore produces no visible defects at reasonable
75 strength levels. When, however, strength approaches infinity, each
76 color tends to its primary component with the highest intensity.
77 This method was invented by Anton Shepelev.
79 Spectral Method
83 This is the default method. It treats color as a spectrum with three
84 bands: one for the intensity of each primary component. Since neutral
85 gray has a uniform (constant) spectrum, saturation can be measured as
86 the difference of the spectrum of the given color from the uniform
87 spectrum of the same total intensity. The spectral method uses one of
88 the simplest measures of such a difference: the difference between the
89 highest and lowest component intensities, which is an additive value
90 and therefore amenable to multiplication with good physical sense.
91 Although a complete hue-saturation model can be dervied from this
92 approach, pamaltsat need not concern itself with it because it always
93 preserves both hue and total intensity.
94 In order to change saturation, pamaltsat first multiplies the intensity
96 of each component by the desired strength. The saturation of the
97 result is the strength times the saturation of the original, and like‐
98 wise the total intensity, but it is then restored by subtraction of the
99 neutral gray with a suitable intensity.
100 The effect of this method on each component intensity may be expressed
102 in the following equation:
103 <span style="font-family: monospace">
104 sat = orig * strength - Iorig * (strength - 1)
105 </span>
106 where <span style="font-family: monospace">sat</span> is the saturated
107 sample, <span style="font-family: monospace">orig</span> the original
108 sample, and <span style="font-family: monospace">Iorig</span> the total
109 intensity of the original color.
110 The method is also related to color integrity because both its opera‐
112 tions are part of that concept: multiplication of component intensities
113 by the same quotient is an important operation because changes bright‐
114 ness but keeps color balance, and subtraction of a constant from all
115 component intensities is described by the inventor of color integrity
116 as 'subtraction of white.'
117 This procedure may produce both negative and over-unity component
119 intensities. For such samples, pamaltsat decreases the strength to the
120 highest value that keeps the resulting color in range.
121 This method was invented by Anton Shepelev.
123
127 In addition to the options common to all programs based on libnetpbm
128 (most notably -quiet, see
129 Common Options ⟨index.html#commonoptions⟩ ), pamaltsat recognizes the
130 following command line options:
131 -method method
135 specifies the saturation method to use:
136 The default is spectrum
139 -strength strength
142 This specifies a real nonnegative factor whereby to modify satu‐
143 ration. A value greater than unity will increase saturation,
144 whereas a value less than unity will decrease it. Unity will
145 leave the image unchanged, and zero will produce greyscale out‐
146 put according to Rec 709.
147 pamaltsat preserves the total intensity of each pixel and never
149 affects neutral gray pixels.
150 This option is mandatory.
152 -linear
155 This tells pamaltsat that the input is the intensity-linear
156 variation of a Netpbm image forat, in which the samples are pro‐
157 portional to light intensity rather than to brightness, as they
158 are in true-or gamma-adjusted- Netpbm image formats.
159
163 Since pamaltsat does not affect neutral colors, you should adjust the
164 color balance before saturation. You can do this with pamlevels.
165
169 pamaltsat is written with an eye to extending it with new saturation
170 methods, which programmers are welcome to contribute. The only
171 requirement is that every new method depend on a single strength param‐
172 eter with the semantics described under the -strength command-line
173 option.
174
178 pambrighten(1), ppmflash(1),
179
183 This program was first submitted by Anton Shepelev
184 (anton.txt@gmail.com).
185
188 pamaltsat was new in Netpbm 10.84 (September 2018).
189
193 ·
194 SYNOPSIS ⟨#synopsis⟩
196 ·
198 DESCRIPTION ⟨#description⟩
200 ·
202 EXAMPLES ⟨#examples⟩
204 ·
206 SATURATION METHODS ⟨#saturation_methods⟩
208 ·
210 OPTIONS ⟨#options⟩
212 ·
214 USAGE NOTES ⟨#usage_notes⟩
216 ·
218 EXTENSIBILITY ⟨#extensibility⟩
220 ·
222 SEE ALSO ⟨#seealso⟩
224 ·
226 AUTHOR ⟨#author⟩
228 ·
230 HISTORY ⟨#history⟩
232
234 This manual page was generated by the Netpbm tool 'makeman' from HTML
235 source. The master documentation is at
236 http://netpbm.sourceforge.net/doc/pamaltsat.html
238netpbm documentation 14 September 2018 Pamaltsat User Manual(0)