1IM_SHARPEN(3) Library Functions Manual IM_SHARPEN(3)
2
6 im_sharpen - simple coring edge enhancement
7
9 #include <vips/vips.h>
10 int
12 im_sharpen( in, out, mask_radius, x1, y2, y3, m1, m2 )
13 IMAGE *in, *out;
14 int mask_radius;
15 double x1, y2, y3;
16 double m1, m2;
17
20 This function selectively sharpens the L* channel of a LABPACK coded
21 image. It is suitable for preparing an image for printing, where edges
22 need to be boosted to compensate for blurring introduced by the half-
23 tone process, and also for giving any additional `crispening' required.
24 The function operates as:
26 in gaussian out
28 --+-> blur with ----> subtract --> LUT --> add ---->
29 | mask_radius ^ ^
30 | | |
31 +----------------------+ |
32 | |
33 +-----------------------------------------+
34 In other words, the L* channel is smoothed with a gaussian average
36 function of radius mask_radius and this smoothed image is subtracted
37 from the original L* to generate a high-frequency signal.
38 This high-frequency signal is passed through a look-up table specified
40 by the x1, y2, y3, m1 and m2 parameters and added back to the original
41 L* channel to generate the sharpened image.
42 The LUT is shaped as:
44 ^
46 y2 |- - - - - -----------
47 | /
48 | / slope m2
49 | .../
50 -x1 | ... |
51 -------------------...---------------------->
52 | ... | x1
53 |... slope m1
54 / |
55 / m2 |
56 / |
57 / |
58 / |
59 / |
60 ______/ _ _ _ _ _ _ | -y3
61 |
62 When trying to understand the meaning of these parameters, it is help‐
64 ful to imagine a cross-section through an image. Sharpening filters
65 boost apparent sharpness by performing the following transformation.
66 Consider a cross-section through a soft edge:
67 ^
69 | ..........
70 | ....
71 | ...
72 | ..
73 |.
74 --------------------+------------------->
75 .|
76 .. |
77 ... |
78 .... |
79 .......... |
80 |
81 This becomes an enhanced edge, perhaps:
83 ^
85 | ... overshoot
86 | . .
87 | . ............
88 | .
89 |.
90 .
91 .
92 --------------------+------------------->
93 .
94 .|
95 .|
96 . |
97 ............. . |
98 . . |
99 undershoot ... |
100 |
101 There are two features: the transition from black to white has become
103 steeper, and there are positive and negative undershoots and over‐
104 shoots.
105 As a general guide, some overshoot is good for printing (it helps exag‐
107 gerate the edge), but not too much, as you will start to see strong
108 white fringes. Undershoot introduces black lines, which are less
109 intrusive, so you can allow more under- than over-shoot.
110 im_sharpen uses the x1 parameter to distinguish between low and high
112 frequency areas. Pixels which differ by less than x1 L* units from
113 their local average are sharpened by factor 1+m1, and pixels which dif‐
114 fer by more than x1 are sharpened by factor 1+m2. y2 and y3 set limits
115 on the amount of positive and negative sharpening we allow.
116 For printing, we recommend the following settings:
118 mask_radius == 7
120 x1 == 1.5
121 y2 == 20 (don't brighten by more than 20 L*)
122 y3 == 50 (can darken by up to 50 L*)
123 m1 == 1 (some sharpening in flat areas)
125 m2 == 2 (more sharpening in jaggy areas)
126 If you want more or less sharpening, we suggest you just change the m1
128 and m2 parameters. For an extreme sharpen, you might try:
129 m1 == 2
131 m2 == 4
132 And for a relatively gentle sharpen, perhaps:
134 m1 == 0.5
136 m2 == 1.5
137 If you want to adjust the x1 parameter, it can be helpful visualise its
139 effect by setting:
140 m1 == -2
142 m2 == 4
143 In other words, heavily blur flat areas, and heavily sharpen elsewhere.
145 This creates a terrible-looking image, but you will be able to see
146 clearly what parts of your image are being classified as flat.
147 The y2 and y3 parameters need not usually be adjusted, unless you wish
149 to reduce the strength of the finges.
150 The mask_radius parameter changes the width of the fringe and can be
152 adjusted according to the output printing resolution. As an approximate
153 guideline, use 3 for 4 pixels/mm (CRT display resolution), 5 for 8 pix‐
154 els/mm, 7 for 12 pixels/mm and 9 for 16 pixels/mm (300 dpi == 12 pix‐
155 els/mm). These figures refer to the image raster, not the half-tone
156 resolution.
157
160 The function returns 0 on success and -1 on error.
161
163 im_conv(3), im_compass(3), im_fastcor(3).
164
166 National Gallery and Birkbeck College, 1995
167
169 Lindsay!
170 16 May 1995 IM_SHARPEN(3)