diff options
Diffstat (limited to '')
| -rw-r--r-- | upstream/opensuse-tumbleweed/man1/pnmhisteq.1 | 221 |
1 files changed, 221 insertions, 0 deletions
diff --git a/upstream/opensuse-tumbleweed/man1/pnmhisteq.1 b/upstream/opensuse-tumbleweed/man1/pnmhisteq.1 new file mode 100644 index 00000000..86475a80 --- /dev/null +++ b/upstream/opensuse-tumbleweed/man1/pnmhisteq.1 @@ -0,0 +1,221 @@ +\ +.\" This man page was generated by the Netpbm tool 'makeman' from HTML source. +.\" Do not hand-hack it! If you have bug fixes or improvements, please find +.\" the corresponding HTML page on the Netpbm website, generate a patch +.\" against that, and send it to the Netpbm maintainer. +.TH "Pnmhisteq User Manual" 0 "22 March 2015" "netpbm documentation" + +.SH NAME + +pnmhisteq - histogram equalize a PNM image + +.UN synopsis +.SH SYNOPSIS + +\fBpnmhisteq\fP + +[\fB-gray\fP] + +[\fB-noblack\fP] +[\fB-nowhite\fP] + +[\fB-rmap\fP \fIpgmfile\fP] + +[\fB-wmap\fP \fIpgmfile\fP] + +[\fB-verbose\fP] + +[\fIpnmfile\fP] + + +.UN description +.SH DESCRIPTION +.PP +This program is part of +.BR "Netpbm" (1)\c +\&. +.PP +\fBpnmhisteq\fP increases the contrast of a PGM or PPM image +through the technique of "histogram equalization."[1] +.PP +\fBpnmhisteq\fP computes a histogram of the luminosity of the +pixels in the image. It then calculates a mapping between each +luminosity and a new luminosity such that it spreads out intensity +levels around histogram peaks and compresses them at troughs. I.e. +it moves pixels around in the histogram so as to make it flat. It +applies that mapping to the input image to produce the output image. +The effect of this is that the image has equal numbers of pixels at each +possible intensity level, which means it uses the available levels of +intensity more efficiently and thereby has more visible detail. +.PP +Mathematically, the luminosity mapping is this: Assume the pixels +are sorted by luminosity into \fIB\fP buckets numbered from 0 (lowest +luminosity) to \fIB\fP-1. \fIN[i]\fP is the number of pixels in +bucket \fIi\fP. \fIT\fP is the total number of pixels (sum of +\fIN[i]\fP over all \fIi\fP). \fIW\fP is the luminosity of white. +.PP +\fBpnmhisteq\fP replaces an input pixel whose luminosity falls into +bucket \fIj\fP with one whose luminosity is: + +.nf + + j + --- + \e + > (N[i] / T) * W + / + --- + i=0 + +.fi +.PP +Considering a grayscale image for simplicity, this means that +pixels in the most luminous bucket become white. Pixels in the 10th +per centile of luminosity become 10% of white. +.PP +\fBpnmhisteq\fP maps a single luminosity in the input to a single +luminosity in the output. That means if pixels A and B both have luminosity +\&.2 in the input, and pixel A has luminosity .4 in the output, pixel B also has +luminosity .4 in the output. And since the luminosities in the input are not +continuous, the luminosities in the output aren't either and \fBpnmhisteq\fP +doesn't meet the ideal of having exactly the same number of pixels of each +luminosity in the output. +.PP +If you're processing a related set of images, for example frames of +an animation, it's generally best to apply the same luminosity mapping +to every frame, since otherwise you'll get distracting frame-to-frame +changes in the brightness of objects. \fBpnmhisteq\fP's \fB-wmap\fP +option allows you to save, as a PGM image, the luminosity map it +computes from an image. The \fB-rmap\fP option causes \fBpnmisteq\fP +to use such an image as its luminosity map. +.PP +So you can run \fBpnmhisteq\fP with \fB-wmap\fP on a composite you +created with \fBpamcat\fP of the images you intend to process. Then, you can +run \fBpnmisteq\fP with \fB-rmap\fP on each of the individual images, using +the luminosity map you generated from the composite. +.PP +Use \fBpnmhistmap\fP to see the result. Run a color image through +\fBppmtopgm\fP first so that you see a histogram of the luminosity instead of +histograms of the three color components. It should generally show a flat +histogram. But because of the quantization effects described above, you might +see high bars interleaved with low bars, with the local average being flat. +To see local averages, use the \fB-width\fP option of \fBpnmhistmap\fP. + + +.UN options +.SH OPTIONS +.PP +In addition to the options common to all programs based on libnetpbm +(most notably \fB-quiet\fP, see +.UR index.html#commonoptions + Common Options +.UE +\&), \fBpnmhisteq\fP recognizes the following +command line options: +.PP +You can abbreviate any option to its shortest unique prefix. + + +.TP +\fB-gray\fP +When processing a color image, only gray pixels (those with identical +red, green, and blue values) are included in the histogram and +modified in the output image. This is a special purpose option +intended for images where the actual data are gray scale, with color +annotations you don't want modified. Weather satellite images that +show continent outlines in color are best processed using this option. +The option has no effect when the input is a graymap. + +.TP +\fB-noblack\fP +Do not include black pixels in the equalization. The black pixels in the +output are exactly the black pixels in the input and the number of black +pixels does not affect the color of any other pixels. +.sp +Sometimes, black isn't as much a color as a background or annotation for +the real colors, so you want to treat it specially this way. Think of a +picture of stars, which is nearly all black, but with lots of stars of +different brightness. You want to change the brightnesses of the stars to +maximize contrast between them, but if you considered the blackness to be +significant, all the stars would end up close to full white. +.sp +This option was new in Netpbm 10.70 (March 2015). + +.TP +\fB-nowhite\fP +.sp +Same as \fB-noblack\fP, but for the white pixels. +.sp +This option was new in Netpbm 10.70 (March 2015). + +.TP +\fB-rmap\fP \fImapfile\fP +Process the image using the luminosity map specified by the PGM +file \fImapfile\fP. + +The PGM image, usually created by an earlier run of \fBpnmhisteq\fP +with the \fB-wmap\fP option, contains a single row with number of +columns equal to the maxval (greatest intensity value) of the image +plus one. Each pixel in the image is transformed by looking up its +luminosity in the corresponding column in the map file (column number += luminosity) and changing it to the value given by that column. + +.TP +\fB-wmap\fP \fImapfile\fP +Creates a PGM file \fImapfile\fP, containing the luminosity map +computed from the histogram of the input image. This map file can be +read on subsequent runs of \fBpnmhisteq\fP with the \fB-rmap\fP +option, allowing a group of images to be processed with an identical +map. + +.TP +\fB-verbose\fP +Prints the histogram and luminosity map on Standard Error. + + + +.UN limitations +.SH LIMITATIONS +.PP +Histogram equalization is effective for increasing the visible +detail in scientific imagery and in some continuous-tone pictures. It +is often too drastic, however, for scanned halftone images, where it +does an excellent job of making halftone artifacts apparent. You +might want to experiment with \fBpnmnorm\fP and \fBpnmgamma\fP for +more subtle contrast enhancement. +.PP +The luminosity map file supplied by the \fB-rmap\fP option must +have the same maxval as the input image. This is always the case when +the map file was created by the \fB-wmap\fP option of +\fBpnmhisteq\fP. If this restriction causes a problem, simply adjust +the maxval of the map with \fBpamdepth\fP to agree with the input +image. +.PP +If the input is a PBM file (on which histogram equalization is an +identity operation), the only effect of passing the file through +\fBpnmhisteq\fP will be the passage of time. + +.UN seealso +.SH SEE ALSO +.BR "pnmnorm" (1)\c +\&, +.BR "pamcat" (1)\c +\&, +.BR "pamdepth" (1)\c +\&, +.BR "pnmgamma" (1)\c +\&, +.BR "pnm" (5)\c +\&, + + +.TP +[1] +Russ, John C. The Image Processing Handbook. Boca Raton: CRC +Press, 1992. Pages 105-110. +.SH DOCUMENT SOURCE +This manual page was generated by the Netpbm tool 'makeman' from HTML +source. The master documentation is at +.IP +.B http://netpbm.sourceforge.net/doc/pnmhisteq.html +.PP
\ No newline at end of file |