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| author | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
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| committer | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
| commit | 1017cbebe5d5edd859e0fddad0a8600f509f4821 (patch) | |
| tree | 78bdf336648566f7a7d55f42837357dea3dd674c /pnmgamma.html | |
| parent | 16f2ac126651015a376eba864a3a35f738b0b25a (diff) | |
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Place user guide into Subversion repository
git-svn-id: http://svn.code.sf.net/p/netpbm/code/userguide@181 9d0c8265-081b-0410-96cb-a4ca84ce46f8
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diff --git a/pnmgamma.html b/pnmgamma.html new file mode 100644 index 00000000..5ce9ceae --- /dev/null +++ b/pnmgamma.html @@ -0,0 +1,314 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<HTML><HEAD><TITLE>Pnmgamma User Manual</TITLE></HEAD> +<BODY> +<H1>pnmgamma</H1> +Updated: 18 February 2006 +<BR> +<A HREF="#index">Table Of Contents</A> + +<H2>NAME</H2> + +pnmgamma - perform gamma correction on a PNM image + +<H2 id="synopsis">SYNOPSIS</H2> + +<B>pnmgamma </B> + +<br> +[ + <B>-bt709ramp</B> | + <B>-srgbramp</B> | +] +[<b>-ungamma</b>] +<br> +[<I>gamma</I> | <i>redgamma</i> <i>greengamma</i> <i>bluegamma</i> +[<I>pnmfile</I>]] + +<b>pnmgamma</b> +{ + <b>-bt709tolinear</b> | + <b>-lineartobt709</b> | + <b>-bt709tosrgb</b> | + <b>-srgbtobt709</b> +} +[<b>-gamma=</b><i>float</i>] +[<b>-rgamma=</b><i>float</i>] +[<b>-ggamma=</b><i>float</i>] +[<b>-bgamma=</b><i>float</i>] + +[<i>pnmfile</i>] + + +<H2 id="description">DESCRIPTION</H2> + +<p>This program is part of <a href="index.html">Netpbm</a>. + +<p><b>Pnmgamma</b> performs gamma correction on pseudo-PNM images. + +<P>The PPM format specification specifies that certain sample values +in a file represent certain light intensities in an image. In +particular, they specify that the sample values are directly +proportional to luminance as defined by ITU-R Recommendation BT.709. + +<P>However, people sometimes work with approximations of PPM and PGM +where the sample values represent intensity in different ways. For +example, the sample value might be directly proportional to radiance +(often called "linear intensity"). Or a different gamma +transfer function may be used. The relationship between the sample +values and radiance is often called a gamma transfer function. + +<P><B>pnmgamma</B> allows you to manipulate the gamma transfer +function, thus working with and/or creating pseudo-PPM files that are +useful for various things. + +<P>For example, if you feed a true PPM to <kbd>pnmgamma -bt709tolinear +</kbd>, you get as output a file which is PPM in every respect except +that the sample values are radiances. If you feed such a file to +<kbd>pnmgamma -linearto709</kbd>, you get back a true PPM. + +<P>The situation for PGM images is analogous. And <B>pnmgamma</B> +treats PBM images as PGM images. + +<P>When you feed a radiance-proportional pseudo-PPM image to a display +program that expects a true PPM, the display appears darker than it +should, so <B>pnmgamma</B> has the effect of lightening the image. +When you feed a true PPM to a display program that expects +radiance-proportional sample values, and therefore does a gamma +correction of its own on them, the display appears lighter than it +should, so <B>pnmgamma</B> with a gamma value less than one (the +multiplicative inverse of whatever gamma value the display program +uses) has the effect of darkening the image. + +<H2 id="parameters">PARAMETERS</H2> + +<p>The form of the parameters depends on whether you're using the old +syntax or the new syntax. With the old syntax, the parameters are +a mixture of gamma values and the input file name. With the new +syntax, the only parameter is the input file name and you specify gamma +values with option. + +<p>You use the old syntax if you specify <b>-bt709ramp</b> (or +its synonym <b>-cieramp</b>) or <b>-srgramp</b> or if you don't specify +any transfer function at all (and thus default to a simple exponential). +Otherwise, you use the new syntax. + +<p>With the old syntax, you may specify a single gamma value or 3 +separate gamma values (red, green, and blue) or no gamma values. In +any case, the meanings of those parameters is the same as the more +modern <b>-gamma</b>, <b>-rgamma</b>, <b>-ggamma</b>, and +<b>-bgamma</b> options described below. + + +<H2 id="options">OPTIONS</H2> + +<DL COMPACT> + +<dt><b>-bt709tolinear</b> + +<dd>Convert the image from ITU Recommendation BT.709 luminance to +radiance (sometimes called "linear"). I.e. convert from +true PPM or PGM to a radiance-linear variation that can be used with +certain tools that need it. + +<p>This option was new in Netpbm 10.32 (February 2006). + +<dt><b>-lineartobt709</b> + +<dd>Convert the image from radiance to ITU Recommendation BT.709 +luminance. I.e. convert to true PPM or PGM from a "linear" +variation. + +<p>This option was new in Netpbm 10.32 (February 2006). + +<dt><b>-bt709tosrgb</b> + +<dd>Convert the image from ITU Recommendation BT.709 luminance to SRGB +luminance. I.e. convert from true PPM or PGM to an SRGB-based +variation that is required by certain tools and display devices. + +<p>This option was new in Netpbm 10.32 (February 2006). + +<dt><b>-srgbtobt709</b> + +<dd>Convert the image from SRGB luminance to ITU Recommendation BT.709 +luminance. I.e. convert to true PPM or PGM from an SRGB-based +variation. + +<p>This option was new in Netpbm 10.32 (February 2006). + + +<DT><B>-bt709ramp</B> + +<DD>Use the ITU-R Recommendation BT.709 gamma transfer function. Note +that it is true BT.709 only if you use the default gamma value +(i.e. don't specify any gamma parameters). This transfer function is +a power function modified with a linear ramp near black. + +<P>If you specify none of the transfer functions the transfer function +defaults to a simple power function. + +<P>This option was renamed in Netpbm 10.32 (February 2006). Before that, +its name is <b>-cieramp</b>. + +<dt><b>-cieramp</b> + +<dd>This is an obsolete synonym for <b>-bt709ramp</b>. + +<P>The name of this option comes from a former belief that this was a +standard of CIE (International Commission On Illumination), but it now +(August 2005) looks like it never was. + +<DT><B>-srgbramp </B> + +<DD>Use the Internation Electrotechnical Commission (IEC) SRGB gamma +transfer function (as specified in the standard IEC 61966-2-1). Note +that it is true SRGB only if you use the default gamma value +(i.e. don't specify any gamma parameters). This transfer function is +like the one selected by <B>-bt709ramp</B>, but with different constants +in it. + +<P>Note that SRGB is often spelled "sRGB". In this +document, we use standard English typography, though, which doesn't +allow for that kind of capitalization. + +<P>If you specify neither <B>-bt709ramp</B> nor <B>-srgbramp</B>, the +transfer function defaults to a simple power function. + +<DT><B>-ungamma</B> + +<DD>Apply the inverse of the specified transfer function (i.e. go from +gamma-corrected luminance to radiance). + +<p>This is valid only with <b>-bt709ramp</b> (aka <b>-cieramp</b>), +<b>-srgbramp</b>, and the default exponential transfer function. + + +<dt><b>-gamma=</b><i>float</i> + +<dd>This specifies the gamma value to use in the transfer function. All +of the transfer functions involve an exponent, and the gamma value is that +exponent. + +<p>The standards specify a particular gamma value. If you use anything +else, you are varying from the standard. + +<p>The default is the standard value. For the simple exponential transfer +function (which is not a standard), the default is 2.2. + +<p>If you specify one of the component-specific options (<b>-rgamma</b>, +etc.), that overrides the <b>-gamma</b> value. + +<p>With the <b>-bt709ramp</b> (aka <b>-cieramp</b>), <b>-srgbramp</b>, +or the default exponentional transfer function, you can't actually use +this option, but you specify the same thing with <a +href="#parameters">parameters.</a> + +<p>This option was new in Netpbm 10.32 (February 2006). + +<dt><b>-rgamma=</b><i>float</i> +<dt><b>-ggamma=</b><i>float</i> +<dt><b>-bgamma=</b><i>float</i> + +<dd>These options are just like <b>-gamma</b>, except they specify the +value for a particular one of the color components. + +<p>If you don't specify this option for a particular color component, +the default is the <b>-gamma</b> value (or <b>-gamma</b>'s default if +you didn't specify that either). + +<p>With the <b>-bt709ramp</b> (aka <b>-cieramp</b>), <b>-srgbramp</b>, +or the default exponentional transfer function, you can't actually use +this option, but you specify the same thing with <a +href="#parameters">parameters.</a> + +<p>This option was new in Netpbm 10.32 (February 2006). + +<dt><b>-maxval=</b><i>maxval</i> + +<dd>This is the maxval of the output image. By default, the maxval of +the output is the same as that of the input. + +<p>Because the transformation is not linear, you need a greater maxval +in the output in order not to lose any information from the input. +For example, if you convert to radiance-linear sample values with with +<kbd>-ungamma -bt709ramp</kbd> and default gamma value, and your maxval is +255 on both input and output, 3 different input sample values all +generate output sample value 254. In order to have a different output +sample value for each input sample value, you would need an output +maxval at least 3 times the input maxval. + +<p>This option was new in Netpbm 10.32 (February 2006). Before that, +you can achieve the same result by increasing the maxval of the input +or decreasing the maxval of the output using <b>pnmdepth</b>. + +</DL> + +<H2 id="gamma">WHAT IS GAMMA?</H2> + +<P>A good explanation of gamma is in Charles Poynton's Gamma FAQ at +<A HREF="http://www.poynton.com/GammaFAQ.html"> +http://www.poynton.com/GammaFAQ.html</A> and Color FAQ at <A +HREF="http://www.poynton.com/ColorFAQ.html"> +http://www.poynton.com/ColorFAQ.html</A>. + +<P>In brief: The simplest way to code an image is by using sample +values that are directly proportional to the radiance of the color +components. Radiance is a physical quantification based on the amount +of power in the light; it is easily measurable in a laboratory, but +does not take into account what the light looks like to a person. It +wastes the sample space because the human eye can't discern +differences between low-radiance colors as well as it can between +high-radiance colors. So instead, we pass the radiance values +through a transfer function that makes it so that changing a sample +value by 1 causes the same level of perceived color change anywhere in +the sample range. We store those resulting values in the image file. +That transfer function is called the gamma transfer function and the +transformation is called gamma correcting. + +<p>The gamma-corrected value, proportional to subjective brightness, +are known as the luminance of the pixel. + +<p>There is no precise objective way to measure luminance, since it's +psychological. Also, perception of brightness varies according to a +variety of factors, including the surrounding in which an image is +viewed. Therefore, there is not just one gamma transfer function. + +<P>Virtually all image formats, either specified or de facto, use +gamma-corrected values for their sample values. + +<P>What's really nice about gamma is that by coincidence, the inverse +function that you have to do to convert the gamma-corrected values +back to radiance is done automatically by CRTs. You just apply a +voltage to the CRT's electron gun that is proportional to the +gamma-corrected sample value, and the radiance of the light that comes +out of the screen is close to the radiance value you had before you +applied the gamma transfer function! + +<P>And when you consider that computer video devices usually want you +to store in video memory a value proportional to the signal voltage +you want to go to the monitor, which the monitor turns into a +proportional drive voltage on the electron gun, it is really +convenient to work with gamma-corrected sample values. + +<H2 id="seealso">SEE ALSO</H2> + +<B><A HREF="pnm.html">pnm</A></B> + +<H2 id="author">AUTHOR</H2> + +Copyright (C) 1991 by Bill Davidson and Jef Poskanzer. + +<HR> +<A NAME="index"> </A> +<H2>Table Of Contents</H2> +<UL> +<LI><A HREF="#synopsis">SYNOPSIS</A> +<LI><A HREF="#description">DESCRIPTION</A> +<LI><A HREF="#parameters">PARAMETERS</A> +<LI><A HREF="#options">OPTIONS</A> +<LI><A HREF="#gamma">WHAT IS GAMMA?</A> +<LI><A HREF="#seealso">SEE ALSO</A> +<LI><A HREF="#author">AUTHOR</A> +</UL> +</BODY> +</HTML> |