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3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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cristyffb69ed2010-12-25 00:06:48 +0000[diff] [blame]12 <title>ImageMagick: The Fx Special Effects Image Operator</title>
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cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]218<h1>Apply Special Effects to an Image with an Fx Expression</h1>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]219<p class="navigation-index">[<a href="#fx">The Fx Special Effects Image Operator</a> &bull; <a href="#anatomy">The Anatomy of an Fx Expression</a>]</p>
220
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]221<a id="fx"></a>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]222<div class="doc-section">
223
224<p>Use the Fx special effects image operator to apply a mathematical expression to an image or image channels. Use Fx to:</p>
225
226<ul>
227 <li>create canvases, gradients, mathematical colormaps</li>
228 <li>move color values between images and channels</li>
229 <li>translate, flip, mirror, rotate, scale, shear and generally distort images</li>
230 <li>merge or composite multiple images together</li>
231 <li>convolve or merge neighboring pixels together</li>
232 <li>generate image metrics or 'fingerprints'</li>
233</ul>
234
235<p>The expression can be simple:</p>
236
cristyf9a6c462011-04-24 14:01:11 +0000[diff] [blame]237<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert -size 64x64 canvas:black -channel blue -fx "1/2" fx_navy.png</span></p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]238<p>Here, we convert a black to a navy blue image:</p>
239
240<p class="image">
241 <a href="../images/black.png"><img src="../images/black.png" width="64" height="64" alt="black" /></a>
242 <img style="margin-top:22px; margin-bottom:22px;" src="../images/right.gif" width="20" height="20" alt="==>" />
243 <a href="../images/navy.png"><img src="../images/navy.png" width="64" height="64" alt="navy" /></a>
244</p>
245
246<p>Or the expression can be complex:</p>
247
248<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg \ <br/>
249 -fx "(1.0/(1.0+exp(10.0*(0.5-u)))-0.006693)*1.0092503" \ <br/>
250 rose-sigmoidal.png</span></p>
251<p>This expression results in a high contrast version of the image:</p>
252
253<p class="image">
254 <a href="../images/rose.jpg"><img src="../images/rose.jpg" width="70" height="46" alt="rose" /></a>
255 <img style="margin-top:13px; margin-bottom:13px;" src="../images/right.gif" width="20" height="20" alt="==>" />
256 <a href="../images/rose-sigmoidal.png"><img src="../images/rose-sigmoidal.png" width="70" height="46" alt="rose-sigmoidal" /></a>
257</p>
258
259<p>The expression can include variable assignments. Assignments, in most cases, reduce the complexity of an expression and permit some operations that might not be possible any other way. For example, lets create a radial gradient:</p>
260
cristyf9a6c462011-04-24 14:01:11 +0000[diff] [blame]261<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert -size 70x70 canvas: \ <br/>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]262 -fx "Xi=i-w/2; Yj=j-h/2; 1.2*(0.5-hypot(Xi,Yj)/70.0)+0.5" \ <br/>
263 radial-gradient.png</span></p>
264<p>The command above returns this image:</p>
265
266<p class="image">
267 <a href="../images/radial-gradient.png"><img src="../images/radial-gradient.png" width="70" height="70" alt="radial-gradient" /></a>
268</p>
cristye9a32c32011-04-07 01:11:05 +0000[diff] [blame]269
270<p>This FX expression adds random noise to an image:</p>
271
272<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert photo.jpg -fx 'iso=32; rone=rand(); rtwo=rand(); \<br />
273 myn=sqrt(-2*ln(rone))*cos(2*Pi*rtwo); myntwo=sqrt(-2*ln(rtwo))* \<br />
274 cos(2*Pi*rone); pnoise=sqrt(p)*myn*sqrt(iso)* \ <br />
275 channel(4.28,3.86,6.68,0)/255; max(0,p+pnoise)' noisy.png</span></p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]276<p>See <a href="http://www.imagemagick.org/Usage/transform/index.html#fx">Using FX, The Special Effects Image Operator</a> for more examples.</p>
277
278<p>The next section discusses the Fx expression language.</p>
279
280</div>
281
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]282<h2><a id="anatomy"></a>The Anatomy of an Fx Expression</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]283<div class="doc-section">
284
285<h3>The Fx Expression Language</h3>
286<div class="doc-section">
287
288<p>The formal Fx expression language is defined here:</p>
289
290<dl class="doc">
291 <dt class="doc"> numbers:</dt>
292 <dd> integer, floating point, or scientfic notation (+/- required, e.g. 3.81469e-06)</dd>
293 <dt class="doc"> constants: </dt>
294 <dd> e, Epsilon, QuantumRange, QuantumScale, Opaque, Pi, Transparent</dd>
295 <dt class="doc"> Fx operators (in order of precedence): </dt>
cristyd92fed52010-10-25 13:48:19 +0000[diff] [blame]296 <dd> ^ (power), unary -, *, /, % (modulo), +, -,
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]297 &lt;&lt;, &gt;&gt;, &lt;, &lt;=, &gt;, &gt;=, ==, !=,
298 &amp; (bitwise AND), | (bitwise OR),
299 &amp;&amp; (logical AND), || (logical OR),
300 ~ (logical NOT), ?: (ternary conditional)</dd>
301 <dt class="doc"> math functions: </dt>
cristy19156992010-03-04 17:04:59 +0000[diff] [blame]302 <dd> abs(), acos(), airy(), alt(), asin(), atan(), atan2(), ceil(), cos(), cosh(), debug(), exp(), floor(),
303 hypot(), int(), j0(), j1(), jinc(), ln(), log(), logtwo(), max(), min(), mod(), pow(), rand(), round(),
cristy3c98a4f2010-08-13 20:09:41 +0000[diff] [blame]304 sign(), sin(), sinc(), sinh(), sqrt(), tan(), tanh(), trunc()</dd>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]305 <dt class="doc"> channel functions: </dt>
306 <dd> channel(r,g,b,a), channel(c,m,y,k,a)</dd>
307 <dt class="doc"> color names:</dt>
308 <dd> red, cyan, black, etc.</dd>
309 <dt class="doc"> color functions:</dt>
310 <dd> rgb(), rgba(), cmyk(), cmyka(), hsl(), hsla()</dd>
311 <dt class="doc"> color hex values:</dt>
312 <dd> #ccc, #cbfed0, #b9e1cc00</dd>
313 <dt class="doc"> symbols:</dt>
314 <dd style="white-space:pre">
315 s = current image in sequence
316 t = scene number of current image in sequence
317 u = first image in sequence
318 v = second image in sequence
319 n = number of images in sequence
320
321 i = column offset
322 j = row offset
323 p = pixel to use (absolute or relative to current pixel)
324
325 w = width of this image
326 h = height of this image
327 z = channel depth
328
329 r = red value (from RGBA), of a specific or current pixel
330 g = green ''
331 b = blue ''
332 a = alpha ''
333 o = opacity ''
334
335 c = cyan value of CMYK color of pixel
336 y = yellow ''
337 m = magenta ''
338 k = black ''
339
340 intensity = pixel intensity
341
342 hue = pixel hue
343 saturation = pixel saturation
344 lightness = pixel lightness
345 luminance = pixel luminance
346
347 page.width = page width
348 page.height = page height
349 page.x = page x offset
350 page.y = page y offset
351
352 resolution.x = x resolution
353 resolution.y = y resolution
354
355 depth = image depth
356 minima = image minima
357 maxima = image maxima
358 mean = image mean
359 standard_deviation = image standard deviation
360 kurtosis = image kurtosis
361 skewness = image skewness
362 (add a channel specifier to compute a statistic for that channel, e.g. depth.r)
363 </dd>
364</dl>
365
366</div>
367
368<h3>The Fx Expression</h3>
369<div class="doc-section">
370
371<p>An Fx expression may include any combination of the following:</p>
372<ul>
373<li> <em>x</em> <kbd>^</kbd> <em>y</em>: exponentiation (<em>x<sup>y</sup></em>)</li>
374<li> <kbd>(</kbd> ... <kbd>)</kbd>: grouping</li>
375<li> <em>x</em> <kbd>*</kbd> <em>y</em>: multiplication (the asterisk <kbd>*</kbd> is optional, for example, <kbd>2u</kbd> or <kbd>2(x+y)</kbd> are acceptable)</li>
376<li> <em>x</em> <kbd>/</kbd> <em>y</em>: division</li>
377<li> <em>x</em> <kbd>%</kbd> <em>y</em>: modulo</li>
378<li> <em>x</em> <kbd>+</kbd> <em>y</em>: addition</li>
379<li> <em>x</em> <kbd>-</kbd> <em>y</em>: subtraction</li>
380<li> <em>x</em> <kbd>&lt;&lt;</kbd> <em>y</em>: left shift</li>
381<li> <em>x</em> <kbd>&gt;&gt;</kbd> <em>y</em>: right shift</li>
382<li> <em>x</em> <kbd>&lt;</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x</em> &lt; <em>y</em>, otherwise 0.0</li>
383<li> <em>x</em> <kbd>&lt;=</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x</em> &lt;= <em>y</em>, otherwise 0.0</li>
384<li> <em>x</em> <kbd>&gt;</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x</em> &gt; <em>y</em>, otherwise 0.0</li>
385<li> <em>x</em> <kbd>&gt;=</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x</em> &gt;= <em>y</em>, otherwise 0.0</li>
386<li> <em>x</em> <kbd>==</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x </em>==<em> y</em>, otherwise 0.0</li>
387<li> <em>x</em> <kbd>!=</kbd> <em>y</em>: boolean relation, return value 1.0 if <em>x </em>!=<em> y</em>, otherwise 0.0</li>
388<li> <em>x</em> <kbd>&amp;</kbd> <em>y</em>: binary AND</li>
389<li> <em>x</em> <kbd>|</kbd> <em>y</em>: binary OR</li>
390<li> <em>x</em> <kbd>&amp;&amp;</kbd> <em>y</em>: logical AND connective, return value 1.0 if <em>x</em> &gt; 0 and <em>y</em> &gt; 0, otherwise 0.0</li>
391<li> <em>x</em> <kbd>||</kbd> <em>y</em>: logical OR connective (inclusive), return value 1.0 if <em>x</em> &gt; 0 or <em>y</em> &gt; 0 (or both), otherwise 0.0</li>
392<li> <kbd>~</kbd><em>x</em>: logical NOT operator, return value 1.0 if <em>not</em> <em>x</em> &gt; 0, otherwise 0.0</li>
393<li> <kbd>+</kbd><em>x</em>: unary plus, return 1.0*value</li>
394<li> <kbd>-</kbd><em>x</em>: unary minus, return -1.0*value</li>
395<li> <em>x</em> <kbd>?</kbd> <em>y</em> : <em>z</em>: ternary conditional expression, return value <em>y</em> if <em>x</em> != 0, otherwise <em>z</em>; only one ternary conditional permitted per statement</li>
396<li> <em>x</em> <kbd>=</kbd> <em>y</em>: assignment; assignment variables are restricted to letter combinations only (e.g. Xi not X1)</li>
397<li> <em>x</em> <kbd>;</kbd> <em>y</em>: statement separator </li>
398<li> <kbd>pi</kbd>: constant (3.141659...)</li>
399<li> <kbd>e</kbd>: constant (2.71828...)</li>
400<li> <kbd>QuantumRange</kbd>: constant maximum pixel value (255 for Q8, 65535 for Q16)</li>
401<li> <kbd>QuantumScale</kbd>: constant 1.0/<kbd>QuantumRange</kbd></li>
402<li> <kbd>intensity</kbd>: pixel intensity; equivalent to 0.299*red+0.587*green+0.114*blue</li>
403<li> <kbd>hue</kbd>: pixel hue</li>
404<li> <kbd>saturation</kbd>: pixel saturation</li>
405<li> <kbd>lightness</kbd>: pixel lightness; equivalent to 0.5*max(red,green,blue) + 0.5*min(red,green,blue)</li>
406<li> <kbd>luminance</kbd>: pixel luminance; equivalent to 0.2126*red + 0.7152*green + 0.0722*blue</li>
407<li> <kbd>red, green, blue</kbd>, etc.: color names</li>
408<li> <kbd>#ccc, #cbfed0, #b9e1cc00</kbd>, etc.: color hex values</li>
409<li> <kbd>rgb(), rgba(), cmyk(), cmyka(), hsl(), hsla()</kbd>: color functions</li>
410<li> <kbd>s, t, u, v, n, i, j, w, h, z, r, g, b, a, o, c, y, m, k</kbd>: symbols</li>
411<li> <kbd>abs(</kbd><em>x</em><kbd>)</kbd>: absolute value function</li>
412<li> <kbd>acos(</kbd><em>x</em><kbd>)</kbd>: arc cosine function</li>
cristy2c839602010-04-03 02:32:08 +0000[diff] [blame]413<li> <kbd>airy(</kbd><em>x</em><kbd>)</kbd>: Airy function (max=1, min=0); <kbd>airy(</kbd><em>x</em><kbd>)</kbd>=<kbd>[jinc(</kbd><em>x</em><kbd>)]<sup>2</sup></kbd>=<kbd>[2*j1(</kbd><em>pi*x</em><kbd>)/(</kbd><em>pi*x</em><kbd>)]<sup>2</sup></kbd></li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]414<li> <kbd>alt(</kbd><em>x</em><kbd>)</kbd>: sign alternation function (return 1.0 if <kbd>int(</kbd><em>x</em><kbd>)</kbd> is even, -1.0 if <kbd>int(</kbd><em>x</em><kbd>)</kbd> is odd)</li>
415<li> <kbd>asin(</kbd><em>x</em><kbd>)</kbd>: arc sine function</li>
416<li> <kbd>atan(</kbd><em>x</em><kbd>)</kbd>: arc tangent function</li>
417<li> <kbd>atan2(</kbd><em>y</em>,<em>x</em><kbd>)</kbd>: arc tangent function of two variables</li>
418<li> <kbd>ceil(</kbd><em>x</em><kbd>)</kbd>: smallest integral value not less than argument</li>
419<li> <kbd>channel(</kbd><em>r</em>,<em>g</em>,<em>b</em>,<em>a</em><kbd>)</kbd>: select numeric argument based on current channel</li>
420<li> <kbd>channel(</kbd><em>c</em>,<em>m</em>,<em>y</em>,<em>k</em>,<em>a</em><kbd>)</kbd>: select numeric argument based on current channel</li>
421<li> <kbd>cos(</kbd><em>x</em><kbd>)</kbd>: cosine function</li>
422<li> <kbd>cosh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic cosine function</li>
423<li> <kbd>debug(</kbd><em>x</em><kbd>)</kbd>: print <em>x</em> (useful for debugging your expression)</li>
424<li> <kbd>exp(</kbd><em>x</em><kbd>)</kbd>: natural exponential function (<em>e<sup>x</sup></em>)</li>
425<li> <kbd>floor(</kbd><em>x</em><kbd>)</kbd>: largest integral value not greater than argument</li>
426<li> <kbd>hypot(</kbd><em>x</em>,<em>y</em><kbd>)</kbd>: the square root of x<sup>2</sup>+y<sup>2</sup></li>
427<li> <kbd>int(</kbd><em>x</em><kbd>)</kbd>: greatest integer function (return greatest integer less than or equal to <em>x</em>)</li>
cristy2c839602010-04-03 02:32:08 +0000[diff] [blame]428<li> <kbd>j0(</kbd><em>x</em><kbd>)</kbd>: Bessel functions of <em>x</em> of the first kind of order 0</li>
429<li> <kbd>j1(</kbd><em>x</em><kbd>)</kbd>: Bessel functions of <em>x</em> of the first kind of order 1</li>
cristy4949d522010-03-12 18:41:34 +0000[diff] [blame]430<li> <kbd>jinc(</kbd><em>x</em><kbd>)</kbd>: jinc function (max=1, min=-0.1323); <kbd>jinc(</kbd><em>x</em><kbd>)</kbd>=<kbd>2*j1(</kbd><em>pi*x</em><kbd>)/(</kbd><em>pi*x</em><kbd>)</kbd></li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]431<li> <kbd>ln(</kbd><em>x</em><kbd>)</kbd>: natural logarithm function</li>
432<li> <kbd>log(</kbd><em>x</em><kbd>)</kbd>: logarithm base 10</li>
433<li> <kbd>logtwo(</kbd><em>x</em><kbd>)</kbd>: logarithm base 2</li>
434<li> <kbd>ln(</kbd><em>x</em><kbd>)</kbd>: natural logarithm</li>
435<li> <kbd>max(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: maximum of <em>x</em> and <em>y</em></li>
436<li> <kbd>min(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: minimum of <em>x</em> and <em>y</em></li>
437<li> <kbd>mod(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: floating-point remainder function</li>
438<li> <kbd>pow(</kbd><em>x</em>,<em>y</em><kbd>)</kbd>: power function (<em>x<sup>y</sup></em>)</li>
439<li> <kbd>rand()</kbd>: value uniformly distributed over the interval [0.0, 1.0) with a 2 to the 128th-1 period</li>
440<li> <kbd>round()</kbd>: round to integral value, regardless of rounding direction</li>
441<li> <kbd>sign(</kbd><em>x</em><kbd>)</kbd>: return -1.0 if <em>x</em> is less than 0.0 otherwise 1.0</li>
442<li> <kbd>sin(</kbd><em>x</em><kbd>)</kbd>: sine function</li>
cristy2c2a4492010-03-16 15:06:03 +0000[diff] [blame]443<li> <kbd>sinc(</kbd><em>x</em><kbd>)</kbd>: sinc function (max=1, min=-0.21); <kbd>sinc(</kbd><em>x</em><kbd>)</kbd>=<kbd>sin(</kbd><em>pi*x</em><kbd>)/(</kbd><em>pi*x</em><kbd>)</kbd></li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]444<li> <kbd>sinh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic sine function</li>
445<li> <kbd>sqrt(</kbd><em>x</em><kbd>)</kbd>: square root function</li>
446<li> <kbd>tan(</kbd><em>x</em><kbd>)</kbd>: tangent function</li>
447<li> <kbd>tanh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic tangent function</li>
cristy3c98a4f2010-08-13 20:09:41 +0000[diff] [blame]448<li> <kbd>trunc(</kbd><em>x</em><kbd>)</kbd>: round to integer, towards zero</li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]449</ul>
450
451<p>The expression semantics include these rules:</p>
452
453<ul>
454<li>symbols are case insensitive</li>
455<li>only one ternary conditional (e.g. x ? y : z) per statement</li>
456<li>statements are assignments or the final expression to return</li>
457<li>an assignment starts a statement, it is not an operator</li>
458<li>assignments to built-ins do not throw an exception and have no effect; e.g. <kbd>r=3.0; r</kbd> returns the pixel red color value, not 3.0</li>
cristyb7b531e2010-10-24 01:41:05 +0000[diff] [blame]459<li>Unary operators have a lower priority than binary operators, that is, the unary minus (negation) has lower precedence than exponentiation, so -3^2 is interpreted as -(3^2) = -9. Use parentheses to clarify your intent (e.g. (-3)^2 = 9).</li>
cristyce66ab12010-10-21 23:36:32 +0000[diff] [blame]460<li>Similarly, care must be exercised when using the slash ('/') symbol. The string of characters <em>1/2x</em> is interpreted as (1/2)x. The contrary interpretation should be written explicitly as 1/(2x). Again, the use of parentheses helps clarify the meaning and should be used whenever there is any chance of misinterpretation.</li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]461</ul>
462
cristyce66ab12010-10-21 23:36:32 +0000[diff] [blame]463
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]464</div>
465
466<h3>Source Images</h3>
467<div class="doc-section">
468
469<p>The symbols <kbd>u</kbd> and <kbd>v</kbd> refer to the first and second images, respectively, in the current image sequence. Refer to a particular image in a sequence by appending its index to any image reference (usually <kbd>u</kbd>), with a zero index for the beginning of the sequence. A negative index counts from the end. For example, <kbd>u[0]</kbd> is the first image in the sequence, <kbd>u[2]</kbd> is the third, <kbd>u[-1]</kbd> is the last image, and <kbd>u[t]</kbd> is the current image. The current image can also be referenced by <kbd>s</kbd>. If the sequence number exceeds the length of the sequence, the count is wrapped. Thus in a 3-image sequence, <kbd>u[-1]</kbd>, <kbd>u[2]</kbd>, and <kbd>u[5]</kbd> all refer to the same (third) image.</p>
470
471<p>As an example, we form an image by averaging the first image and third images:</p>
472
473<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert image1.jpg image2.jpg image3.jpg -fx "(u+u[2])/2.0" image.jpg</span></p>
474<p>By default, the image to which <kbd>p</kbd>, <kbd>r</kbd>, <kbd>g</kbd>, <kbd>b</kbd>, <kbd>a</kbd>, etc., are applied is the first image <kbd>u</kbd> in the image list, which may also be referenced using <kbd>s</kbd>. </p>
475
cristy19156992010-03-04 17:04:59 +0000[diff] [blame]476<p>It is important to note the special role played by the first image. This is the only image in the image sequence that is modified, other images are used only for their data. As an illustrative example, consider the following, and note that the setting <a href="../www/command-line-options.html#channel">-channel red</a> instructs <a href="../www/command-line-options.html#fx">-fx</a> to modify only the red channel; nothing in the green or blue channels will change. It is instructive to ponder why the result is not symmetric.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]477
478<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert -channel red logo: -flop logo: -resize "20%" -fx "(u+v)/2" image.jpg</span></p>
479<div class="image">
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]480 <a href="../images/logo-sm-flop.png"><img src="../images/logo-sm-flop.png" width="128" height="96" alt="logo-sm-flop.png" /></a>
481 <a href="../images/logo-sm.png"><img src="../images/logo-sm.png" width="128" height="96" alt="logo-sm.png" /></a>
482<img style="margin-top:38px; margin-bottom:38px;" src="../images/right.gif" width="20" height="20" alt="==>" />
483 <a href="../images/logo-sm-fx.png"><img src="../images/logo-sm-fx.png" width="128" height="96" alt="logo-sm-fx.png" /></a>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]484</div>
485
486
487</div>
488
489<h3>Accessing Pixels</h3>
490<div class="doc-section">
491
492<p>All color values are normalized to the range of 0.0 to 1.0. The alpha channel ranges from 0.0 (fully transparent) to 1.0 (fully opaque).</p>
493
494<p>The pixels are processed one at a time, but a different pixel of an image can be specified using a pixel index represented by <kbd>p</kbd>. For example,</p>
495
496<pre class="text">
497 p[-1].g green value of pixel to the immediate left of the current pixel
498 p[-1,-1].r red value of the pixel diagonally left and up from current pixel
499</pre>
500
501<p>To specify an absolute position, use braces, rather than brackets.</p>
502
503<pre class="text">
cristyee742642011-02-26 02:49:59 +0000[diff] [blame]504 p{0,0}.r red value of the pixel in the upper left corner of the image
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]505 p{12,34}.b blue pixel value at column number 12, row 34 of the image
506</pre>
507
508<p>Integer values of the position retrieve the color of the pixel referenced, while non-integer position values return a blended color according to the current <a href="../www/command-line-options.html#interpolate">-interpolate</a> setting.</p>
509
510<p>A position outside the boundary of the image retrieves a value dictated by the <a href="../www/command-line-options.html#virtual-pixel">-virtual-pixel</a> option setting.</p>
511
512</div>
513
514<h3>Apply an Expression to Select Image Channels</h3>
515<div class="doc-section">
516
517<p>Use the <a href="../www/command-line-options.html#channel">-channel</a> setting to specify the output channel of the result. If no output channel is given, the result is set over all channels except the opacity channel. For example, to replace the red channel of <kbd>alpha.png</kbd> with the average of the green channels from the images <kbd>alpha.png</kbd> and <kbd>beta.png</kbd>, use:</p>
518
519<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert alpha.png beta.png -channel red -fx "(u.g+v.g)/2" gamma.png</span></p>
520</div>
521
522<h3>Results</h3>
523<div class="doc-section">
524
525<p>The <a href="../www/command-line-options.html#fx">-fx</a> operator evaluates the given expression for each channel (set by <a href="../www/command-line-options.html#channel">-channel</a>) of each pixel in the first image (<kbd>u</kbd>) in the sequence. The computed values are temporarily stored in a copy (clone) of that first image until all the pixels have been processed, after which this single new image replaces the list of images in the current image sequence. As such, in the previous example the updated version of <kbd>alpha.png</kbd> replaces both of the original images, <kbd>alpha.png</kbd> and <kbd>beta.png</kbd>, before being saved as <kbd>gamma.png</kbd>.</p>
526
527<p>The current image <kbd>s</kbd> is set to the first image in the sequence (<kbd>u</kbd>), and <kbd>t</kbd> to its index, 0. The symbols <kbd>i</kbd> and <kbd>j</kbd> reference the current pixel being processed.</p>
528
529
530<p>For use with <a href="../www/command-line-options.html#format_identify_">-format</a>, the value-escape <kbd>%[fx: ]</kbd> is evaluated just once for each image in the current image sequence. As each image in the sequence is being evaluated, <kbd>s</kbd> and <kbd>t</kbd> successively refer to the current image and its index, while <kbd>i</kbd> and <kbd>j</kbd> are set to zero, and the current channel set to red (<a href="../www/command-line-options.html#channel">-channel</a> is ignored). An example:</p>
531
cristyf9a6c462011-04-24 14:01:11 +0000[diff] [blame]532<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert canvas:'rgb(25%,50%,75%)' rose: -colorspace rgb \ <br/>
cristy3cd7cc92011-04-01 01:00:28 +0000[diff] [blame]533 -format 'Red channel of NW corner of image #%[fx:t] is %[fx: s]' info:</span><span class='crtout'>Red channel of NW corner of image #0 is 0.453758 <br/>
534 Red channel of NW corner of image #1 is 0.184588</span></p>
535<p>Here we use the image indexes to <em>rotate</em> each image differently, and use <kbd>-set</kbd> with the image index to set a different <em>pause delay</em> on the first image in the animation:</p>
536
537<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose: -duplicate 29 -virtual-pixel Gray -distort SRT '%[fx:360.0*t/n]' \ <br />
538 -set delay '%[fx:t == 0 ? 240 : 10]' -loop 0 rose.gif</span></p><p>The color-escape <kbd>%[pixel: ]</kbd> is evaluated once per image and per color channel in that image (<a href="../www/command-line-options.html#channel">-channel</a> is ignored), The values generated are then converted into a color string (a named color or hex color value). The symbols <kbd>i</kbd> and <kbd>j</kbd> are set to zero, and <kbd>s</kbd> and <kbd>t</kbd> refer to each successively current image and index.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]539
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