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cristyffb69ed2010-12-25 00:06:48 +0000[diff] [blame]8 <title>ImageMagick: The Fx Special Effects Image Operator</title>
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189
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]190<h1>Apply Special Effects to an Image with an Fx Expression</h1>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]191<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>
192
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]193<a id="fx"></a>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]194<div class="doc-section">
195
196<p>Use the Fx special effects image operator to apply a mathematical expression to an image or image channels. Use Fx to:</p>
197
198<ul>
199 <li>create canvases, gradients, mathematical colormaps</li>
200 <li>move color values between images and channels</li>
201 <li>translate, flip, mirror, rotate, scale, shear and generally distort images</li>
202 <li>merge or composite multiple images together</li>
203 <li>convolve or merge neighboring pixels together</li>
204 <li>generate image metrics or 'fingerprints'</li>
205</ul>
206
207<p>The expression can be simple:</p>
208
209<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert -size 64x64 xc:black -channel blue -fx "1/2" fx_navy.png</span></p>
210<p>Here, we convert a black to a navy blue image:</p>
211
212<p class="image">
213 <a href="../images/black.png"><img src="../images/black.png" width="64" height="64" alt="black" /></a>
214 <img style="margin-top:22px; margin-bottom:22px;" src="../images/right.gif" width="20" height="20" alt="==>" />
215 <a href="../images/navy.png"><img src="../images/navy.png" width="64" height="64" alt="navy" /></a>
216</p>
217
218<p>Or the expression can be complex:</p>
219
220<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg \ <br/>
221 -fx "(1.0/(1.0+exp(10.0*(0.5-u)))-0.006693)*1.0092503" \ <br/>
222 rose-sigmoidal.png</span></p>
223<p>This expression results in a high contrast version of the image:</p>
224
225<p class="image">
226 <a href="../images/rose.jpg"><img src="../images/rose.jpg" width="70" height="46" alt="rose" /></a>
227 <img style="margin-top:13px; margin-bottom:13px;" src="../images/right.gif" width="20" height="20" alt="==>" />
228 <a href="../images/rose-sigmoidal.png"><img src="../images/rose-sigmoidal.png" width="70" height="46" alt="rose-sigmoidal" /></a>
229</p>
230
231<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>
232
233<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert -size 70x70 xc: \ <br/>
234 -fx "Xi=i-w/2; Yj=j-h/2; 1.2*(0.5-hypot(Xi,Yj)/70.0)+0.5" \ <br/>
235 radial-gradient.png</span></p>
236<p>The command above returns this image:</p>
237
238<p class="image">
239 <a href="../images/radial-gradient.png"><img src="../images/radial-gradient.png" width="70" height="70" alt="radial-gradient" /></a>
240</p>
241<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>
242
243<p>The next section discusses the Fx expression language.</p>
244
245</div>
246
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]247<h2><a id="anatomy"></a>The Anatomy of an Fx Expression</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]248<div class="doc-section">
249
250<h3>The Fx Expression Language</h3>
251<div class="doc-section">
252
253<p>The formal Fx expression language is defined here:</p>
254
255<dl class="doc">
256 <dt class="doc"> numbers:</dt>
257 <dd> integer, floating point, or scientfic notation (+/- required, e.g. 3.81469e-06)</dd>
258 <dt class="doc"> constants: </dt>
259 <dd> e, Epsilon, QuantumRange, QuantumScale, Opaque, Pi, Transparent</dd>
260 <dt class="doc"> Fx operators (in order of precedence): </dt>
cristyd92fed52010-10-25 13:48:19 +0000[diff] [blame]261 <dd> ^ (power), unary -, *, /, % (modulo), +, -,
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]262 &lt;&lt;, &gt;&gt;, &lt;, &lt;=, &gt;, &gt;=, ==, !=,
263 &amp; (bitwise AND), | (bitwise OR),
264 &amp;&amp; (logical AND), || (logical OR),
265 ~ (logical NOT), ?: (ternary conditional)</dd>
266 <dt class="doc"> math functions: </dt>
cristy19156992010-03-04 17:04:59 +0000[diff] [blame]267 <dd> abs(), acos(), airy(), alt(), asin(), atan(), atan2(), ceil(), cos(), cosh(), debug(), exp(), floor(),
268 hypot(), int(), j0(), j1(), jinc(), ln(), log(), logtwo(), max(), min(), mod(), pow(), rand(), round(),
cristy3c98a4f2010-08-13 20:09:41 +0000[diff] [blame]269 sign(), sin(), sinc(), sinh(), sqrt(), tan(), tanh(), trunc()</dd>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]270 <dt class="doc"> channel functions: </dt>
271 <dd> channel(r,g,b,a), channel(c,m,y,k,a)</dd>
272 <dt class="doc"> color names:</dt>
273 <dd> red, cyan, black, etc.</dd>
274 <dt class="doc"> color functions:</dt>
275 <dd> rgb(), rgba(), cmyk(), cmyka(), hsl(), hsla()</dd>
276 <dt class="doc"> color hex values:</dt>
277 <dd> #ccc, #cbfed0, #b9e1cc00</dd>
278 <dt class="doc"> symbols:</dt>
279 <dd style="white-space:pre">
280 s = current image in sequence
281 t = scene number of current image in sequence
282 u = first image in sequence
283 v = second image in sequence
284 n = number of images in sequence
285
286 i = column offset
287 j = row offset
288 p = pixel to use (absolute or relative to current pixel)
289
290 w = width of this image
291 h = height of this image
292 z = channel depth
293
294 r = red value (from RGBA), of a specific or current pixel
295 g = green ''
296 b = blue ''
297 a = alpha ''
298 o = opacity ''
299
300 c = cyan value of CMYK color of pixel
301 y = yellow ''
302 m = magenta ''
303 k = black ''
304
305 intensity = pixel intensity
306
307 hue = pixel hue
308 saturation = pixel saturation
309 lightness = pixel lightness
310 luminance = pixel luminance
311
312 page.width = page width
313 page.height = page height
314 page.x = page x offset
315 page.y = page y offset
316
317 resolution.x = x resolution
318 resolution.y = y resolution
319
320 depth = image depth
321 minima = image minima
322 maxima = image maxima
323 mean = image mean
324 standard_deviation = image standard deviation
325 kurtosis = image kurtosis
326 skewness = image skewness
327 (add a channel specifier to compute a statistic for that channel, e.g. depth.r)
328 </dd>
329</dl>
330
331</div>
332
333<h3>The Fx Expression</h3>
334<div class="doc-section">
335
336<p>An Fx expression may include any combination of the following:</p>
337<ul>
338<li> <em>x</em> <kbd>^</kbd> <em>y</em>: exponentiation (<em>x<sup>y</sup></em>)</li>
339<li> <kbd>(</kbd> ... <kbd>)</kbd>: grouping</li>
340<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>
341<li> <em>x</em> <kbd>/</kbd> <em>y</em>: division</li>
342<li> <em>x</em> <kbd>%</kbd> <em>y</em>: modulo</li>
343<li> <em>x</em> <kbd>+</kbd> <em>y</em>: addition</li>
344<li> <em>x</em> <kbd>-</kbd> <em>y</em>: subtraction</li>
345<li> <em>x</em> <kbd>&lt;&lt;</kbd> <em>y</em>: left shift</li>
346<li> <em>x</em> <kbd>&gt;&gt;</kbd> <em>y</em>: right shift</li>
347<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>
348<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>
349<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>
350<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>
351<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>
352<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>
353<li> <em>x</em> <kbd>&amp;</kbd> <em>y</em>: binary AND</li>
354<li> <em>x</em> <kbd>|</kbd> <em>y</em>: binary OR</li>
355<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>
356<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>
357<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>
358<li> <kbd>+</kbd><em>x</em>: unary plus, return 1.0*value</li>
359<li> <kbd>-</kbd><em>x</em>: unary minus, return -1.0*value</li>
360<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>
361<li> <em>x</em> <kbd>=</kbd> <em>y</em>: assignment; assignment variables are restricted to letter combinations only (e.g. Xi not X1)</li>
362<li> <em>x</em> <kbd>;</kbd> <em>y</em>: statement separator </li>
363<li> <kbd>pi</kbd>: constant (3.141659...)</li>
364<li> <kbd>e</kbd>: constant (2.71828...)</li>
365<li> <kbd>QuantumRange</kbd>: constant maximum pixel value (255 for Q8, 65535 for Q16)</li>
366<li> <kbd>QuantumScale</kbd>: constant 1.0/<kbd>QuantumRange</kbd></li>
367<li> <kbd>intensity</kbd>: pixel intensity; equivalent to 0.299*red+0.587*green+0.114*blue</li>
368<li> <kbd>hue</kbd>: pixel hue</li>
369<li> <kbd>saturation</kbd>: pixel saturation</li>
370<li> <kbd>lightness</kbd>: pixel lightness; equivalent to 0.5*max(red,green,blue) + 0.5*min(red,green,blue)</li>
371<li> <kbd>luminance</kbd>: pixel luminance; equivalent to 0.2126*red + 0.7152*green + 0.0722*blue</li>
372<li> <kbd>red, green, blue</kbd>, etc.: color names</li>
373<li> <kbd>#ccc, #cbfed0, #b9e1cc00</kbd>, etc.: color hex values</li>
374<li> <kbd>rgb(), rgba(), cmyk(), cmyka(), hsl(), hsla()</kbd>: color functions</li>
375<li> <kbd>s, t, u, v, n, i, j, w, h, z, r, g, b, a, o, c, y, m, k</kbd>: symbols</li>
376<li> <kbd>abs(</kbd><em>x</em><kbd>)</kbd>: absolute value function</li>
377<li> <kbd>acos(</kbd><em>x</em><kbd>)</kbd>: arc cosine function</li>
cristy2c839602010-04-03 02:32:08 +0000[diff] [blame]378<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]379<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>
380<li> <kbd>asin(</kbd><em>x</em><kbd>)</kbd>: arc sine function</li>
381<li> <kbd>atan(</kbd><em>x</em><kbd>)</kbd>: arc tangent function</li>
382<li> <kbd>atan2(</kbd><em>y</em>,<em>x</em><kbd>)</kbd>: arc tangent function of two variables</li>
383<li> <kbd>ceil(</kbd><em>x</em><kbd>)</kbd>: smallest integral value not less than argument</li>
384<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>
385<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>
386<li> <kbd>cos(</kbd><em>x</em><kbd>)</kbd>: cosine function</li>
387<li> <kbd>cosh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic cosine function</li>
388<li> <kbd>debug(</kbd><em>x</em><kbd>)</kbd>: print <em>x</em> (useful for debugging your expression)</li>
389<li> <kbd>exp(</kbd><em>x</em><kbd>)</kbd>: natural exponential function (<em>e<sup>x</sup></em>)</li>
390<li> <kbd>floor(</kbd><em>x</em><kbd>)</kbd>: largest integral value not greater than argument</li>
391<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>
392<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]393<li> <kbd>j0(</kbd><em>x</em><kbd>)</kbd>: Bessel functions of <em>x</em> of the first kind of order 0</li>
394<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]395<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]396<li> <kbd>ln(</kbd><em>x</em><kbd>)</kbd>: natural logarithm function</li>
397<li> <kbd>log(</kbd><em>x</em><kbd>)</kbd>: logarithm base 10</li>
398<li> <kbd>logtwo(</kbd><em>x</em><kbd>)</kbd>: logarithm base 2</li>
399<li> <kbd>ln(</kbd><em>x</em><kbd>)</kbd>: natural logarithm</li>
400<li> <kbd>max(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: maximum of <em>x</em> and <em>y</em></li>
401<li> <kbd>min(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: minimum of <em>x</em> and <em>y</em></li>
402<li> <kbd>mod(</kbd><em>x</em>, <em>y</em><kbd>)</kbd>: floating-point remainder function</li>
403<li> <kbd>pow(</kbd><em>x</em>,<em>y</em><kbd>)</kbd>: power function (<em>x<sup>y</sup></em>)</li>
404<li> <kbd>rand()</kbd>: value uniformly distributed over the interval [0.0, 1.0) with a 2 to the 128th-1 period</li>
405<li> <kbd>round()</kbd>: round to integral value, regardless of rounding direction</li>
406<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>
407<li> <kbd>sin(</kbd><em>x</em><kbd>)</kbd>: sine function</li>
cristy2c2a4492010-03-16 15:06:03 +0000[diff] [blame]408<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]409<li> <kbd>sinh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic sine function</li>
410<li> <kbd>sqrt(</kbd><em>x</em><kbd>)</kbd>: square root function</li>
411<li> <kbd>tan(</kbd><em>x</em><kbd>)</kbd>: tangent function</li>
412<li> <kbd>tanh(</kbd><em>x</em><kbd>)</kbd>: hyperbolic tangent function</li>
cristy3c98a4f2010-08-13 20:09:41 +0000[diff] [blame]413<li> <kbd>trunc(</kbd><em>x</em><kbd>)</kbd>: round to integer, towards zero</li>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]414</ul>
415
416<p>The expression semantics include these rules:</p>
417
418<ul>
419<li>symbols are case insensitive</li>
420<li>only one ternary conditional (e.g. x ? y : z) per statement</li>
421<li>statements are assignments or the final expression to return</li>
422<li>an assignment starts a statement, it is not an operator</li>
423<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]424<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]425<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]426</ul>
427
cristyce66ab12010-10-21 23:36:32 +0000[diff] [blame]428
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]429</div>
430
431<h3>Source Images</h3>
432<div class="doc-section">
433
434<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>
435
436<p>As an example, we form an image by averaging the first image and third images:</p>
437
438<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>
439<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>
440
cristy19156992010-03-04 17:04:59 +0000[diff] [blame]441<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]442
443<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>
444<div class="image">
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]445 <a href="../images/logo-sm-flop.png"><img src="../images/logo-sm-flop.png" width="128" height="96" alt="logo-sm-flop.png" /></a>
446 <a href="../images/logo-sm.png"><img src="../images/logo-sm.png" width="128" height="96" alt="logo-sm.png" /></a>
447<img style="margin-top:38px; margin-bottom:38px;" src="../images/right.gif" width="20" height="20" alt="==>" />
448 <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]449</div>
450
451
452</div>
453
454<h3>Accessing Pixels</h3>
455<div class="doc-section">
456
457<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>
458
459<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>
460
461<pre class="text">
462 p[-1].g green value of pixel to the immediate left of the current pixel
463 p[-1,-1].r red value of the pixel diagonally left and up from current pixel
464</pre>
465
466<p>To specify an absolute position, use braces, rather than brackets.</p>
467
468<pre class="text">
469 p{0,0}.r red value of the pixel in the upper left corner of the image
470 p{12,34}.b blue pixel value at column number 12, row 34 of the image
471</pre>
472
473<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>
474
475<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>
476
477</div>
478
479<h3>Apply an Expression to Select Image Channels</h3>
480<div class="doc-section">
481
482<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>
483
484<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>
485</div>
486
487<h3>Results</h3>
488<div class="doc-section">
489
490<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>
491
492<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>
493
494
495<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>
496
497<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert xc:'rgb(25%,50%,75%)' rose: -colorspace rgb \ <br/>
498 -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 0.250004 <br/>
499 Red channel of NW corner of image # 1 is 0.188235</span></p>
500<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>
501
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