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216
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]217<h1>Module quantize Methods</h1>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]218<p class="navigation-index">[<a href="#QuantizeImage">QuantizeImage</a> &bull; <a href="#AcquireQuantizeInfo">AcquireQuantizeInfo</a> &bull; <a href="#CloneQuantizeInfo">CloneQuantizeInfo</a> &bull; <a href="#CompressImageColormap">CompressImageColormap</a> &bull; <a href="#DestroyQuantizeInfo">DestroyQuantizeInfo</a> &bull; <a href="#GetImageQuantizeError">GetImageQuantizeError</a> &bull; <a href="#GetQuantizeInfo">GetQuantizeInfo</a> &bull; <a href="#PosterizeImage">PosterizeImage</a> &bull; <a href="#QuantizeImage">QuantizeImage</a> &bull; <a href="#QuantizeImages">QuantizeImages</a> &bull; <a href="#RemapImage">RemapImage</a> &bull; <a href="#RemapImages">RemapImages</a> &bull; <a href="#SetGrayscaleImage">SetGrayscaleImage</a>]</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]219
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]220<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImage">QuantizeImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]221<div class="doc-section">
222
223<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
224
225<p>For purposes of color allocation, an image is a set of n pixels, where each pixel is a point in RGB space. RGB space is a 3-dimensional vector space, and each pixel, Pi, is defined by an ordered triple of red, green, and blue coordinates, (Ri, Gi, Bi).</p>
226
227<p>Each primary color component (red, green, or blue) represents an intensity which varies linearly from 0 to a maximum value, Cmax, which corresponds to full saturation of that color. Color allocation is defined over a domain consisting of the cube in RGB space with opposite vertices at (0,0,0) and (Cmax, Cmax, Cmax). QUANTIZE requires Cmax = 255.</p>
228
229<p>The algorithm maps this domain onto a tree in which each node represents a cube within that domain. In the following discussion these cubes are defined by the coordinate of two opposite vertices:</p>
230
231<pre class="text">
232 the origin.
233</pre>
234
235<p>The tree's root node represents the entire domain, (0,0,0) through (Cmax,Cmax,Cmax). Each lower level in the tree is generated by subdividing one node's cube into eight smaller cubes of equal size. This corresponds to bisecting the parent cube with planes passing through the midpoints of each edge.</p>
236
237<p>The basic algorithm operates in three phases: Classification, Reduction, and Assignment. Classification builds a color description tree for the image. Reduction collapses the tree until the number it represents, at most, the number of colors desired in the output image. Assignment defines the output image's color map and sets each pixel's color by restorage_class in the reduced tree. Our goal is to minimize the numerical discrepancies between the original colors and quantized colors (quantization error).</p>
238
239<p>Classification begins by initializing a color description tree of sufficient depth to represent each possible input color in a leaf. However, it is impractical to generate a fully-formed color description tree in the storage_class phase for realistic values of Cmax. If colors components in the input image are quantized to k-bit precision, so that Cmax= 2k-1, the tree would need k levels below the root node to allow representing each possible input color in a leaf. This becomes prohibitive because the tree's total number of nodes is 1 + sum(i=1, k, 8k).</p>
240
241<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
242<h5>avoid building a fully populated tree, QUANTIZE</h5>
243<p>(1) Initializes data structures for nodes only as they are needed; (2) Chooses a maximum depth for the tree as a function of the desired number of colors in the output image (currently log2(colormap size)).</p>
244
245<p>For each pixel in the input image, storage_class scans downward from the root of the color description tree. At each level of the tree it identifies the single node which represents a cube in RGB space containing the pixel's color. It updates the following data for each such node:</p>
246
247<pre class="text">
248 n1: Number of pixels whose color is contained in the RGB cube which
249 this node represents;
250</pre>
251
252<p>n2: Number of pixels whose color is not represented in a node at lower depth in the tree; initially, n2 = 0 for all nodes except leaves of the tree.</p>
253
254<p>Sr, Sg, Sb: Sums of the red, green, and blue component values for all pixels not classified at a lower depth. The combination of these sums and n2 will ultimately characterize the mean color of a set of pixels represented by this node.</p>
255
256<p>E: the distance squared in RGB space between each pixel contained within a node and the nodes' center. This represents the quantization error for a node.</p>
257
258<p>Reduction repeatedly prunes the tree until the number of nodes with n2 > 0 is less than or equal to the maximum number of colors allowed in the output image. On any given iteration over the tree, it selects those nodes whose E count is minimal for pruning and merges their color statistics upward. It uses a pruning threshold, Ep, to govern node selection as follows:</p>
259
260<p>Ep = 0 while number of nodes with (n2 > 0) > required maximum number of colors prune all nodes such that E <= Ep Set Ep to minimum E in remaining nodes</p>
261
262<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
263
264<p>When a node to be pruned has offspring, the pruning procedure invokes itself recursively in order to prune the tree from the leaves upward. n2, Sr, Sg, and Sb in a node being pruned are always added to the corresponding data in that node's parent. This retains the pruned node's color characteristics for later averaging.</p>
265
266<p>For each node, n2 pixels exist for which that node represents the smallest volume in RGB space containing those pixel's colors. When n2 > 0 the node will uniquely define a color in the output image. At the beginning of reduction, n2 = 0 for all nodes except a the leaves of the tree which represent colors present in the input image.</p>
267
268<p>The other pixel count, n1, indicates the total number of colors within the cubic volume which the node represents. This includes n1 - n2 pixels whose colors should be defined by nodes at a lower level in the tree.</p>
269
270<p>Assignment generates the output image from the pruned tree. The output</p>
271<h5>parts</h5>
272<p>(1) A color map, which is an array of color descriptions (RGB triples) for each color present in the output image; (2) A pixel array, which represents each pixel as an index into the color map array.</p>
273
274<p>First, the assignment phase makes one pass over the pruned color description tree to establish the image's color map. For each node with n2 > 0, it divides Sr, Sg, and Sb by n2 . This produces the mean color of all pixels that classify no lower than this node. Each of these colors becomes an entry in the color map.</p>
275
276<p>Finally, the assignment phase reclassifies each pixel in the pruned tree to identify the deepest node containing the pixel's color. The pixel's value in the pixel array becomes the index of this node's mean color in the color map.</p>
277
278<p>This method is based on a similar algorithm written by Paul Raveling.</p>
279
280 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]281<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]282<div class="doc-section">
283
284<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
285
286<p>The format of the AcquireQuantizeInfo method is:</p>
287
288<pre class="code">
289 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
290</pre>
291
292<p>A description of each parameter follows:</p>
293
294<h5>image_info</h5>
295<p>the image info.</p>
296
297 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]298<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]299<div class="doc-section">
300
301<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
302
303<p>The format of the CloneQuantizeInfo method is:</p>
304
305<pre class="code">
306 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
307</pre>
308
309<p>A description of each parameter follows:</p>
310
311<h5>clone_info</h5>
312<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
313
314<h5>quantize_info</h5>
315<p>a structure of type info.</p>
316
317 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]318<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CompressImageColormap">CompressImageColormap</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]319<div class="doc-section">
320
321<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
322
323<p>The format of the CompressImageColormap method is:</p>
324
325<pre class="code">
326 MagickBooleanType CompressImageColormap(Image *image)
327</pre>
328
329<p>A description of each parameter follows:</p>
330
331<h5>image</h5>
332<p>the image.</p>
333
334 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]335<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]336<div class="doc-section">
337
338<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
339
340<p>The format of the DestroyQuantizeInfo method is:</p>
341
342<pre class="code">
343 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
344</pre>
345
346<p>A description of each parameter follows:</p>
347
348<h5>quantize_info</h5>
349<p>Specifies a pointer to an QuantizeInfo structure.</p>
350
351 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]352<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetImageQuantizeError">GetImageQuantizeError</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]353<div class="doc-section">
354
355<p>GetImageQuantizeError() measures the difference between the original and quantized images. This difference is the total quantization error. The error is computed by summing over all pixels in an image the distance squared in RGB space between each reference pixel value and its quantized value. These values are computed:</p>
356
357<pre class="text">
358 o mean_error_per_pixel: This value is the mean error for any single
359 pixel in the image.
360</pre>
361
362<h5>normalized_mean_square_error</h5>
363<p>This value is the normalized mean quantization error for any single pixel in the image. This distance measure is normalized to a range between 0 and 1. It is independent of the range of red, green, and blue values in the image.</p>
364
365<h5>normalized_maximum_square_error</h5>
366<p>Thsi value is the normalized maximum quantization error for any single pixel in the image. This distance measure is normalized to a range between 0 and 1. It is independent of the range of red, green, and blue values in your image.</p>
367
368<p>The format of the GetImageQuantizeError method is:</p>
369
370<pre class="code">
371 MagickBooleanType GetImageQuantizeError(Image *image)
372</pre>
373
374<p>A description of each parameter follows.</p>
375
376<h5>image</h5>
377<p>the image.</p>
378
379 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]380<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetQuantizeInfo">GetQuantizeInfo</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]381<div class="doc-section">
382
383<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
384
385<p>The format of the GetQuantizeInfo method is:</p>
386
387<pre class="code">
388 GetQuantizeInfo(QuantizeInfo *quantize_info)
389</pre>
390
391<p>A description of each parameter follows:</p>
392
393<h5>quantize_info</h5>
394<p>Specifies a pointer to a QuantizeInfo structure.</p>
395
396 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]397<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="PosterizeImage">PosterizeImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]398<div class="doc-section">
399
400<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
401
402<p>The format of the PosterizeImage method is:</p>
403
404<pre class="code">
405 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
406 const MagickBooleanType dither)
407 MagickBooleanType PosterizeImageChannel(Image *image,
408 const ChannelType channel,const size_t levels,
409 const MagickBooleanType dither)
410</pre>
411
412<p>A description of each parameter follows:</p>
413
414<h5>image</h5>
415<p>Specifies a pointer to an Image structure.</p>
416
417<h5>levels</h5>
418<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
419
420<h5>dither</h5>
421<p>Set this integer value to something other than zero to dither the mapped image.</p>
422
423 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]424<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImage">QuantizeImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]425<div class="doc-section">
426
427<p>QuantizeImage() analyzes the colors within a reference image and chooses a fixed number of colors to represent the image. The goal of the algorithm is to minimize the color difference between the input and output image while minimizing the processing time.</p>
428
429<p>The format of the QuantizeImage method is:</p>
430
431<pre class="code">
432 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
433 Image *image)
434</pre>
435
436<p>A description of each parameter follows:</p>
437
438<h5>quantize_info</h5>
439<p>Specifies a pointer to an QuantizeInfo structure.</p>
440
441<h5>image</h5>
442<p>the image.</p>
443
444 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]445<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImages">QuantizeImages</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]446<div class="doc-section">
447
448<p>QuantizeImages() analyzes the colors within a set of reference images and chooses a fixed number of colors to represent the set. The goal of the algorithm is to minimize the color difference between the input and output images while minimizing the processing time.</p>
449
450<p>The format of the QuantizeImages method is:</p>
451
452<pre class="code">
453 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
454 Image *images)
455</pre>
456
457<p>A description of each parameter follows:</p>
458
459<h5>quantize_info</h5>
460<p>Specifies a pointer to an QuantizeInfo structure.</p>
461
462<h5>images</h5>
463<p>Specifies a pointer to a list of Image structures.</p>
464
465 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]466<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImage">RemapImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]467<div class="doc-section">
468
469<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
470
471<p>The format of the RemapImage method is:</p>
472
473<pre class="code">
474 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
475 Image *image,const Image *remap_image)
476</pre>
477
478<p>A description of each parameter follows:</p>
479
480<h5>quantize_info</h5>
481<p>Specifies a pointer to an QuantizeInfo structure.</p>
482
483<h5>image</h5>
484<p>the image.</p>
485
486<h5>remap_image</h5>
487<p>the reference image.</p>
488
489 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]490<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImages">RemapImages</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]491<div class="doc-section">
492
493<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p>
494
495<p>The format of the RemapImage method is:</p>
496
497<pre class="code">
498 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
499 Image *images,Image *remap_image)
500</pre>
501
502<p>A description of each parameter follows:</p>
503
504<h5>quantize_info</h5>
505<p>Specifies a pointer to an QuantizeInfo structure.</p>
506
507<h5>images</h5>
508<p>the image sequence.</p>
509
510<h5>remap_image</h5>
511<p>the reference image.</p>
512
513 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]514<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="SetGrayscaleImage">SetGrayscaleImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]515<div class="doc-section">
516
517<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
518
519<p>The format of the SetGrayscaleImage method is:</p>
520
521<pre class="code">
522 MagickBooleanType SetGrayscaleImage(Image *image)
523</pre>
524
525<p>A description of each parameter follows:</p>
526
527<h5>image</h5>
528<p>The image.</p>
529
530 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]531
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