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207
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]208<h1>Module quantize Methods</h1>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]209<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]210
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]211<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]212<div class="doc-section">
213
214<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
215
216<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>
217
218<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>
219
220<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>
221
222<pre class="text">
223 the origin.
224</pre>
225
226<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>
227
228<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>
229
230<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>
231
232<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
233<h5>avoid building a fully populated tree, QUANTIZE</h5>
234<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>
235
236<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>
237
238<pre class="text">
239 n1: Number of pixels whose color is contained in the RGB cube which
240 this node represents;
241</pre>
242
243<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>
244
245<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>
246
247<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>
248
249<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>
250
251<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>
252
253<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
254
255<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>
256
257<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>
258
259<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>
260
261<p>Assignment generates the output image from the pruned tree. The output</p>
262<h5>parts</h5>
263<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>
264
265<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>
266
267<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>
268
269<p>This method is based on a similar algorithm written by Paul Raveling.</p>
270
271 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]272<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]273<div class="doc-section">
274
275<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
276
277<p>The format of the AcquireQuantizeInfo method is:</p>
278
279<pre class="code">
280 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
281</pre>
282
283<p>A description of each parameter follows:</p>
284
285<h5>image_info</h5>
286<p>the image info.</p>
287
288 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]289<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]290<div class="doc-section">
291
292<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
293
294<p>The format of the CloneQuantizeInfo method is:</p>
295
296<pre class="code">
297 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
298</pre>
299
300<p>A description of each parameter follows:</p>
301
302<h5>clone_info</h5>
303<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
304
305<h5>quantize_info</h5>
306<p>a structure of type info.</p>
307
308 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]309<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]310<div class="doc-section">
311
312<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
313
314<p>The format of the CompressImageColormap method is:</p>
315
316<pre class="code">
317 MagickBooleanType CompressImageColormap(Image *image)
318</pre>
319
320<p>A description of each parameter follows:</p>
321
322<h5>image</h5>
323<p>the image.</p>
324
325 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]326<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]327<div class="doc-section">
328
329<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
330
331<p>The format of the DestroyQuantizeInfo method is:</p>
332
333<pre class="code">
334 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
335</pre>
336
337<p>A description of each parameter follows:</p>
338
339<h5>quantize_info</h5>
340<p>Specifies a pointer to an QuantizeInfo structure.</p>
341
342 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]343<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]344<div class="doc-section">
345
346<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>
347
348<pre class="text">
349 o mean_error_per_pixel: This value is the mean error for any single
350 pixel in the image.
351</pre>
352
353<h5>normalized_mean_square_error</h5>
354<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>
355
356<h5>normalized_maximum_square_error</h5>
357<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>
358
359<p>The format of the GetImageQuantizeError method is:</p>
360
361<pre class="code">
362 MagickBooleanType GetImageQuantizeError(Image *image)
363</pre>
364
365<p>A description of each parameter follows.</p>
366
367<h5>image</h5>
368<p>the image.</p>
369
370 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]371<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]372<div class="doc-section">
373
374<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
375
376<p>The format of the GetQuantizeInfo method is:</p>
377
378<pre class="code">
379 GetQuantizeInfo(QuantizeInfo *quantize_info)
380</pre>
381
382<p>A description of each parameter follows:</p>
383
384<h5>quantize_info</h5>
385<p>Specifies a pointer to a QuantizeInfo structure.</p>
386
387 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]388<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]389<div class="doc-section">
390
391<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
392
393<p>The format of the PosterizeImage method is:</p>
394
395<pre class="code">
396 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
397 const MagickBooleanType dither)
398 MagickBooleanType PosterizeImageChannel(Image *image,
399 const ChannelType channel,const size_t levels,
400 const MagickBooleanType dither)
401</pre>
402
403<p>A description of each parameter follows:</p>
404
405<h5>image</h5>
406<p>Specifies a pointer to an Image structure.</p>
407
408<h5>levels</h5>
409<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
410
411<h5>dither</h5>
412<p>Set this integer value to something other than zero to dither the mapped image.</p>
413
414 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]415<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]416<div class="doc-section">
417
418<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>
419
420<p>The format of the QuantizeImage method is:</p>
421
422<pre class="code">
423 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
424 Image *image)
425</pre>
426
427<p>A description of each parameter follows:</p>
428
429<h5>quantize_info</h5>
430<p>Specifies a pointer to an QuantizeInfo structure.</p>
431
432<h5>image</h5>
433<p>the image.</p>
434
435 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]436<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]437<div class="doc-section">
438
439<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>
440
441<p>The format of the QuantizeImages method is:</p>
442
443<pre class="code">
444 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
445 Image *images)
446</pre>
447
448<p>A description of each parameter follows:</p>
449
450<h5>quantize_info</h5>
451<p>Specifies a pointer to an QuantizeInfo structure.</p>
452
453<h5>images</h5>
454<p>Specifies a pointer to a list of Image structures.</p>
455
456 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]457<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]458<div class="doc-section">
459
460<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
461
462<p>The format of the RemapImage method is:</p>
463
464<pre class="code">
465 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
466 Image *image,const Image *remap_image)
467</pre>
468
469<p>A description of each parameter follows:</p>
470
471<h5>quantize_info</h5>
472<p>Specifies a pointer to an QuantizeInfo structure.</p>
473
474<h5>image</h5>
475<p>the image.</p>
476
477<h5>remap_image</h5>
478<p>the reference image.</p>
479
480 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]481<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]482<div class="doc-section">
483
484<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p>
485
486<p>The format of the RemapImage method is:</p>
487
488<pre class="code">
489 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
490 Image *images,Image *remap_image)
491</pre>
492
493<p>A description of each parameter follows:</p>
494
495<h5>quantize_info</h5>
496<p>Specifies a pointer to an QuantizeInfo structure.</p>
497
498<h5>images</h5>
499<p>the image sequence.</p>
500
501<h5>remap_image</h5>
502<p>the reference image.</p>
503
504 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]505<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]506<div class="doc-section">
507
508<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
509
510<p>The format of the SetGrayscaleImage method is:</p>
511
512<pre class="code">
513 MagickBooleanType SetGrayscaleImage(Image *image)
514</pre>
515
516<p>A description of each parameter follows:</p>
517
518<h5>image</h5>
519<p>The image.</p>
520
521 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]522
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