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cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]196<h1>Module quantize Methods</h1>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]197<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]198
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]199<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]200<div class="doc-section">
201
202<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
203
204<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>
205
206<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>
207
208<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>
209
210<pre class="text">
211 the origin.
212</pre>
213
214<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>
215
216<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>
217
218<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>
219
220<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
221<h5>avoid building a fully populated tree, QUANTIZE</h5>
222<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>
223
224<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>
225
226<pre class="text">
227 n1: Number of pixels whose color is contained in the RGB cube which
228 this node represents;
229</pre>
230
231<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>
232
233<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>
234
235<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>
236
237<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>
238
239<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>
240
241<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
242
243<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>
244
245<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>
246
247<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>
248
249<p>Assignment generates the output image from the pruned tree. The output</p>
250<h5>parts</h5>
251<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>
252
253<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>
254
255<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>
256
257<p>This method is based on a similar algorithm written by Paul Raveling.</p>
258
259 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]260<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]261<div class="doc-section">
262
263<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
264
265<p>The format of the AcquireQuantizeInfo method is:</p>
266
267<pre class="code">
268 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
269</pre>
270
271<p>A description of each parameter follows:</p>
272
273<h5>image_info</h5>
274<p>the image info.</p>
275
276 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]277<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]278<div class="doc-section">
279
280<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
281
282<p>The format of the CloneQuantizeInfo method is:</p>
283
284<pre class="code">
285 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
286</pre>
287
288<p>A description of each parameter follows:</p>
289
290<h5>clone_info</h5>
291<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
292
293<h5>quantize_info</h5>
294<p>a structure of type info.</p>
295
296 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]297<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]298<div class="doc-section">
299
300<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
301
302<p>The format of the CompressImageColormap method is:</p>
303
304<pre class="code">
305 MagickBooleanType CompressImageColormap(Image *image)
306</pre>
307
308<p>A description of each parameter follows:</p>
309
310<h5>image</h5>
311<p>the image.</p>
312
313 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]314<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]315<div class="doc-section">
316
317<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
318
319<p>The format of the DestroyQuantizeInfo method is:</p>
320
321<pre class="code">
322 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
323</pre>
324
325<p>A description of each parameter follows:</p>
326
327<h5>quantize_info</h5>
328<p>Specifies a pointer to an QuantizeInfo structure.</p>
329
330 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]331<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]332<div class="doc-section">
333
334<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>
335
336<pre class="text">
337 o mean_error_per_pixel: This value is the mean error for any single
338 pixel in the image.
339</pre>
340
341<h5>normalized_mean_square_error</h5>
342<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>
343
344<h5>normalized_maximum_square_error</h5>
345<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>
346
347<p>The format of the GetImageQuantizeError method is:</p>
348
349<pre class="code">
350 MagickBooleanType GetImageQuantizeError(Image *image)
351</pre>
352
353<p>A description of each parameter follows.</p>
354
355<h5>image</h5>
356<p>the image.</p>
357
358 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]359<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]360<div class="doc-section">
361
362<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
363
364<p>The format of the GetQuantizeInfo method is:</p>
365
366<pre class="code">
367 GetQuantizeInfo(QuantizeInfo *quantize_info)
368</pre>
369
370<p>A description of each parameter follows:</p>
371
372<h5>quantize_info</h5>
373<p>Specifies a pointer to a QuantizeInfo structure.</p>
374
375 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]376<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]377<div class="doc-section">
378
379<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
380
381<p>The format of the PosterizeImage method is:</p>
382
383<pre class="code">
384 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
385 const MagickBooleanType dither)
386 MagickBooleanType PosterizeImageChannel(Image *image,
387 const ChannelType channel,const size_t levels,
388 const MagickBooleanType dither)
389</pre>
390
391<p>A description of each parameter follows:</p>
392
393<h5>image</h5>
394<p>Specifies a pointer to an Image structure.</p>
395
396<h5>levels</h5>
397<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
398
399<h5>dither</h5>
400<p>Set this integer value to something other than zero to dither the mapped image.</p>
401
402 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]403<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]404<div class="doc-section">
405
406<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>
407
408<p>The format of the QuantizeImage method is:</p>
409
410<pre class="code">
411 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
412 Image *image)
413</pre>
414
415<p>A description of each parameter follows:</p>
416
417<h5>quantize_info</h5>
418<p>Specifies a pointer to an QuantizeInfo structure.</p>
419
420<h5>image</h5>
421<p>the 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="QuantizeImages">QuantizeImages</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]425<div class="doc-section">
426
427<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>
428
429<p>The format of the QuantizeImages method is:</p>
430
431<pre class="code">
432 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
433 Image *images)
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>images</h5>
442<p>Specifies a pointer to a list of Image structures.</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="RemapImage">RemapImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]446<div class="doc-section">
447
448<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
449
450<p>The format of the RemapImage method is:</p>
451
452<pre class="code">
453 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
454 Image *image,const Image *remap_image)
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>image</h5>
463<p>the image.</p>
464
465<h5>remap_image</h5>
466<p>the reference image.</p>
467
468 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]469<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]470<div class="doc-section">
471
472<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p>
473
474<p>The format of the RemapImage method is:</p>
475
476<pre class="code">
477 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
478 Image *images,Image *remap_image)
479</pre>
480
481<p>A description of each parameter follows:</p>
482
483<h5>quantize_info</h5>
484<p>Specifies a pointer to an QuantizeInfo structure.</p>
485
486<h5>images</h5>
487<p>the image sequence.</p>
488
489<h5>remap_image</h5>
490<p>the reference image.</p>
491
492 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]493<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]494<div class="doc-section">
495
496<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
497
498<p>The format of the SetGrayscaleImage method is:</p>
499
500<pre class="code">
501 MagickBooleanType SetGrayscaleImage(Image *image)
502</pre>
503
504<p>A description of each parameter follows:</p>
505
506<h5>image</h5>
507<p>The image.</p>
508
509 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]510
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