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201
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]202<h1>Module quantize Methods</h1>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]203<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]204
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]205<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]206<div class="doc-section">
207
208<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
209
210<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>
211
212<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>
213
214<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>
215
216<pre class="text">
217 the origin.
218</pre>
219
220<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>
221
222<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>
223
224<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>
225
226<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
227<h5>avoid building a fully populated tree, QUANTIZE</h5>
228<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>
229
230<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>
231
232<pre class="text">
233 n1: Number of pixels whose color is contained in the RGB cube which
234 this node represents;
235</pre>
236
237<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>
238
239<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>
240
241<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>
242
243<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>
244
245<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>
246
247<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
248
249<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>
250
251<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>
252
253<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>
254
255<p>Assignment generates the output image from the pruned tree. The output</p>
256<h5>parts</h5>
257<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>
258
259<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>
260
261<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>
262
263<p>This method is based on a similar algorithm written by Paul Raveling.</p>
264
265 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]266<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]267<div class="doc-section">
268
269<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
270
271<p>The format of the AcquireQuantizeInfo method is:</p>
272
273<pre class="code">
274 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
275</pre>
276
277<p>A description of each parameter follows:</p>
278
279<h5>image_info</h5>
280<p>the image info.</p>
281
282 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]283<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]284<div class="doc-section">
285
286<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
287
288<p>The format of the CloneQuantizeInfo method is:</p>
289
290<pre class="code">
291 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
292</pre>
293
294<p>A description of each parameter follows:</p>
295
296<h5>clone_info</h5>
297<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
298
299<h5>quantize_info</h5>
300<p>a structure of type info.</p>
301
302 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]303<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]304<div class="doc-section">
305
306<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
307
308<p>The format of the CompressImageColormap method is:</p>
309
310<pre class="code">
311 MagickBooleanType CompressImageColormap(Image *image)
312</pre>
313
314<p>A description of each parameter follows:</p>
315
316<h5>image</h5>
317<p>the image.</p>
318
319 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]320<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]321<div class="doc-section">
322
323<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
324
325<p>The format of the DestroyQuantizeInfo method is:</p>
326
327<pre class="code">
328 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
329</pre>
330
331<p>A description of each parameter follows:</p>
332
333<h5>quantize_info</h5>
334<p>Specifies a pointer to an QuantizeInfo structure.</p>
335
336 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]337<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]338<div class="doc-section">
339
340<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>
341
342<pre class="text">
343 o mean_error_per_pixel: This value is the mean error for any single
344 pixel in the image.
345</pre>
346
347<h5>normalized_mean_square_error</h5>
348<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>
349
350<h5>normalized_maximum_square_error</h5>
351<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>
352
353<p>The format of the GetImageQuantizeError method is:</p>
354
355<pre class="code">
356 MagickBooleanType GetImageQuantizeError(Image *image)
357</pre>
358
359<p>A description of each parameter follows.</p>
360
361<h5>image</h5>
362<p>the image.</p>
363
364 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]365<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]366<div class="doc-section">
367
368<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
369
370<p>The format of the GetQuantizeInfo method is:</p>
371
372<pre class="code">
373 GetQuantizeInfo(QuantizeInfo *quantize_info)
374</pre>
375
376<p>A description of each parameter follows:</p>
377
378<h5>quantize_info</h5>
379<p>Specifies a pointer to a QuantizeInfo structure.</p>
380
381 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]382<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]383<div class="doc-section">
384
385<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
386
387<p>The format of the PosterizeImage method is:</p>
388
389<pre class="code">
390 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
391 const MagickBooleanType dither)
392 MagickBooleanType PosterizeImageChannel(Image *image,
393 const ChannelType channel,const size_t levels,
394 const MagickBooleanType dither)
395</pre>
396
397<p>A description of each parameter follows:</p>
398
399<h5>image</h5>
400<p>Specifies a pointer to an Image structure.</p>
401
402<h5>levels</h5>
403<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
404
405<h5>dither</h5>
406<p>Set this integer value to something other than zero to dither the mapped image.</p>
407
408 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]409<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]410<div class="doc-section">
411
412<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>
413
414<p>The format of the QuantizeImage method is:</p>
415
416<pre class="code">
417 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
418 Image *image)
419</pre>
420
421<p>A description of each parameter follows:</p>
422
423<h5>quantize_info</h5>
424<p>Specifies a pointer to an QuantizeInfo structure.</p>
425
426<h5>image</h5>
427<p>the image.</p>
428
429 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]430<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]431<div class="doc-section">
432
433<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>
434
435<p>The format of the QuantizeImages method is:</p>
436
437<pre class="code">
438 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
439 Image *images)
440</pre>
441
442<p>A description of each parameter follows:</p>
443
444<h5>quantize_info</h5>
445<p>Specifies a pointer to an QuantizeInfo structure.</p>
446
447<h5>images</h5>
448<p>Specifies a pointer to a list of Image structures.</p>
449
450 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]451<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]452<div class="doc-section">
453
454<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
455
456<p>The format of the RemapImage method is:</p>
457
458<pre class="code">
459 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
460 Image *image,const Image *remap_image)
461</pre>
462
463<p>A description of each parameter follows:</p>
464
465<h5>quantize_info</h5>
466<p>Specifies a pointer to an QuantizeInfo structure.</p>
467
468<h5>image</h5>
469<p>the image.</p>
470
471<h5>remap_image</h5>
472<p>the reference image.</p>
473
474 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]475<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]476<div class="doc-section">
477
478<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p>
479
480<p>The format of the RemapImage method is:</p>
481
482<pre class="code">
483 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
484 Image *images,Image *remap_image)
485</pre>
486
487<p>A description of each parameter follows:</p>
488
489<h5>quantize_info</h5>
490<p>Specifies a pointer to an QuantizeInfo structure.</p>
491
492<h5>images</h5>
493<p>the image sequence.</p>
494
495<h5>remap_image</h5>
496<p>the reference image.</p>
497
498 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame]499<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]500<div class="doc-section">
501
502<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
503
504<p>The format of the SetGrayscaleImage method is:</p>
505
506<pre class="code">
507 MagickBooleanType SetGrayscaleImage(Image *image)
508</pre>
509
510<p>A description of each parameter follows:</p>
511
512<h5>image</h5>
513<p>The image.</p>
514
515 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]516
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