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192
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]193<h1>Module quantize Methods</h1>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]194<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]195
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]196<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]197<div class="doc-section">
198
199<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
200
201<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>
202
203<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>
204
205<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>
206
207<pre class="text">
208 the origin.
209</pre>
210
211<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>
212
213<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>
214
215<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>
216
217<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
218<h5>avoid building a fully populated tree, QUANTIZE</h5>
219<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>
220
221<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>
222
223<pre class="text">
224 n1: Number of pixels whose color is contained in the RGB cube which
225 this node represents;
226</pre>
227
228<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>
229
230<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>
231
232<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>
233
234<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>
235
236<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>
237
238<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
239
240<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>
241
242<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>
243
244<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>
245
246<p>Assignment generates the output image from the pruned tree. The output</p>
247<h5>parts</h5>
248<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>
249
250<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>
251
252<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>
253
254<p>This method is based on a similar algorithm written by Paul Raveling.</p>
255
256 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]257<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]258<div class="doc-section">
259
260<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
261
262<p>The format of the AcquireQuantizeInfo method is:</p>
263
264<pre class="code">
265 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
266</pre>
267
268<p>A description of each parameter follows:</p>
269
270<h5>image_info</h5>
271<p>the image info.</p>
272
273 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]274<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]275<div class="doc-section">
276
277<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
278
279<p>The format of the CloneQuantizeInfo method is:</p>
280
281<pre class="code">
282 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
283</pre>
284
285<p>A description of each parameter follows:</p>
286
287<h5>clone_info</h5>
288<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
289
290<h5>quantize_info</h5>
291<p>a structure of type info.</p>
292
293 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]294<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]295<div class="doc-section">
296
297<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
298
299<p>The format of the CompressImageColormap method is:</p>
300
301<pre class="code">
302 MagickBooleanType CompressImageColormap(Image *image)
303</pre>
304
305<p>A description of each parameter follows:</p>
306
307<h5>image</h5>
308<p>the image.</p>
309
310 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]311<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]312<div class="doc-section">
313
314<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
315
316<p>The format of the DestroyQuantizeInfo method is:</p>
317
318<pre class="code">
319 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
320</pre>
321
322<p>A description of each parameter follows:</p>
323
324<h5>quantize_info</h5>
325<p>Specifies a pointer to an QuantizeInfo structure.</p>
326
327 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]328<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]329<div class="doc-section">
330
331<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>
332
333<pre class="text">
334 o mean_error_per_pixel: This value is the mean error for any single
335 pixel in the image.
336</pre>
337
338<h5>normalized_mean_square_error</h5>
339<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>
340
341<h5>normalized_maximum_square_error</h5>
342<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>
343
344<p>The format of the GetImageQuantizeError method is:</p>
345
346<pre class="code">
347 MagickBooleanType GetImageQuantizeError(Image *image)
348</pre>
349
350<p>A description of each parameter follows.</p>
351
352<h5>image</h5>
353<p>the image.</p>
354
355 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]356<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]357<div class="doc-section">
358
359<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
360
361<p>The format of the GetQuantizeInfo method is:</p>
362
363<pre class="code">
364 GetQuantizeInfo(QuantizeInfo *quantize_info)
365</pre>
366
367<p>A description of each parameter follows:</p>
368
369<h5>quantize_info</h5>
370<p>Specifies a pointer to a QuantizeInfo structure.</p>
371
372 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]373<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]374<div class="doc-section">
375
376<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
377
378<p>The format of the PosterizeImage method is:</p>
379
380<pre class="code">
381 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
382 const MagickBooleanType dither)
383 MagickBooleanType PosterizeImageChannel(Image *image,
384 const ChannelType channel,const size_t levels,
385 const MagickBooleanType dither)
386</pre>
387
388<p>A description of each parameter follows:</p>
389
390<h5>image</h5>
391<p>Specifies a pointer to an Image structure.</p>
392
393<h5>levels</h5>
394<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
395
396<h5>dither</h5>
397<p>Set this integer value to something other than zero to dither the mapped image.</p>
398
399 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]400<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]401<div class="doc-section">
402
403<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>
404
405<p>The format of the QuantizeImage method is:</p>
406
407<pre class="code">
408 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
409 Image *image)
410</pre>
411
412<p>A description of each parameter follows:</p>
413
414<h5>quantize_info</h5>
415<p>Specifies a pointer to an QuantizeInfo structure.</p>
416
417<h5>image</h5>
418<p>the image.</p>
419
420 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]421<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]422<div class="doc-section">
423
424<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>
425
426<p>The format of the QuantizeImages method is:</p>
427
428<pre class="code">
429 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
430 Image *images)
431</pre>
432
433<p>A description of each parameter follows:</p>
434
435<h5>quantize_info</h5>
436<p>Specifies a pointer to an QuantizeInfo structure.</p>
437
438<h5>images</h5>
439<p>Specifies a pointer to a list of Image structures.</p>
440
441 </div>
cristy350dea42011-02-15 17:31:04 +0000[diff] [blame^]442<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]443<div class="doc-section">
444
445<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
446
447<p>The format of the RemapImage method is:</p>
448
449<pre class="code">
450 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
451 Image *image,const Image *remap_image)
452</pre>
453
454<p>A description of each parameter follows:</p>
455
456<h5>quantize_info</h5>
457<p>Specifies a pointer to an QuantizeInfo structure.</p>
458
459<h5>image</h5>
460<p>the image.</p>
461
462<h5>remap_image</h5>
463<p>the reference image.</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="RemapImages">RemapImages</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]467<div class="doc-section">
468
469<p>RemapImages() replaces the colors of a sequence of images 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 RemapImages(const QuantizeInfo *quantize_info,
475 Image *images,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>images</h5>
484<p>the image sequence.</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="SetGrayscaleImage">SetGrayscaleImage</a></h2>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]491<div class="doc-section">
492
493<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
494
495<p>The format of the SetGrayscaleImage method is:</p>
496
497<pre class="code">
498 MagickBooleanType SetGrayscaleImage(Image *image)
499</pre>
500
501<p>A description of each parameter follows:</p>
502
503<h5>image</h5>
504<p>The image.</p>
505
506 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]507
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