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167
168<div class="main">
169
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]170<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>
171
172<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
173_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
174<div class="doc-section">
175
176<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p></ol>
177
178<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></ol>
179
180<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></ol>
181
182<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>
183
184<pre class="text">
185 the origin.
186</pre>
187
188<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></ol>
189
190<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></ol>
191
192<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></ol>
193
194<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
195<h5>avoid building a fully populated tree, QUANTIZE</h5>
196<ol><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></ol>
197
198<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>
199
200<pre class="text">
201 n1: Number of pixels whose color is contained in the RGB cube which
202 this node represents;
203</pre>
204
205<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></ol>
206
207<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></ol>
208
209<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></ol>
210
211<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></ol>
212
213<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></ol>
214
215<p>This has the effect of minimizing any quantization error when merging two nodes together.</p></ol>
216
217<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></ol>
218
219<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></ol>
220
221<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></ol>
222
223<p>Assignment generates the output image from the pruned tree. The output</p>
224<h5>parts</h5>
225<ol><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></ol>
226
227<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></ol>
228
229<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></ol>
230
231<p>This method is based on a similar algorithm written by Paul Raveling.</p></ol>
232
233 </div>
234<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
235_8c.html" target="source" name="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
236<div class="doc-section">
237
238<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p></ol>
239
240<p>The format of the AcquireQuantizeInfo method is:</p>
241
242<pre class="code">
243 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
244</pre>
245
246<p>A description of each parameter follows:</p></ol>
247
248<h5>image_info</h5>
249<ol><p>the image info.</p></ol>
250
251 </div>
252<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
253_8c.html" target="source" name="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
254<div class="doc-section">
255
256<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p></ol>
257
258<p>The format of the CloneQuantizeInfo method is:</p>
259
260<pre class="code">
261 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
262</pre>
263
264<p>A description of each parameter follows:</p></ol>
265
266<h5>clone_info</h5>
267<ol><p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p></ol>
268
269<h5>quantize_info</h5>
270<ol><p>a structure of type info.</p></ol>
271
272 </div>
273<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
274_8c.html" target="source" name="CompressImageColormap">CompressImageColormap</a></h2>
275<div class="doc-section">
276
277<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p></ol>
278
279<p>The format of the CompressImageColormap method is:</p>
280
281<pre class="code">
282 MagickBooleanType CompressImageColormap(Image *image)
283</pre>
284
285<p>A description of each parameter follows:</p></ol>
286
287<h5>image</h5>
288<ol><p>the image.</p></ol>
289
290 </div>
291<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
292_8c.html" target="source" name="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
293<div class="doc-section">
294
295<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p></ol>
296
297<p>The format of the DestroyQuantizeInfo method is:</p>
298
299<pre class="code">
300 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
301</pre>
302
303<p>A description of each parameter follows:</p></ol>
304
305<h5>quantize_info</h5>
306<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
307
308 </div>
309<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
310_8c.html" target="source" name="GetImageQuantizeError">GetImageQuantizeError</a></h2>
311<div class="doc-section">
312
313<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>
314
315<pre class="text">
316 o mean_error_per_pixel: This value is the mean error for any single
317 pixel in the image.
318</pre>
319
320<h5>normalized_mean_square_error</h5>
321<ol><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></ol>
322
323<h5>normalized_maximum_square_error</h5>
324<ol><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></ol>
325
326<p>The format of the GetImageQuantizeError method is:</p>
327
328<pre class="code">
329 MagickBooleanType GetImageQuantizeError(Image *image)
330</pre>
331
332<p>A description of each parameter follows.</p></ol>
333
334<h5>image</h5>
335<ol><p>the image.</p></ol>
336
337 </div>
338<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
339_8c.html" target="source" name="GetQuantizeInfo">GetQuantizeInfo</a></h2>
340<div class="doc-section">
341
342<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p></ol>
343
344<p>The format of the GetQuantizeInfo method is:</p>
345
346<pre class="code">
347 GetQuantizeInfo(QuantizeInfo *quantize_info)
348</pre>
349
350<p>A description of each parameter follows:</p></ol>
351
352<h5>quantize_info</h5>
353<ol><p>Specifies a pointer to a QuantizeInfo structure.</p></ol>
354
355 </div>
356<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
357_8c.html" target="source" name="PosterizeImage">PosterizeImage</a></h2>
358<div class="doc-section">
359
360<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p></ol>
361
362<p>The format of the PosterizeImage method is:</p>
363
364<pre class="code">
cristybb503372010-05-27 20:51:26 +0000[diff] [blame]365 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]366 const MagickBooleanType dither)
367</pre>
368
369<p>A description of each parameter follows:</p></ol>
370
371<h5>image</h5>
372<ol><p>Specifies a pointer to an Image structure.</p></ol>
373
374<h5>levels</h5>
375<ol><p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p></ol>
376
377<h5>dither</h5>
378<ol><p>Set this integer value to something other than zero to dither the mapped image.</p></ol>
379
380 </div>
381<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
382_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
383<div class="doc-section">
384
385<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></ol>
386
387<p>The format of the QuantizeImage method is:</p>
388
389<pre class="code">
390 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
391 Image *image)
392</pre>
393
394<p>A description of each parameter follows:</p></ol>
395
396<h5>quantize_info</h5>
397<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
398
399<h5>image</h5>
400<ol><p>the image.</p></ol>
401
402 </div>
403<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
404_8c.html" target="source" name="QuantizeImages">QuantizeImages</a></h2>
405<div class="doc-section">
406
407<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></ol>
408
409<p>The format of the QuantizeImages method is:</p>
410
411<pre class="code">
412 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
413 Image *images)
414</pre>
415
416<p>A description of each parameter follows:</p></ol>
417
418<h5>quantize_info</h5>
419<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
420
421<h5>images</h5>
422<ol><p>Specifies a pointer to a list of Image structures.</p></ol>
423
424 </div>
425<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
426_8c.html" target="source" name="RemapImage">RemapImage</a></h2>
427<div class="doc-section">
428
429<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p></ol>
430
431<p>The format of the RemapImage method is:</p>
432
433<pre class="code">
434 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
435 Image *image,const Image *remap_image)
436</pre>
437
438<p>A description of each parameter follows:</p></ol>
439
440<h5>quantize_info</h5>
441<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
442
443<h5>image</h5>
444<ol><p>the image.</p></ol>
445
446<h5>remap_image</h5>
447<ol><p>the reference image.</p></ol>
448
449 </div>
450<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
451_8c.html" target="source" name="RemapImages">RemapImages</a></h2>
452<div class="doc-section">
453
454<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p></ol>
455
456<p>The format of the RemapImage method is:</p>
457
458<pre class="code">
459 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
460 Image *images,Image *remap_image)
461</pre>
462
463<p>A description of each parameter follows:</p></ol>
464
465<h5>quantize_info</h5>
466<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
467
468<h5>images</h5>
469<ol><p>the image sequence.</p></ol>
470
471<h5>remap_image</h5>
472<ol><p>the reference image.</p></ol>
473
474 </div>
475<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
476_8c.html" target="source" name="SetGrayscaleImage">SetGrayscaleImage</a></h2>
477<div class="doc-section">
478
479<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p></ol>
480
481<p>The format of the SetGrayscaleImage method is:</p>
482
483<pre class="code">
484 MagickBooleanType SetGrayscaleImage(Image *image)
485</pre>
486
487<p>A description of each parameter follows:</p></ol>
488
489<h5>image</h5>
490<ol><p>The image.</p></ol>
491
492 </div>
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