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12 <title>ImageMagick: MagickCore, C API for ImageMagick: Reduce the Number of Unique Colors in an Image</title>
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162
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164
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]165<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>
166
167<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
168_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
169<div class="doc-section">
170
171<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p></ol>
172
173<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>
174
175<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>
176
177<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>
178
179<pre class="text">
180 the origin.
181</pre>
182
183<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>
184
185<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>
186
187<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>
188
189<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
190<h5>avoid building a fully populated tree, QUANTIZE</h5>
191<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>
192
193<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>
194
195<pre class="text">
196 n1: Number of pixels whose color is contained in the RGB cube which
197 this node represents;
198</pre>
199
200<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>
201
202<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>
203
204<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>
205
206<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>
207
208<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>
209
210<p>This has the effect of minimizing any quantization error when merging two nodes together.</p></ol>
211
212<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>
213
214<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>
215
216<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>
217
218<p>Assignment generates the output image from the pruned tree. The output</p>
219<h5>parts</h5>
220<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>
221
222<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>
223
224<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>
225
226<p>This method is based on a similar algorithm written by Paul Raveling.</p></ol>
227
228 </div>
229<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
230_8c.html" target="source" name="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
231<div class="doc-section">
232
233<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p></ol>
234
235<p>The format of the AcquireQuantizeInfo method is:</p>
236
237<pre class="code">
238 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
239</pre>
240
241<p>A description of each parameter follows:</p></ol>
242
243<h5>image_info</h5>
244<ol><p>the image info.</p></ol>
245
246 </div>
247<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
248_8c.html" target="source" name="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
249<div class="doc-section">
250
251<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p></ol>
252
253<p>The format of the CloneQuantizeInfo method is:</p>
254
255<pre class="code">
256 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
257</pre>
258
259<p>A description of each parameter follows:</p></ol>
260
261<h5>clone_info</h5>
262<ol><p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p></ol>
263
264<h5>quantize_info</h5>
265<ol><p>a structure of type info.</p></ol>
266
267 </div>
268<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
269_8c.html" target="source" name="CompressImageColormap">CompressImageColormap</a></h2>
270<div class="doc-section">
271
272<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p></ol>
273
274<p>The format of the CompressImageColormap method is:</p>
275
276<pre class="code">
277 MagickBooleanType CompressImageColormap(Image *image)
278</pre>
279
280<p>A description of each parameter follows:</p></ol>
281
282<h5>image</h5>
283<ol><p>the image.</p></ol>
284
285 </div>
286<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
287_8c.html" target="source" name="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
288<div class="doc-section">
289
290<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p></ol>
291
292<p>The format of the DestroyQuantizeInfo method is:</p>
293
294<pre class="code">
295 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
296</pre>
297
298<p>A description of each parameter follows:</p></ol>
299
300<h5>quantize_info</h5>
301<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
302
303 </div>
304<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
305_8c.html" target="source" name="GetImageQuantizeError">GetImageQuantizeError</a></h2>
306<div class="doc-section">
307
308<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>
309
310<pre class="text">
311 o mean_error_per_pixel: This value is the mean error for any single
312 pixel in the image.
313</pre>
314
315<h5>normalized_mean_square_error</h5>
316<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>
317
318<h5>normalized_maximum_square_error</h5>
319<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>
320
321<p>The format of the GetImageQuantizeError method is:</p>
322
323<pre class="code">
324 MagickBooleanType GetImageQuantizeError(Image *image)
325</pre>
326
327<p>A description of each parameter follows.</p></ol>
328
329<h5>image</h5>
330<ol><p>the image.</p></ol>
331
332 </div>
333<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
334_8c.html" target="source" name="GetQuantizeInfo">GetQuantizeInfo</a></h2>
335<div class="doc-section">
336
337<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p></ol>
338
339<p>The format of the GetQuantizeInfo method is:</p>
340
341<pre class="code">
342 GetQuantizeInfo(QuantizeInfo *quantize_info)
343</pre>
344
345<p>A description of each parameter follows:</p></ol>
346
347<h5>quantize_info</h5>
348<ol><p>Specifies a pointer to a QuantizeInfo structure.</p></ol>
349
350 </div>
351<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
352_8c.html" target="source" name="PosterizeImage">PosterizeImage</a></h2>
353<div class="doc-section">
354
355<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p></ol>
356
357<p>The format of the PosterizeImage method is:</p>
358
359<pre class="code">
cristybb503372010-05-27 20:51:26 +0000[diff] [blame]360 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]361 const MagickBooleanType dither)
362</pre>
363
364<p>A description of each parameter follows:</p></ol>
365
366<h5>image</h5>
367<ol><p>Specifies a pointer to an Image structure.</p></ol>
368
369<h5>levels</h5>
370<ol><p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p></ol>
371
372<h5>dither</h5>
373<ol><p>Set this integer value to something other than zero to dither the mapped image.</p></ol>
374
375 </div>
376<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
377_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
378<div class="doc-section">
379
380<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>
381
382<p>The format of the QuantizeImage method is:</p>
383
384<pre class="code">
385 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
386 Image *image)
387</pre>
388
389<p>A description of each parameter follows:</p></ol>
390
391<h5>quantize_info</h5>
392<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
393
394<h5>image</h5>
395<ol><p>the image.</p></ol>
396
397 </div>
398<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
399_8c.html" target="source" name="QuantizeImages">QuantizeImages</a></h2>
400<div class="doc-section">
401
402<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>
403
404<p>The format of the QuantizeImages method is:</p>
405
406<pre class="code">
407 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
408 Image *images)
409</pre>
410
411<p>A description of each parameter follows:</p></ol>
412
413<h5>quantize_info</h5>
414<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
415
416<h5>images</h5>
417<ol><p>Specifies a pointer to a list of Image structures.</p></ol>
418
419 </div>
420<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
421_8c.html" target="source" name="RemapImage">RemapImage</a></h2>
422<div class="doc-section">
423
424<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p></ol>
425
426<p>The format of the RemapImage method is:</p>
427
428<pre class="code">
429 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
430 Image *image,const Image *remap_image)
431</pre>
432
433<p>A description of each parameter follows:</p></ol>
434
435<h5>quantize_info</h5>
436<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
437
438<h5>image</h5>
439<ol><p>the image.</p></ol>
440
441<h5>remap_image</h5>
442<ol><p>the reference image.</p></ol>
443
444 </div>
445<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
446_8c.html" target="source" name="RemapImages">RemapImages</a></h2>
447<div class="doc-section">
448
449<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p></ol>
450
451<p>The format of the RemapImage method is:</p>
452
453<pre class="code">
454 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
455 Image *images,Image *remap_image)
456</pre>
457
458<p>A description of each parameter follows:</p></ol>
459
460<h5>quantize_info</h5>
461<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
462
463<h5>images</h5>
464<ol><p>the image sequence.</p></ol>
465
466<h5>remap_image</h5>
467<ol><p>the reference image.</p></ol>
468
469 </div>
470<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
471_8c.html" target="source" name="SetGrayscaleImage">SetGrayscaleImage</a></h2>
472<div class="doc-section">
473
474<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p></ol>
475
476<p>The format of the SetGrayscaleImage method is:</p>
477
478<pre class="code">
479 MagickBooleanType SetGrayscaleImage(Image *image)
480</pre>
481
482<p>A description of each parameter follows:</p></ol>
483
484<h5>image</h5>
485<ol><p>The image.</p></ol>
486
487 </div>
cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]488
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