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11 <title>ImageMagick: MagickCore, C API for ImageMagick: Reduce the Number of Unique Colors in an Image</title>
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150
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]151<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>
152
153<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
154_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
155<div class="doc-section">
156
157<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p></ol>
158
159<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>
160
161<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>
162
163<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>
164
165<pre class="text">
166 the origin.
167</pre>
168
169<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>
170
171<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>
172
173<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>
174
175<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
176<h5>avoid building a fully populated tree, QUANTIZE</h5>
177<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>
178
179<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>
180
181<pre class="text">
182 n1: Number of pixels whose color is contained in the RGB cube which
183 this node represents;
184</pre>
185
186<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>
187
188<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>
189
190<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>
191
192<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>
193
194<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>
195
196<p>This has the effect of minimizing any quantization error when merging two nodes together.</p></ol>
197
198<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>
199
200<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>
201
202<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>
203
204<p>Assignment generates the output image from the pruned tree. The output</p>
205<h5>parts</h5>
206<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>
207
208<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>
209
210<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>
211
212<p>This method is based on a similar algorithm written by Paul Raveling.</p></ol>
213
214 </div>
215<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
216_8c.html" target="source" name="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
217<div class="doc-section">
218
219<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p></ol>
220
221<p>The format of the AcquireQuantizeInfo method is:</p>
222
223<pre class="code">
224 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
225</pre>
226
227<p>A description of each parameter follows:</p></ol>
228
229<h5>image_info</h5>
230<ol><p>the image info.</p></ol>
231
232 </div>
233<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
234_8c.html" target="source" name="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
235<div class="doc-section">
236
237<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p></ol>
238
239<p>The format of the CloneQuantizeInfo method is:</p>
240
241<pre class="code">
242 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
243</pre>
244
245<p>A description of each parameter follows:</p></ol>
246
247<h5>clone_info</h5>
248<ol><p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p></ol>
249
250<h5>quantize_info</h5>
251<ol><p>a structure of type info.</p></ol>
252
253 </div>
254<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
255_8c.html" target="source" name="CompressImageColormap">CompressImageColormap</a></h2>
256<div class="doc-section">
257
258<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p></ol>
259
260<p>The format of the CompressImageColormap method is:</p>
261
262<pre class="code">
263 MagickBooleanType CompressImageColormap(Image *image)
264</pre>
265
266<p>A description of each parameter follows:</p></ol>
267
268<h5>image</h5>
269<ol><p>the image.</p></ol>
270
271 </div>
272<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
273_8c.html" target="source" name="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
274<div class="doc-section">
275
276<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p></ol>
277
278<p>The format of the DestroyQuantizeInfo method is:</p>
279
280<pre class="code">
281 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
282</pre>
283
284<p>A description of each parameter follows:</p></ol>
285
286<h5>quantize_info</h5>
287<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
288
289 </div>
290<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
291_8c.html" target="source" name="GetImageQuantizeError">GetImageQuantizeError</a></h2>
292<div class="doc-section">
293
294<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>
295
296<pre class="text">
297 o mean_error_per_pixel: This value is the mean error for any single
298 pixel in the image.
299</pre>
300
301<h5>normalized_mean_square_error</h5>
302<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>
303
304<h5>normalized_maximum_square_error</h5>
305<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>
306
307<p>The format of the GetImageQuantizeError method is:</p>
308
309<pre class="code">
310 MagickBooleanType GetImageQuantizeError(Image *image)
311</pre>
312
313<p>A description of each parameter follows.</p></ol>
314
315<h5>image</h5>
316<ol><p>the image.</p></ol>
317
318 </div>
319<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
320_8c.html" target="source" name="GetQuantizeInfo">GetQuantizeInfo</a></h2>
321<div class="doc-section">
322
323<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p></ol>
324
325<p>The format of the GetQuantizeInfo method is:</p>
326
327<pre class="code">
328 GetQuantizeInfo(QuantizeInfo *quantize_info)
329</pre>
330
331<p>A description of each parameter follows:</p></ol>
332
333<h5>quantize_info</h5>
334<ol><p>Specifies a pointer to a QuantizeInfo structure.</p></ol>
335
336 </div>
337<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
338_8c.html" target="source" name="PosterizeImage">PosterizeImage</a></h2>
339<div class="doc-section">
340
341<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p></ol>
342
343<p>The format of the PosterizeImage method is:</p>
344
345<pre class="code">
346 MagickBooleanType PosterizeImage(Image *image,const unsigned long levels,
347 const MagickBooleanType dither)
348</pre>
349
350<p>A description of each parameter follows:</p></ol>
351
352<h5>image</h5>
353<ol><p>Specifies a pointer to an Image structure.</p></ol>
354
355<h5>levels</h5>
356<ol><p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p></ol>
357
358<h5>dither</h5>
359<ol><p>Set this integer value to something other than zero to dither the mapped image.</p></ol>
360
361 </div>
362<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
363_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
364<div class="doc-section">
365
366<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>
367
368<p>The format of the QuantizeImage method is:</p>
369
370<pre class="code">
371 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
372 Image *image)
373</pre>
374
375<p>A description of each parameter follows:</p></ol>
376
377<h5>quantize_info</h5>
378<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
379
380<h5>image</h5>
381<ol><p>the image.</p></ol>
382
383 </div>
384<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
385_8c.html" target="source" name="QuantizeImages">QuantizeImages</a></h2>
386<div class="doc-section">
387
388<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>
389
390<p>The format of the QuantizeImages method is:</p>
391
392<pre class="code">
393 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
394 Image *images)
395</pre>
396
397<p>A description of each parameter follows:</p></ol>
398
399<h5>quantize_info</h5>
400<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
401
402<h5>images</h5>
403<ol><p>Specifies a pointer to a list of Image structures.</p></ol>
404
405 </div>
406<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
407_8c.html" target="source" name="RemapImage">RemapImage</a></h2>
408<div class="doc-section">
409
410<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p></ol>
411
412<p>The format of the RemapImage method is:</p>
413
414<pre class="code">
415 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
416 Image *image,const Image *remap_image)
417</pre>
418
419<p>A description of each parameter follows:</p></ol>
420
421<h5>quantize_info</h5>
422<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
423
424<h5>image</h5>
425<ol><p>the image.</p></ol>
426
427<h5>remap_image</h5>
428<ol><p>the reference image.</p></ol>
429
430 </div>
431<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
432_8c.html" target="source" name="RemapImages">RemapImages</a></h2>
433<div class="doc-section">
434
435<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p></ol>
436
437<p>The format of the RemapImage method is:</p>
438
439<pre class="code">
440 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
441 Image *images,Image *remap_image)
442</pre>
443
444<p>A description of each parameter follows:</p></ol>
445
446<h5>quantize_info</h5>
447<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
448
449<h5>images</h5>
450<ol><p>the image sequence.</p></ol>
451
452<h5>remap_image</h5>
453<ol><p>the reference image.</p></ol>
454
455 </div>
456<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
457_8c.html" target="source" name="SetGrayscaleImage">SetGrayscaleImage</a></h2>
458<div class="doc-section">
459
460<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p></ol>
461
462<p>The format of the SetGrayscaleImage method is:</p>
463
464<pre class="code">
465 MagickBooleanType SetGrayscaleImage(Image *image)
466</pre>
467
468<p>A description of each parameter follows:</p></ol>
469
470<h5>image</h5>
471<ol><p>The image.</p></ol>
472
473 </div>
cristyf3bb4782009-09-08 13:10:04 +0000[diff] [blame^]474
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