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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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149
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151
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]152<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>
153
154<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
155_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
156<div class="doc-section">
157
158<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p></ol>
159
160<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>
161
162<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>
163
164<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>
165
166<pre class="text">
167 the origin.
168</pre>
169
170<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>
171
172<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>
173
174<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>
175
176<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
177<h5>avoid building a fully populated tree, QUANTIZE</h5>
178<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>
179
180<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>
181
182<pre class="text">
183 n1: Number of pixels whose color is contained in the RGB cube which
184 this node represents;
185</pre>
186
187<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>
188
189<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>
190
191<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>
192
193<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>
194
195<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>
196
197<p>This has the effect of minimizing any quantization error when merging two nodes together.</p></ol>
198
199<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>
200
201<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>
202
203<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>
204
205<p>Assignment generates the output image from the pruned tree. The output</p>
206<h5>parts</h5>
207<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>
208
209<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>
210
211<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>
212
213<p>This method is based on a similar algorithm written by Paul Raveling.</p></ol>
214
215 </div>
216<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
217_8c.html" target="source" name="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
218<div class="doc-section">
219
220<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p></ol>
221
222<p>The format of the AcquireQuantizeInfo method is:</p>
223
224<pre class="code">
225 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
226</pre>
227
228<p>A description of each parameter follows:</p></ol>
229
230<h5>image_info</h5>
231<ol><p>the image info.</p></ol>
232
233 </div>
234<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
235_8c.html" target="source" name="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
236<div class="doc-section">
237
238<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p></ol>
239
240<p>The format of the CloneQuantizeInfo method is:</p>
241
242<pre class="code">
243 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
244</pre>
245
246<p>A description of each parameter follows:</p></ol>
247
248<h5>clone_info</h5>
249<ol><p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p></ol>
250
251<h5>quantize_info</h5>
252<ol><p>a structure of type info.</p></ol>
253
254 </div>
255<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
256_8c.html" target="source" name="CompressImageColormap">CompressImageColormap</a></h2>
257<div class="doc-section">
258
259<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p></ol>
260
261<p>The format of the CompressImageColormap method is:</p>
262
263<pre class="code">
264 MagickBooleanType CompressImageColormap(Image *image)
265</pre>
266
267<p>A description of each parameter follows:</p></ol>
268
269<h5>image</h5>
270<ol><p>the image.</p></ol>
271
272 </div>
273<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
274_8c.html" target="source" name="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
275<div class="doc-section">
276
277<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p></ol>
278
279<p>The format of the DestroyQuantizeInfo method is:</p>
280
281<pre class="code">
282 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
283</pre>
284
285<p>A description of each parameter follows:</p></ol>
286
287<h5>quantize_info</h5>
288<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
289
290 </div>
291<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
292_8c.html" target="source" name="GetImageQuantizeError">GetImageQuantizeError</a></h2>
293<div class="doc-section">
294
295<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>
296
297<pre class="text">
298 o mean_error_per_pixel: This value is the mean error for any single
299 pixel in the image.
300</pre>
301
302<h5>normalized_mean_square_error</h5>
303<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>
304
305<h5>normalized_maximum_square_error</h5>
306<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>
307
308<p>The format of the GetImageQuantizeError method is:</p>
309
310<pre class="code">
311 MagickBooleanType GetImageQuantizeError(Image *image)
312</pre>
313
314<p>A description of each parameter follows.</p></ol>
315
316<h5>image</h5>
317<ol><p>the image.</p></ol>
318
319 </div>
320<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
321_8c.html" target="source" name="GetQuantizeInfo">GetQuantizeInfo</a></h2>
322<div class="doc-section">
323
324<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p></ol>
325
326<p>The format of the GetQuantizeInfo method is:</p>
327
328<pre class="code">
329 GetQuantizeInfo(QuantizeInfo *quantize_info)
330</pre>
331
332<p>A description of each parameter follows:</p></ol>
333
334<h5>quantize_info</h5>
335<ol><p>Specifies a pointer to a QuantizeInfo structure.</p></ol>
336
337 </div>
338<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
339_8c.html" target="source" name="PosterizeImage">PosterizeImage</a></h2>
340<div class="doc-section">
341
342<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p></ol>
343
344<p>The format of the PosterizeImage method is:</p>
345
346<pre class="code">
347 MagickBooleanType PosterizeImage(Image *image,const unsigned long levels,
348 const MagickBooleanType dither)
349</pre>
350
351<p>A description of each parameter follows:</p></ol>
352
353<h5>image</h5>
354<ol><p>Specifies a pointer to an Image structure.</p></ol>
355
356<h5>levels</h5>
357<ol><p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p></ol>
358
359<h5>dither</h5>
360<ol><p>Set this integer value to something other than zero to dither the mapped image.</p></ol>
361
362 </div>
363<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
364_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
365<div class="doc-section">
366
367<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>
368
369<p>The format of the QuantizeImage method is:</p>
370
371<pre class="code">
372 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
373 Image *image)
374</pre>
375
376<p>A description of each parameter follows:</p></ol>
377
378<h5>quantize_info</h5>
379<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
380
381<h5>image</h5>
382<ol><p>the image.</p></ol>
383
384 </div>
385<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
386_8c.html" target="source" name="QuantizeImages">QuantizeImages</a></h2>
387<div class="doc-section">
388
389<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>
390
391<p>The format of the QuantizeImages method is:</p>
392
393<pre class="code">
394 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
395 Image *images)
396</pre>
397
398<p>A description of each parameter follows:</p></ol>
399
400<h5>quantize_info</h5>
401<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
402
403<h5>images</h5>
404<ol><p>Specifies a pointer to a list of Image structures.</p></ol>
405
406 </div>
407<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
408_8c.html" target="source" name="RemapImage">RemapImage</a></h2>
409<div class="doc-section">
410
411<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p></ol>
412
413<p>The format of the RemapImage method is:</p>
414
415<pre class="code">
416 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
417 Image *image,const Image *remap_image)
418</pre>
419
420<p>A description of each parameter follows:</p></ol>
421
422<h5>quantize_info</h5>
423<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
424
425<h5>image</h5>
426<ol><p>the image.</p></ol>
427
428<h5>remap_image</h5>
429<ol><p>the reference image.</p></ol>
430
431 </div>
432<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
433_8c.html" target="source" name="RemapImages">RemapImages</a></h2>
434<div class="doc-section">
435
436<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p></ol>
437
438<p>The format of the RemapImage method is:</p>
439
440<pre class="code">
441 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
442 Image *images,Image *remap_image)
443</pre>
444
445<p>A description of each parameter follows:</p></ol>
446
447<h5>quantize_info</h5>
448<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
449
450<h5>images</h5>
451<ol><p>the image sequence.</p></ol>
452
453<h5>remap_image</h5>
454<ol><p>the reference image.</p></ol>
455
456 </div>
457<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
458_8c.html" target="source" name="SetGrayscaleImage">SetGrayscaleImage</a></h2>
459<div class="doc-section">
460
461<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p></ol>
462
463<p>The format of the SetGrayscaleImage method is:</p>
464
465<pre class="code">
466 MagickBooleanType SetGrayscaleImage(Image *image)
467</pre>
468
469<p>A description of each parameter follows:</p></ol>
470
471<h5>image</h5>
472<ol><p>The image.</p></ol>
473
474 </div>
cristyf3bb4782009-09-08 13:10:04 +0000[diff] [blame]475
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