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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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cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]154<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>
155
156<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
157_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
158<div class="doc-section">
159
160<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p></ol>
161
162<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>
163
164<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>
165
166<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>
167
168<pre class="text">
169 the origin.
170</pre>
171
172<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>
173
174<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>
175
176<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>
177
178<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
179<h5>avoid building a fully populated tree, QUANTIZE</h5>
180<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>
181
182<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>
183
184<pre class="text">
185 n1: Number of pixels whose color is contained in the RGB cube which
186 this node represents;
187</pre>
188
189<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>
190
191<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>
192
193<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>
194
195<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>
196
197<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>
198
199<p>This has the effect of minimizing any quantization error when merging two nodes together.</p></ol>
200
201<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>
202
203<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>
204
205<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>
206
207<p>Assignment generates the output image from the pruned tree. The output</p>
208<h5>parts</h5>
209<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>
210
211<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>
212
213<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>
214
215<p>This method is based on a similar algorithm written by Paul Raveling.</p></ol>
216
217 </div>
218<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
219_8c.html" target="source" name="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
220<div class="doc-section">
221
222<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p></ol>
223
224<p>The format of the AcquireQuantizeInfo method is:</p>
225
226<pre class="code">
227 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
228</pre>
229
230<p>A description of each parameter follows:</p></ol>
231
232<h5>image_info</h5>
233<ol><p>the image info.</p></ol>
234
235 </div>
236<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
237_8c.html" target="source" name="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
238<div class="doc-section">
239
240<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p></ol>
241
242<p>The format of the CloneQuantizeInfo method is:</p>
243
244<pre class="code">
245 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
246</pre>
247
248<p>A description of each parameter follows:</p></ol>
249
250<h5>clone_info</h5>
251<ol><p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p></ol>
252
253<h5>quantize_info</h5>
254<ol><p>a structure of type info.</p></ol>
255
256 </div>
257<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
258_8c.html" target="source" name="CompressImageColormap">CompressImageColormap</a></h2>
259<div class="doc-section">
260
261<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p></ol>
262
263<p>The format of the CompressImageColormap method is:</p>
264
265<pre class="code">
266 MagickBooleanType CompressImageColormap(Image *image)
267</pre>
268
269<p>A description of each parameter follows:</p></ol>
270
271<h5>image</h5>
272<ol><p>the image.</p></ol>
273
274 </div>
275<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
276_8c.html" target="source" name="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
277<div class="doc-section">
278
279<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p></ol>
280
281<p>The format of the DestroyQuantizeInfo method is:</p>
282
283<pre class="code">
284 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
285</pre>
286
287<p>A description of each parameter follows:</p></ol>
288
289<h5>quantize_info</h5>
290<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
291
292 </div>
293<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
294_8c.html" target="source" name="GetImageQuantizeError">GetImageQuantizeError</a></h2>
295<div class="doc-section">
296
297<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>
298
299<pre class="text">
300 o mean_error_per_pixel: This value is the mean error for any single
301 pixel in the image.
302</pre>
303
304<h5>normalized_mean_square_error</h5>
305<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>
306
307<h5>normalized_maximum_square_error</h5>
308<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>
309
310<p>The format of the GetImageQuantizeError method is:</p>
311
312<pre class="code">
313 MagickBooleanType GetImageQuantizeError(Image *image)
314</pre>
315
316<p>A description of each parameter follows.</p></ol>
317
318<h5>image</h5>
319<ol><p>the image.</p></ol>
320
321 </div>
322<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
323_8c.html" target="source" name="GetQuantizeInfo">GetQuantizeInfo</a></h2>
324<div class="doc-section">
325
326<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p></ol>
327
328<p>The format of the GetQuantizeInfo method is:</p>
329
330<pre class="code">
331 GetQuantizeInfo(QuantizeInfo *quantize_info)
332</pre>
333
334<p>A description of each parameter follows:</p></ol>
335
336<h5>quantize_info</h5>
337<ol><p>Specifies a pointer to a QuantizeInfo structure.</p></ol>
338
339 </div>
340<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
341_8c.html" target="source" name="PosterizeImage">PosterizeImage</a></h2>
342<div class="doc-section">
343
344<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p></ol>
345
346<p>The format of the PosterizeImage method is:</p>
347
348<pre class="code">
349 MagickBooleanType PosterizeImage(Image *image,const unsigned long levels,
350 const MagickBooleanType dither)
351</pre>
352
353<p>A description of each parameter follows:</p></ol>
354
355<h5>image</h5>
356<ol><p>Specifies a pointer to an Image structure.</p></ol>
357
358<h5>levels</h5>
359<ol><p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p></ol>
360
361<h5>dither</h5>
362<ol><p>Set this integer value to something other than zero to dither the mapped image.</p></ol>
363
364 </div>
365<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
366_8c.html" target="source" name="QuantizeImage">QuantizeImage</a></h2>
367<div class="doc-section">
368
369<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>
370
371<p>The format of the QuantizeImage method is:</p>
372
373<pre class="code">
374 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
375 Image *image)
376</pre>
377
378<p>A description of each parameter follows:</p></ol>
379
380<h5>quantize_info</h5>
381<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
382
383<h5>image</h5>
384<ol><p>the image.</p></ol>
385
386 </div>
387<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
388_8c.html" target="source" name="QuantizeImages">QuantizeImages</a></h2>
389<div class="doc-section">
390
391<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>
392
393<p>The format of the QuantizeImages method is:</p>
394
395<pre class="code">
396 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
397 Image *images)
398</pre>
399
400<p>A description of each parameter follows:</p></ol>
401
402<h5>quantize_info</h5>
403<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
404
405<h5>images</h5>
406<ol><p>Specifies a pointer to a list of Image structures.</p></ol>
407
408 </div>
409<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
410_8c.html" target="source" name="RemapImage">RemapImage</a></h2>
411<div class="doc-section">
412
413<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p></ol>
414
415<p>The format of the RemapImage method is:</p>
416
417<pre class="code">
418 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
419 Image *image,const Image *remap_image)
420</pre>
421
422<p>A description of each parameter follows:</p></ol>
423
424<h5>quantize_info</h5>
425<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
426
427<h5>image</h5>
428<ol><p>the image.</p></ol>
429
430<h5>remap_image</h5>
431<ol><p>the reference image.</p></ol>
432
433 </div>
434<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
435_8c.html" target="source" name="RemapImages">RemapImages</a></h2>
436<div class="doc-section">
437
438<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p></ol>
439
440<p>The format of the RemapImage method is:</p>
441
442<pre class="code">
443 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
444 Image *images,Image *remap_image)
445</pre>
446
447<p>A description of each parameter follows:</p></ol>
448
449<h5>quantize_info</h5>
450<ol><p>Specifies a pointer to an QuantizeInfo structure.</p></ol>
451
452<h5>images</h5>
453<ol><p>the image sequence.</p></ol>
454
455<h5>remap_image</h5>
456<ol><p>the reference image.</p></ol>
457
458 </div>
459<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize
460_8c.html" target="source" name="SetGrayscaleImage">SetGrayscaleImage</a></h2>
461<div class="doc-section">
462
463<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p></ol>
464
465<p>The format of the SetGrayscaleImage method is:</p>
466
467<pre class="code">
468 MagickBooleanType SetGrayscaleImage(Image *image)
469</pre>
470
471<p>A description of each parameter follows:</p></ol>
472
473<h5>image</h5>
474<ol><p>The image.</p></ol>
475
476 </div>
cristyf3bb4782009-09-08 13:10:04 +0000[diff] [blame]477
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