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gerrit-public.fairphone.software / platform / external / ImageMagick / 6ad765e3f7a12d4b1b71a745a120c51549d81125 / . / www / api / quantize.html
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cristy20398432011-08-11 01:31:50 +0000[diff] [blame]231<h1>Module quantize Methods</h1>
cristy72da67d2013-02-24 21:40:48 +0000[diff] [blame]232<p class="navigation-index">[<a href="quantize.php#QuantizeImage">QuantizeImage</a> &bull; <a href="quantize.php#AcquireQuantizeInfo">AcquireQuantizeInfo</a> &bull; <a href="quantize.php#CloneQuantizeInfo">CloneQuantizeInfo</a> &bull; <a href="quantize.php#CompressImageColormap">CompressImageColormap</a> &bull; <a href="quantize.php#DestroyQuantizeInfo">DestroyQuantizeInfo</a> &bull; <a href="quantize.php#GetImageQuantizeError">GetImageQuantizeError</a> &bull; <a href="quantize.php#GetQuantizeInfo">GetQuantizeInfo</a> &bull; <a href="quantize.php#PosterizeImage">PosterizeImage</a> &bull; <a href="quantize.php#QuantizeImage">QuantizeImage</a> &bull; <a href="quantize.php#QuantizeImages">QuantizeImages</a> &bull; <a href="quantize.php#RemapImage">RemapImage</a> &bull; <a href="quantize.php#RemapImages">RemapImages</a> &bull; <a href="quantize.php#SetGrayscaleImage">SetGrayscaleImage</a>]</p>
cristy6f77f692011-02-15 15:31:39 +0000[diff] [blame]233
cristy20398432011-08-11 01:31:50 +0000[diff] [blame]234<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImage">QuantizeImage</a></h2>
235<div class="doc-section">
236
237<p>QuantizeImage() takes a standard RGB or monochrome images and quantizes them down to some fixed number of colors.</p>
238
239<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>
240
241<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>
242
243<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>
244
245<pre class="text">
246 the origin.
247</pre>
248
249<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>
250
251<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>
252
253<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>
254
255<p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p>
256<h5>avoid building a fully populated tree, QUANTIZE</h5>
257<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>
258
259<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>
260
261<pre class="text">
262 n1: Number of pixels whose color is contained in the RGB cube which
263 this node represents;
264</pre>
265
266<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>
267
268<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>
269
270<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>
271
272<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>
273
274<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>
275
276<p>This has the effect of minimizing any quantization error when merging two nodes together.</p>
277
278<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>
279
280<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>
281
282<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>
283
284<p>Assignment generates the output image from the pruned tree. The output</p>
285<h5>parts</h5>
286<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>
287
288<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>
289
290<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>
291
292<p>This method is based on a similar algorithm written by Paul Raveling.</p>
293
294 </div>
295<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2>
296<div class="doc-section">
297
298<p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p>
299
300<p>The format of the AcquireQuantizeInfo method is:</p>
301
302<pre class="code">
303 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info)
304</pre>
305
306<p>A description of each parameter follows:</p>
307
308<h5>image_info</h5>
309<p>the image info.</p>
310
311 </div>
312<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CloneQuantizeInfo">CloneQuantizeInfo</a></h2>
313<div class="doc-section">
314
315<p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p>
316
317<p>The format of the CloneQuantizeInfo method is:</p>
318
319<pre class="code">
320 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info)
321</pre>
322
323<p>A description of each parameter follows:</p>
324
325<h5>clone_info</h5>
326<p>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one.</p>
327
328<h5>quantize_info</h5>
329<p>a structure of type info.</p>
330
331 </div>
332<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CompressImageColormap">CompressImageColormap</a></h2>
333<div class="doc-section">
334
335<p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p>
336
337<p>The format of the CompressImageColormap method is:</p>
338
339<pre class="code">
340 MagickBooleanType CompressImageColormap(Image *image)
341</pre>
342
343<p>A description of each parameter follows:</p>
344
345<h5>image</h5>
346<p>the image.</p>
347
348 </div>
349<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2>
350<div class="doc-section">
351
352<p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p>
353
354<p>The format of the DestroyQuantizeInfo method is:</p>
355
356<pre class="code">
357 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info)
358</pre>
359
360<p>A description of each parameter follows:</p>
361
362<h5>quantize_info</h5>
363<p>Specifies a pointer to an QuantizeInfo structure.</p>
364
365 </div>
366<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetImageQuantizeError">GetImageQuantizeError</a></h2>
367<div class="doc-section">
368
369<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>
370
371<pre class="text">
372 o mean_error_per_pixel: This value is the mean error for any single
373 pixel in the image.
374</pre>
375
376<h5>normalized_mean_square_error</h5>
377<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>
378
379<h5>normalized_maximum_square_error</h5>
380<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>
381
382<p>The format of the GetImageQuantizeError method is:</p>
383
384<pre class="code">
385 MagickBooleanType GetImageQuantizeError(Image *image)
386</pre>
387
388<p>A description of each parameter follows.</p>
389
390<h5>image</h5>
391<p>the image.</p>
392
393 </div>
394<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetQuantizeInfo">GetQuantizeInfo</a></h2>
395<div class="doc-section">
396
397<p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p>
398
399<p>The format of the GetQuantizeInfo method is:</p>
400
401<pre class="code">
402 GetQuantizeInfo(QuantizeInfo *quantize_info)
403</pre>
404
405<p>A description of each parameter follows:</p>
406
407<h5>quantize_info</h5>
408<p>Specifies a pointer to a QuantizeInfo structure.</p>
409
410 </div>
411<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="PosterizeImage">PosterizeImage</a></h2>
412<div class="doc-section">
413
414<p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p>
415
416<p>The format of the PosterizeImage method is:</p>
417
418<pre class="code">
419 MagickBooleanType PosterizeImage(Image *image,const size_t levels,
420 const MagickBooleanType dither)
421 MagickBooleanType PosterizeImageChannel(Image *image,
422 const ChannelType channel,const size_t levels,
423 const MagickBooleanType dither)
424</pre>
425
426<p>A description of each parameter follows:</p>
427
428<h5>image</h5>
429<p>Specifies a pointer to an Image structure.</p>
430
431<h5>levels</h5>
432<p>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect.</p>
433
434<h5>dither</h5>
435<p>Set this integer value to something other than zero to dither the mapped image.</p>
436
437 </div>
438<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImage">QuantizeImage</a></h2>
439<div class="doc-section">
440
441<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>
442
443<p>The format of the QuantizeImage method is:</p>
444
445<pre class="code">
446 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info,
447 Image *image)
448</pre>
449
450<p>A description of each parameter follows:</p>
451
452<h5>quantize_info</h5>
453<p>Specifies a pointer to an QuantizeInfo structure.</p>
454
455<h5>image</h5>
456<p>the image.</p>
457
458 </div>
459<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImages">QuantizeImages</a></h2>
460<div class="doc-section">
461
462<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>
463
464<p>The format of the QuantizeImages method is:</p>
465
466<pre class="code">
467 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info,
468 Image *images)
469</pre>
470
471<p>A description of each parameter follows:</p>
472
473<h5>quantize_info</h5>
474<p>Specifies a pointer to an QuantizeInfo structure.</p>
475
476<h5>images</h5>
477<p>Specifies a pointer to a list of Image structures.</p>
478
479 </div>
480<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImage">RemapImage</a></h2>
481<div class="doc-section">
482
483<p>RemapImage() replaces the colors of an image with the closest color from a reference image.</p>
484
485<p>The format of the RemapImage method is:</p>
486
487<pre class="code">
488 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info,
489 Image *image,const Image *remap_image)
490</pre>
491
492<p>A description of each parameter follows:</p>
493
494<h5>quantize_info</h5>
495<p>Specifies a pointer to an QuantizeInfo structure.</p>
496
497<h5>image</h5>
498<p>the image.</p>
499
500<h5>remap_image</h5>
501<p>the reference image.</p>
502
503 </div>
504<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImages">RemapImages</a></h2>
505<div class="doc-section">
506
507<p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p>
508
509<p>The format of the RemapImage method is:</p>
510
511<pre class="code">
512 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info,
513 Image *images,Image *remap_image)
514</pre>
515
516<p>A description of each parameter follows:</p>
517
518<h5>quantize_info</h5>
519<p>Specifies a pointer to an QuantizeInfo structure.</p>
520
521<h5>images</h5>
522<p>the image sequence.</p>
523
524<h5>remap_image</h5>
525<p>the reference image.</p>
526
527 </div>
528<h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="SetGrayscaleImage">SetGrayscaleImage</a></h2>
529<div class="doc-section">
530
531<p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p>
532
533<p>The format of the SetGrayscaleImage method is:</p>
534
535<pre class="code">
536 MagickBooleanType SetGrayscaleImage(Image *image)
537</pre>
538
539<p>A description of each parameter follows:</p>
540
541<h5>image</h5>
542<p>The image.</p>
543
544 </div>
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