1 2 3 4 5 <!DOCTYPE html> 6 <html lang="en"> 7 <head> 8 <title>ImageMagick: MagickCore, C API for ImageMagick: Reduce the Number of Unique Colors in an Image</title> 9 <meta charset="utf-8" /> 10 <meta http-equiv="X-UA-Compatible" content="IE=edge" /> 11 <meta name="viewport" content="width=device-width, initial-scale=1" /> 12 <meta http-equiv="content-type" content="text/html; charset=utf-8"/> 13 <meta name="application-name" content="ImageMagick"/> 14 <meta name="description" content="ImageMagick is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats (over 200) including PNG, JPEG, JPEG-2000, GIF, WebP, Postscript, PDF, and SVG. 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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> 66 67 <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> 68 69 <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 (vertex nearest the origin in RGB space and the vertex farthest from the origin).</p> 70 71 <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> 72 73 <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> 74 75 <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> 76 77 <p>A complete tree would require 19,173,961 nodes for k = 8, Cmax = 255.</p> 78 <dt>avoid building a fully populated tree, QUANTIZE</dt> 79 <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> 80 81 <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> 82 83 <pre class="text"> 84 n1: Number of pixels whose color is contained in the RGB cube which 85 this node represents; 86 </pre> 87 88 <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> 89 90 <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> 91 92 <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> 93 94 <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> 95 96 <dd> 97 </dd> 98 99 <dd> 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 </dd> 100 101 <dd> This has the effect of minimizing any quantization error when merging two nodes together. </dd> 102 103 <dd> 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. </dd> 104 105 <dd> 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. </dd> 106 107 <dd> 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. </dd> 108 109 <dd> Assignment generates the output image from the pruned tree. The output </dd> 110 <dl class="dl-horizontal"> 111 <dt>parts</dt> 112 <dd>(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. </dd> 113 114 <dd> 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. </dd> 115 116 <dd> 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. </dd> 117 118 <dd> This method is based on a similar algorithm written by Paul Raveling. </dd> 119 120 <dd> </dd> 121 </dl> 122 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="AcquireQuantizeInfo">AcquireQuantizeInfo</a></h2> 123 124 <p>AcquireQuantizeInfo() allocates the QuantizeInfo structure.</p> 125 126 <p>The format of the AcquireQuantizeInfo method is:</p> 127 128 <pre class="text"> 129 QuantizeInfo *AcquireQuantizeInfo(const ImageInfo *image_info) 130 </pre> 131 132 <p>A description of each parameter follows:</p> 133 134 <dd> 135 </dd> 136 137 <dd> </dd> 138 <dl class="dl-horizontal"> 139 <dt>image_info</dt> 140 <dd>the image info. </dd> 141 142 <dd> </dd> 143 </dl> 144 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CloneQuantizeInfo">CloneQuantizeInfo</a></h2> 145 146 <p>CloneQuantizeInfo() makes a duplicate of the given quantize info structure, or if quantize info is NULL, a new one.</p> 147 148 <p>The format of the CloneQuantizeInfo method is:</p> 149 150 <pre class="text"> 151 QuantizeInfo *CloneQuantizeInfo(const QuantizeInfo *quantize_info) 152 </pre> 153 154 <p>A description of each parameter follows:</p> 155 156 <dd> 157 </dd> 158 159 <dd> </dd> 160 <dl class="dl-horizontal"> 161 <dt>clone_info</dt> 162 <dd>Method CloneQuantizeInfo returns a duplicate of the given quantize info, or if image info is NULL a new one. </dd> 163 164 <dd> </dd> 165 <dt>quantize_info</dt> 166 <dd>a structure of type info. </dd> 167 168 <dd> </dd> 169 </dl> 170 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="CompressImageColormap">CompressImageColormap</a></h2> 171 172 <p>CompressImageColormap() compresses an image colormap by removing any duplicate or unused color entries.</p> 173 174 <p>The format of the CompressImageColormap method is:</p> 175 176 <pre class="text"> 177 MagickBooleanType CompressImageColormap(Image *image, 178 ExceptionInfo *exception) 179 </pre> 180 181 <p>A description of each parameter follows:</p> 182 183 <dd> 184 </dd> 185 186 <dd> </dd> 187 <dl class="dl-horizontal"> 188 <dt>image</dt> 189 <dd>the image. </dd> 190 191 <dd> </dd> 192 <dt>exception</dt> 193 <dd>return any errors or warnings in this structure. </dd> 194 195 <dd> </dd> 196 </dl> 197 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="DestroyQuantizeInfo">DestroyQuantizeInfo</a></h2> 198 199 <p>DestroyQuantizeInfo() deallocates memory associated with an QuantizeInfo structure.</p> 200 201 <p>The format of the DestroyQuantizeInfo method is:</p> 202 203 <pre class="text"> 204 QuantizeInfo *DestroyQuantizeInfo(QuantizeInfo *quantize_info) 205 </pre> 206 207 <p>A description of each parameter follows:</p> 208 209 <dd> 210 </dd> 211 212 <dd> </dd> 213 <dl class="dl-horizontal"> 214 <dt>quantize_info</dt> 215 <dd>Specifies a pointer to an QuantizeInfo structure. </dd> 216 217 <dd> </dd> 218 </dl> 219 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetImageQuantizeError">GetImageQuantizeError</a></h2> 220 221 <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> 222 223 <pre class="text"> 224 o mean_error_per_pixel: This value is the mean error for any single 225 pixel in the image. 226 </pre> 227 228 <dt>normalized_mean_square_error</dt> 229 <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> 230 231 <dt>normalized_maximum_square_error</dt> 232 <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> 233 234 <p>The format of the GetImageQuantizeError method is:</p> 235 236 <pre class="text"> 237 MagickBooleanType GetImageQuantizeError(Image *image, 238 ExceptionInfo *exception) 239 </pre> 240 241 <p>A description of each parameter follows.</p> 242 243 <dt>image</dt> 244 <p>the image.</p> 245 246 <dt>exception</dt> 247 <p>return any errors or warnings in this structure.</p> 248 249 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="GetQuantizeInfo">GetQuantizeInfo</a></h2> 250 251 <p>GetQuantizeInfo() initializes the QuantizeInfo structure.</p> 252 253 <p>The format of the GetQuantizeInfo method is:</p> 254 255 <pre class="text"> 256 GetQuantizeInfo(QuantizeInfo *quantize_info) 257 </pre> 258 259 <p>A description of each parameter follows:</p> 260 261 <dd> 262 </dd> 263 264 <dd> </dd> 265 <dl class="dl-horizontal"> 266 <dt>quantize_info</dt> 267 <dd>Specifies a pointer to a QuantizeInfo structure. </dd> 268 269 <dd> </dd> 270 </dl> 271 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="PosterizeImage">PosterizeImage</a></h2> 272 273 <p>PosterizeImage() reduces the image to a limited number of colors for a "poster" effect.</p> 274 275 <p>The format of the PosterizeImage method is:</p> 276 277 <pre class="text"> 278 MagickBooleanType PosterizeImage(Image *image,const size_t levels, 279 const DitherMethod dither_method,ExceptionInfo *exception) 280 </pre> 281 282 <p>A description of each parameter follows:</p> 283 284 <dd> 285 </dd> 286 287 <dd> </dd> 288 <dl class="dl-horizontal"> 289 <dt>image</dt> 290 <dd>Specifies a pointer to an Image structure. </dd> 291 292 <dd> </dd> 293 <dt>levels</dt> 294 <dd>Number of color levels allowed in each channel. Very low values (2, 3, or 4) have the most visible effect. </dd> 295 296 <dd> </dd> 297 <dt>dither_method</dt> 298 <dd>choose from UndefinedDitherMethod, NoDitherMethod, RiemersmaDitherMethod, FloydSteinbergDitherMethod. </dd> 299 300 <dd> </dd> 301 <dt>exception</dt> 302 <dd>return any errors or warnings in this structure. </dd> 303 304 <dd> </dd> 305 </dl> 306 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImage">QuantizeImage</a></h2> 307 308 <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> 309 310 <p>The format of the QuantizeImage method is:</p> 311 312 <pre class="text"> 313 MagickBooleanType QuantizeImage(const QuantizeInfo *quantize_info, 314 Image *image,ExceptionInfo *exception) 315 </pre> 316 317 <p>A description of each parameter follows:</p> 318 319 <dd> 320 </dd> 321 322 <dd> </dd> 323 <dl class="dl-horizontal"> 324 <dt>quantize_info</dt> 325 <dd>Specifies a pointer to an QuantizeInfo structure. </dd> 326 327 <dd> </dd> 328 <dt>image</dt> 329 <dd>the image. </dd> 330 331 <dd> </dd> 332 <dt>exception</dt> 333 <dd>return any errors or warnings in this structure. </dd> 334 335 <dd> </dd> 336 </dl> 337 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="QuantizeImages">QuantizeImages</a></h2> 338 339 <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> 340 341 <p>The format of the QuantizeImages method is:</p> 342 343 <pre class="text"> 344 MagickBooleanType QuantizeImages(const QuantizeInfo *quantize_info, 345 Image *images,ExceptionInfo *exception) 346 </pre> 347 348 <p>A description of each parameter follows:</p> 349 350 <dd> 351 </dd> 352 353 <dd> </dd> 354 <dl class="dl-horizontal"> 355 <dt>quantize_info</dt> 356 <dd>Specifies a pointer to an QuantizeInfo structure. </dd> 357 358 <dd> </dd> 359 <dt>images</dt> 360 <dd>Specifies a pointer to a list of Image structures. </dd> 361 362 <dd> </dd> 363 <dt>exception</dt> 364 <dd>return any errors or warnings in this structure. </dd> 365 366 <dd> </dd> 367 </dl> 368 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImage">RemapImage</a></h2> 369 370 <p>RemapImage() replaces the colors of an image with the closest of the colors from the reference image.</p> 371 372 <p>The format of the RemapImage method is:</p> 373 374 <pre class="text"> 375 MagickBooleanType RemapImage(const QuantizeInfo *quantize_info, 376 Image *image,const Image *remap_image,ExceptionInfo *exception) 377 </pre> 378 379 <p>A description of each parameter follows:</p> 380 381 <dd> 382 </dd> 383 384 <dd> </dd> 385 <dl class="dl-horizontal"> 386 <dt>quantize_info</dt> 387 <dd>Specifies a pointer to an QuantizeInfo structure. </dd> 388 389 <dd> </dd> 390 <dt>image</dt> 391 <dd>the image. </dd> 392 393 <dd> </dd> 394 <dt>remap_image</dt> 395 <dd>the reference image. </dd> 396 397 <dd> </dd> 398 <dt>exception</dt> 399 <dd>return any errors or warnings in this structure. </dd> 400 401 <dd> </dd> 402 </dl> 403 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="RemapImages">RemapImages</a></h2> 404 405 <p>RemapImages() replaces the colors of a sequence of images with the closest color from a reference image.</p> 406 407 <p>The format of the RemapImage method is:</p> 408 409 <pre class="text"> 410 MagickBooleanType RemapImages(const QuantizeInfo *quantize_info, 411 Image *images,Image *remap_image,ExceptionInfo *exception) 412 </pre> 413 414 <p>A description of each parameter follows:</p> 415 416 <dd> 417 </dd> 418 419 <dd> </dd> 420 <dl class="dl-horizontal"> 421 <dt>quantize_info</dt> 422 <dd>Specifies a pointer to an QuantizeInfo structure. </dd> 423 424 <dd> </dd> 425 <dt>images</dt> 426 <dd>the image sequence. </dd> 427 428 <dd> </dd> 429 <dt>remap_image</dt> 430 <dd>the reference image. </dd> 431 432 <dd> </dd> 433 <dt>exception</dt> 434 <dd>return any errors or warnings in this structure. </dd> 435 436 <dd> </dd> 437 </dl> 438 <h2><a href="http://www.imagemagick.org/api/MagickCore/quantize_8c.html" id="SetGrayscaleImage">SetGrayscaleImage</a></h2> 439 440 <p>SetGrayscaleImage() converts an image to a PseudoClass grayscale image.</p> 441 442 <p>The format of the SetGrayscaleImage method is:</p> 443 444 <pre class="text"> 445 MagickBooleanType SetGrayscaleImage(Image *image, 446 ExceptionInfo *exception) 447 </pre> 448 449 <p>A description of each parameter follows:</p> 450 451 <dd> 452 </dd> 453 454 <dd> </dd> 455 <dl class="dl-horizontal"> 456 <dt>image</dt> 457 <dd>The image. </dd> 458 459 <dd> </dd> 460 <dt>exception</dt> 461 <dd>return any errors or warnings in this structure. </dd> 462 463 <dd> </dd> 464 </dl> 465 </div> 466 <footer class="magick-footer"> 467 <p><a href="../support.html">Donate</a> 468 <a href="../sitemap.html">Sitemap</a> 469 <a href="../links.html">Related</a> 470 <a href="../architecture.html">Architecture</a> 471 </p> 472 <p><a href="quantize.html#">Back to top</a> 473 <a href="http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x89AB63D48277377A">Public Key</a> 474 <a href="http://www.imagemagick.org/script/contact.php">Contact Us</a></p> 475 <p><small> 1999-2016 ImageMagick Studio LLC</small></p> 476 </footer> 477 </div><!-- /.container --> 478 479 <script src="https://localhost/ajax/libs/jquery/1.11.3/jquery.min.js"></script> 480 <script src="../js/magick.html"></script> 481 </div> 482 </body> 483 </html> 484