c - ImageMagick 中使用的算法

Question

我在我的项目中使用了 Imagemagick。我使用 ImageMagick 的比较命令实现了一个子图像检测系统。它运行良好，效果良好。通过阅读文章，我了解到 ImageMagick 会比较大图像像素内每个可能位置的小图像像素。而且我还知道 ImageMagick 使用模糊因子检测旋转图像和缩放图像。我能够找到源代码与比较命令相关。

const Image *reconstruct_image,double *distortion,ExceptionInfo *exception)
  585 {
  586   CacheView
  587     *image_view,
  588     *reconstruct_view;
  589
  590   double
  591     area;
  592
  593   MagickBooleanType
  594     status;
  595
  596   register ssize_t
  597     j;
  598
  599   size_t
  600     columns,
  601     rows;
  602
  603   ssize_t
  604     y;
  605
  606   status=MagickTrue;
  607   rows=MagickMax(image->rows,reconstruct_image->rows);
  608   columns=MagickMax(image->columns,reconstruct_image->columns);
  609   area=0.0;
  610   image_view=AcquireVirtualCacheView(image,exception);
  611   reconstruct_view=AcquireVirtualCacheView(reconstruct_image,exception);
  612 #if defined(MAGICKCORE_OPENMP_SUPPORT)
  613   #pragma omp parallel for schedule(static,4) shared(status) \
  614     magick_threads(image,image,rows,1) reduction(+:area)
  615 #endif
  616   for (y=0; y < (ssize_t) rows; y++)
  617   {
  618     double
  619       channel_distortion[MaxPixelChannels+1];
  620
  621     register const Quantum
  622       *magick_restrict p,
  623       *magick_restrict q;
  624
  625     register ssize_t
  626       x;
  627
  628     if (status == MagickFalse)
  629       continue;
  630     p=GetCacheViewVirtualPixels(image_view,0,y,columns,1,exception);
  631     q=GetCacheViewVirtualPixels(reconstruct_view,0,y,columns,1,exception);
  632     if ((p == (const Quantum *) NULL) || (q == (const Quantum *) NULL))
  633       {
  634         status=MagickFalse;
  635         continue;
  636       }
  637     (void) ResetMagickMemory(channel_distortion,0,sizeof(channel_distortion));
  638     for (x=0; x < (ssize_t) columns; x++)
  639     {
  640       double
  641         Da,
  642         Sa;
  643
  644       register ssize_t
  645         i;
  646
  647       if ((GetPixelReadMask(image,p) == 0) ||
  648           (GetPixelReadMask(reconstruct_image,q) == 0))
  649         {
  650           p+=GetPixelChannels(image);
  651           q+=GetPixelChannels(reconstruct_image);
  652           continue;
  653         }
  654       Sa=QuantumScale*GetPixelAlpha(image,p);
  655       Da=QuantumScale*GetPixelAlpha(reconstruct_image,q);
  656       for (i=0; i < (ssize_t) GetPixelChannels(image); i++)
  657       {
  658         double
  659           distance;
  660
  661         PixelChannel channel=GetPixelChannelChannel(image,i);
  662         PixelTrait traits=GetPixelChannelTraits(image,channel);
  663         PixelTrait reconstruct_traits=GetPixelChannelTraits(reconstruct_image,
  664           channel);
  665         if ((traits == UndefinedPixelTrait) ||
  666             (reconstruct_traits == UndefinedPixelTrait) ||
  667             ((reconstruct_traits & UpdatePixelTrait) == 0))
  668           continue;
  669         distance=QuantumScale*fabs(Sa*p[i]-Da*GetPixelChannel(reconstruct_image,
  670           channel,q));
  671         channel_distortion[i]+=distance;
  672         channel_distortion[CompositePixelChannel]+=distance;
  673       }
  674       area++;
  675       p+=GetPixelChannels(image);
  676       q+=GetPixelChannels(reconstruct_image);
  677     }
  678 #if defined(MAGICKCORE_OPENMP_SUPPORT)
  679     #pragma omp critical (MagickCore_GetMeanAbsoluteError)
  680 #endif
  681     for (j=0; j <= MaxPixelChannels; j++)
  682       distortion[j]+=channel_distortion[j];
  683   }
  684   reconstruct_view=DestroyCacheView(reconstruct_view);




  685   image_view=DestroyCacheView(image_view);
  686   area=PerceptibleReciprocal(area);
  687   for (j=0; j <= MaxPixelChannels; j++)
  688     distortion[j]*=area;
  689   distortion[CompositePixelChannel]/=(double) GetImageChannels(image);
  690   return(status);
  691 }

有人知道他们在以下代码片段中搜索的条件是什么吗？

if ((traits == UndefinedPixelTrait) ||
  666             (reconstruct_traits == UndefinedPixelTrait) ||
  667             ((reconstruct_traits & UpdatePixelTrait) == 0))

Answer 1

如果您想知道如何使用这些值:

http://www.learncpp.com/cpp-tutorial/3-8a-bit-flags-and-bit-masks/

这些值只不过是不同的位。它们是不同的，因此您可以将它们组合起来并以明确的方式检查它们。

如果您不知道 UndefinedPixelTraits 等的含义，只需用 google 搜索该词即可找到 ImageMagick 文档:

https://www.imagemagick.org/include/porting.php

Pixel Traits

Each pixel channel includes one or more of these traits:

Undefined no traits associated with this pixel channel Copy do not update this pixel channel, just copy it Update update this pixel channel Blend blend this pixel channel with the alpha mask if it's enabled We provide these methods to set and get pixel traits:

GetPixelAlphaTraits() SetPixelAlphaTraits() GetPixelBlackTraits()
SetPixelBlackTraits() GetPixelBlueTraits() SetPixelBlueTraits() GetPixelCbTraits() SetPixelCbTraits() GetPixelChannelTraits() SetPixelChannelTraits() GetPixelCrTraits() SetPixelCrTraits() GetPixelGrayTraits() SetPixelGrayTraits() GetPixelGreenTraits()
SetPixelGreenTraits() GetPixelIndexTraits() SetPixelIndexTraits() GetPixelMagentaTraits() SetPixelMagentaTraits() GetPixelRedTraits()
SetPixelRedTraits() GetPixelYellowTraits() SetPixelYellowTraits() GetPixelYTraits() SetPixelYTraits() For convenience you can set the active trait for a set of pixel channels with a channel mask and this method:

SetImageChannelMask() Previously MagickCore methods had channel analogs, for example, NegateImage() and NegateImageChannels(). The channel analog methods are no longer necessary because the pixel channel traits specify whether to act on a particular pixel channel or whether to blend with the alpha mask. For example, instead of

NegateImageChannel(image,channel); we use:

channel_mask=SetImageChannelMask(image,channel); NegateImage(image,exception); (void) SetImageChannelMask(image,channel_mask);

如果您想了解每个方法如何以及为何处理这些标志，请阅读相应的文档或代码本身。