ImageMagick | MagickWand, C API
Description: Recommended interface between the C programming language and the ImageMagick image processing libraries.

$site=strtolower("ImageMagick");

The MagickWand API is the recommended interface between the C programming language and the ImageMagick image processing libraries. Unlike the MagickCore C API, MagickWand uses only a few opaque types. Accessors are available to set or get important wand properties. A description of the MagickWand public methods are found on GitHub:

- Magick Wand Methods
- Set or Get Magick Wand Properties
- Magick Wand Image Methods
- Pixel Iterator Methods
- Pixel Wand Methods
- Image Vector Drawing
- Command-line Interface
- Wand View Methods
- Deprecated Methods
- Error and Warning Codes

After you write your MagickWand program, compile it like this:

    $ cc `MagickWand-config --cflags --cppflags` -O2 -o wand wand.c `MagickWand-config --ldflags --libs`

Set the `PKG_CONFIG_PATH` environment variable if ImageMagick is not in your default system path:

    $ export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig

Here is a example program that utilizes the MagickWand API to get you started, wand.c. It reads an image, creates a thumbnail, and writes the result to disk.

    #include <stdio.h>
    #include <stdlib.h>
    #include <MagickWand/MagickWand.h>

    int main(int argc,char **argv)
    {
    #define ThrowWandException(wand) \
    { \
      char \
        *description; \
     \
      ExceptionType \
        severity; \
     \
      description=MagickGetException(wand,&severity); \
      (void) fprintf(stderr,"%s %s %lu %s\n",GetMagickModule(),description); \
      description=(char *) MagickRelinquishMemory(description); \
      exit(-1); \
    }

      MagickBooleanType
        status;

      MagickWand
        *magick_wand;

      if (argc != 3)
        {
          (void) fprintf(stdout,"Usage: %s image thumbnail\n",argv[0]);
          exit(0);
        }
      /*
        Read an image.
      */
      MagickWandGenesis();
      magick_wand=NewMagickWand();
      status=MagickReadImage(magick_wand,argv[1]);
      if (status == MagickFalse)
        ThrowWandException(magick_wand);
      /*
        Turn the images into a thumbnail sequence.
      */
      MagickResetIterator(magick_wand);
      while (MagickNextImage(magick_wand) != MagickFalse)
        MagickResizeImage(magick_wand,106,80,LanczosFilter,1.0);
      /*
        Write the image then destroy it.
      */
      status=MagickWriteImages(magick_wand,argv[2],MagickTrue);
      if (status == MagickFalse)
        ThrowWandException(magick_wand);
      magick_wand=DestroyMagickWand(magick_wand);
      MagickWandTerminus();
      return(0);
    }

Here is another program that shows one way to get and set image pixels with the MagickWand API, contrast.c. It reads an image, applies sigmoidal non-linearity contrast control, and writes the result to disk.

    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <MagickWand/MagickWand.h>

    int main(int argc,char **argv)
    {
    #define SigmoidalContrast(x) \
      (QuantumRange*(1.0/(1+exp(10.0*(0.5-QuantumScale*x)))-0.0066928509)*1.0092503)
    #define ThrowWandException(wand) \
    { \
      char \
        *description; \
     \
      ExceptionType \
        severity; \
     \
      description=MagickGetException(wand,&severity); \
      (void) fprintf(stderr,"%s %s %lu %s\n",GetMagickModule(),description); \
      description=(char *) MagickRelinquishMemory(description); \
      exit(-1); \
    }

      long
        y;

      MagickBooleanType
        status;

      MagickWand
        *contrast_wand,
        *image_wand;

      PixelInfo
        pixel;

      PixelIterator
        *contrast_iterator,
        *iterator;

      PixelWand
        **contrast_pixels,
        **pixels;

      register long
        x;

      unsigned long
        width;

      if (argc != 3)
        {
          (void) fprintf(stdout,"Usage: %s image sigmoidal-image\n",argv[0]);
          exit(0);
        }
      /*
        Read an image.
      */
      MagickWandGenesis();
      image_wand=NewMagickWand();
      status=MagickReadImage(image_wand,argv[1]);
      if (status == MagickFalse)
        ThrowWandException(image_wand);
      contrast_wand=CloneMagickWand(image_wand);
      /*
        Sigmoidal non-linearity contrast control.
      */
      iterator=NewPixelIterator(image_wand);
      contrast_iterator=NewPixelIterator(contrast_wand);
      if ((iterator == (PixelIterator *) NULL) ||
          (contrast_iterator == (PixelIterator *) NULL))
        ThrowWandException(image_wand);
      for (y=0; y < (long) MagickGetImageHeight(image_wand); y++)
      {
        pixels=PixelGetNextIteratorRow(iterator,&width);
        contrast_pixels=PixelGetNextIteratorRow(contrast_iterator,&width);
        if ((pixels == (PixelWand **) NULL) ||
            (contrast_pixels == (PixelWand **) NULL))
          break;
        for (x=0; x < (long) width; x++)
        {
          PixelGetMagickColor(pixels[x],&pixel);
          pixel.red=SigmoidalContrast(pixel.red);
          pixel.green=SigmoidalContrast(pixel.green);
          pixel.blue=SigmoidalContrast(pixel.blue);
          pixel.index=SigmoidalContrast(pixel.index);
          PixelSetPixelColor(contrast_pixels[x],&pixel);
        }
        (void) PixelSyncIterator(contrast_iterator);
      }
      if (y < (long) MagickGetImageHeight(image_wand))
        ThrowWandException(image_wand);
      contrast_iterator=DestroyPixelIterator(contrast_iterator);
      iterator=DestroyPixelIterator(iterator);
      image_wand=DestroyMagickWand(image_wand);
      /*
        Write the image then destroy it.
      */
      status=MagickWriteImages(contrast_wand,argv[2],MagickTrue);
      if (status == MagickFalse)
        ThrowWandException(image_wand);
      contrast_wand=DestroyMagickWand(contrast_wand);
      MagickWandTerminus();
      return(0);
    }

Now lets perform the same contrast enhancement while taking advantage of our dual or quad-core processing system by running the algorithm in parallel utilizing wand views. The sigmoidal-contrast.c module reads an image, applies sigmoidal non-linearity contrast control, and writes the result to disk just like the previous contrast enhancement program, but now it does its work in parallel (assumes ImageMagick is built with OpenMP support).

    #include <stdio.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <MagickWand/MagickWand.h>

    static MagickBooleanType SigmoidalContrast(WandView *contrast_view,
      const ssize_t y,const int thread_id,void *context)
    {
    #define SigmoidalContrast(x) \
      (QuantumRange*(1.0/(1+exp(10.0*(0.5-QuantumScale*x)))-0.0066928509)*1.0092503)

      PixelInfo
        pixel;

      PixelWand
        **pixels;

      RectangleInfo
        extent;

      register ssize_t
        x;

      extent=GetWandViewExtent(contrast_view);
      pixels=GetWandViewPixels(contrast_view);
      for (x=0; x < (ssize_t) extent.width; x++)
      {
        PixelGetMagickColor(pixels[x],&pixel);
        pixel.red=SigmoidalContrast(pixel.red);
        pixel.green=SigmoidalContrast(pixel.green);
        pixel.blue=SigmoidalContrast(pixel.blue);
        pixel.index=SigmoidalContrast(pixel.index);
        PixelSetPixelColor(pixels[x],&pixel);
      }
      return(MagickTrue);
    }

    int main(int argc,char **argv)
    {
    #define ThrowWandException(wand) \
    { \
      char \
        *description; \
     \
      ExceptionType \
        severity; \
     \
      description=MagickGetException(wand,&severity); \
      (void) fprintf(stderr,"%s %s %lu %s\n",GetMagickModule(),description); \
      description=(char *) MagickRelinquishMemory(description); \
      exit(-1); \
    }

      MagickBooleanType
        status;

      MagickWand
        *contrast_wand;

      PixelInfo
        pixel;

      WandView
        *contrast_view;

      if (argc != 3)
        {
          (void) fprintf(stdout,"Usage: %s image sigmoidal-image\n",argv[0]);
          exit(0);
        }
      /*
        Read an image.
      */
      MagickWandGenesis();
      contrast_wand=NewMagickWand();
      status=MagickReadImage(contrast_wand,argv[1]);
      if (status == MagickFalse)
        ThrowWandException(contrast_wand);
      /*
        Sigmoidal non-linearity contrast control.
      */
      contrast_view=NewWandView(contrast_wand);
      if (contrast_view == (WandView *) NULL)
        ThrowWandException(contrast_wand);
      status=UpdateWandViewIterator(contrast_view,SigmoidalContrast,(void *) NULL);
      if (status == MagickFalse)
        ThrowWandException(contrast_wand);
      contrast_view=DestroyWandView(contrast_view);
      /*
        Write the image then destroy it.
      */
      status=MagickWriteImages(contrast_wand,argv[2],MagickTrue);
      if (status == MagickFalse)
        ThrowWandException(contrast_wand);
      contrast_wand=DestroyMagickWand(contrast_wand);
      MagickWandTerminus();
      return(0);
    }

`MagickWandTerminus()` is a function in the ImageMagick library that is used to clean up and release resources when shutting down an application that uses ImageMagick. This function should be called in the primary thread of the application's process during the shutdown process. It's crucial that this function is invoked only after any threads that are using ImageMagick functions have terminated.

ImageMagick might internally use threads via OpenMP (a method for parallel programming). As a result, it's important to ensure that any function calls into ImageMagick have completed before calling `MagickWandTerminus()`. This prevents issues with OpenMP worker threads accessing resources that are destroyed by this termination function.

If OpenMP is being used (starting from version 5.0), the OpenMP implementation itself handles starting and stopping worker threads and allocating and freeing resources using its own methods. This means that after calling `MagickWandTerminus()`, some OpenMP resources and worker threads might still remain allocated. To address this, the function `omp_pause_resource_all(omp_pause_hard)` can be invoked. This function, introduced in OpenMP version 5.0, ensures that any resources allocated by OpenMP (such as threads and thread-specific memory) are freed. It's recommended to call this function after `MagickWandTerminus()` has completed its execution.

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