ImageMagick Version 7 Porting Guide

ImageMagick Version 7

The design of ImageMagick is an evolutionary process, with the design and implementation efforts serving to influence and guide further progress in the other. With ImageMagick version 7 we aim to improve the design based on lessons learned from the version 6 implementation. ImageMagick was originally designed to display RGB images to an X Windows server. Over time we extended support to RGBA images and then to the CMYK and CMYKA image format. With ImageMagick version 7, we extend support to arbitrary colorspaces with an arbitrary number of pixel channels. Other design changes are in the works and we will document them here so be sure to revisit periodically.

To support variable pixel channels in the MagickCore API, pixel handling has changed when getting or setting the pixel channels. You can access channels as an array, pixel[i], or use an accessor such as GetPixelRed() or SetPixelRed(). There are some modest changes to the MagickWand API. Magick++ and PerlMagick should behave exactly as it does for ImageMagick version 6.

We intend to make ImageMagick version 7 available as an Alpha release by the mid-year 2012. Look for a Beta release sometime in late 2012. An official ImageMagick version 7 release depends on how smoothly the Beta cycle progresses. During the Beta cycle, version 6 developers can attempt to port their software to version 7.

During the ImageMagick version 7 development cycle and release, we will continue to support and enhance version 6 for a minimum of 10 years.

Pixel Channels

A pixel is comprised of one or more color values, or channels (e.g. red pixel channel).

Prior versions of ImageMagick (4-6), support 4 to 5 pixel channels (RGBA or CMYKA). The first 4 channels are accessed with the PixelPacket data structure. The structure includes 4 members of type Quantum (typically 16-bits) of red, green, blue, and opacity. The black channel or colormap indexes are supported by a separate method and structure, IndexPacket. As an example, here is a code snippet from ImageMagick version 6 that negates the color components (but not the alpha component) of the image pixels:

  for (y=0; y < (ssize_t) image->rows; y++)
  {
    register IndexPacket
      *indexes;

    register PixelPacket
      *q;

    q=GetCacheViewAuthenticPixels(image_view,0,y,image->columns,1,exception);
    if (q == (PixelPacket *) NULL)
      {
        status=MagickFalse;
        continue;
      }
    indexes=GetCacheViewAuthenticIndexQueue(image_view);
    for (x=0; x < (ssize_t) image->columns; x++)
    {
      if ((channel & RedChannel) != 0)
        q->red=(Quantum) QuantumRange-q->red;
      if ((channel & GreenChannel) != 0)
        q->green=(Quantum) QuantumRange-q->green;
      if ((channel & BlueChannel) != 0)
        q->blue=(Quantum) QuantumRange-q->blue;
      if (((channel & IndexChannel) != 0) &&
          (image->colorspace == CMYKColorspace))
        indexes[x]=(IndexPacket) QuantumRange-indexes[x];
      q++;
    }
    if (SyncCacheViewAuthenticPixels(image_view,exception) == MagickFalse)
      status=MagickFalse;
  }

ImageMagick version 7 supports any number of channels from 1 to 32 (and beyond) and simplifies access with a single method that returns an array of pixel channels of type Quantum. Source code that compiles against prior versions of ImageMagick requires refactoring to work with ImageMagick version 7. We illustrate with an example. Let's naively refactor the version 6 code snippet from above so it works with the ImageMagick version 7 API:

  for (y=0; y < (ssize_t) image->rows; y++)
  {
    register Quantum
      *q;

    q=GetCacheViewAuthenticPixels(image_view,0,y,image->columns,1,exception);
    if (q == (Quantum *) NULL)
      {
        status=MagickFalse;
        continue;
      }
    for (x=0; x < (ssize_t) image->columns; x++)
    {
      if ((GetPixelRedTraits(image) & UpdatePixelTrait) != 0)
        SetPixelRed(image,QuantumRange-GetPixelRed(image,q),q);
      if ((GetPixelGreenTraits(image) & UpdatePixelTrait) != 0)
        SetPixelGreen(image,QuantumRange-GetPixelGreen(image,q),q);
      if ((GetPixelBlueTraits(image) & UpdatePixelTrait) != 0)
        SetPixelBlue(image,QuantumRange-GetPixelBlue(image,q),q);
      if ((GetPixelBlackTraits(image) & UpdatePixelTrait) != 0)
        SetPixelBlack(image,QuantumRange-GetPixelBlack(image,q),q);
      if ((GetPixelAlphaTraits(image) & UpdatePixelTrait) != 0)
        SetPixelAlpha(image,QuantumRange-GetPixelAlpha(image,q),q);
      q+=GetPixelChannels(image);
    }
    if (SyncCacheViewAuthenticPixels(image_view,exception) == MagickFalse)
      status=MagickFalse;
  }

Let's do that again but take full advantage of the new variable pixel channel support:

  for (y=0; y < (ssize_t) image->rows; y++)
  {
    register Quantum
      *q;

    q=GetCacheViewAuthenticPixels(image_view,0,y,image->columns,1,exception);
    if (q == (Quantum *) NULL)
      {
        status=MagickFalse;
        continue;
      }
    for (x=0; x < (ssize_t) image->columns; x++)
    {
      register ssize_t
        i;

      for (i=0; i < (ssize_t) GetPixelChannels(image); i++)
      {
        PixelChannel
          channel;

        PixelTrait
          traits;

        channel=GetPixelChannelMapChannel(image,i);
        traits=GetPixelChannelMapTraits(image,channel);
        if ((traits & UpdatePixelTrait) != 0)
          q[i]=QuantumRange-q[i];
      }
      q+=GetPixelChannels(image);
    }
    if (SyncCacheViewAuthenticPixels(image_view,exception) == MagickFalse)
      status=MagickFalse;
  }

Note, how we use GetPixelChannels() to advance to the next set of pixel channels.

The colormap indexes and black pixel channel (for the CMYK colorspace) are no longer stored in the index channel, previously accessed with GetAuthenticIndexQueue() and GetCacheViewAuthenticIndexQueue((). Instead they are now a pixel channel and accessed with the convenience pixel accessor methods GetPixelIndex(), SetPixelIndex(), GetPixelBlack(), and SetPixelBlack().

Pixel Accessors

Use accessors to get or set pixel channels:

  GetPixelAlpha()
  GetPixelBlack()
  GetPixelBlue()
  GetPixelCb()
  GetPixelChannels()
  GetPixelCr()
  GetPixelCyan()
  GetPixelGray()
  GetPixelGreen()
  GetPixelIndex()
  GetPixelInfoIntensity()
  GetPixelInfoLuminance()
  GetPixelMagenta()
  GetPixelRed()
  GetPixelPacket()
  GetPixelPacketIntensity()
  GetPixelY()
  GetPixelYellow()
  GetPixelIntensity()
  SetPixelAlpha()
  SetPixelBlack()
  SetPixelBlue()
  SetPixelCb()
  SetPixelChannels()
  SetPixelCr()
  SetPixelCyan()
  SetPixelGray()
  SetPixelGreen()
  SetPixelIndex()
  SetPixelInfo()
  SetPixelInfoBias()
  SetPixelInfoPacket()
  SetPixelMagenta()
  SetPixelRed()
  SetPixelPacket()
  SetPixelPixelInfo()
  SetPixelYellow()
  SetPixelY()

You can find these accessors defined in the header file, MagickCore/pixel-accessor.h

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()
  GetPixelBlackTraits()
  GetPixelBlueTraits()
  GetPixelCbTraits()
  GetPixelCrTraits()
  GetPixelCyanTraits()
  GetPixelGrayTraits()
  GetPixelGreenTraits()
  GetPixelIndexTraits()
  GetPixelMagentaTraits()
  GetPixelRedTraits()
  GetPixelChannelTraits()
  GetPixelYTraits()
  GetPixelYellowTraits()
  SetPixelAlphaTraits()
  SetPixelBlackTraits()
  SetPixelBlueTraits()
  SetPixelCbTraits()
  SetPixelChannelTraits()
  SetPixelCrTraits()
  SetPixelGrayTraits()
  SetPixelGreenTraits()
  SetPixelIndexTraits()
  SetPixelMagentaTraits()
  SetPixelRedTraits()
  SetPixelYellowTraits()
  SetPixelYTraits()

For convenience you can set the active trait for a set of pixel channels with a channel mask and these methods:

  PopPixelChannelMap()
  SetPixelChannelMap()
  SetPixelChannelMask()

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 and whether to blend with the alpha mask. For example, instead of

  NegateImageChannel(image,channel);

we use:

  channel_mask=SetPixelChannelMask(image,channel);
  NegateImage(image,exception);
  (void) SetPixelChannelMap(image,channel_mask);

Pixel User Channels

In version 7, we introduce pixel user channels. Traditionally we utilize 4 channels, red, green, blue, and alpha. For CMYK we also have a black channel. User channels are designed to contain whatever additional channel information that makes sense for your application. Some examples include extra channels in TIFF or PSD images or perhaps you require a channel with infrared information for the pixel. You can associate traits with the user channels so that they when they are acted upon by an image processing algorithm (e.g. blur) the pixels are copied, acted upon by the algorithm, or even blended with the alpha channel if that makes sense.

Pixel Metacontent

In version 7, we introduce pixel metacontent. Metacontent is content about content. So rather than being the content itself, it's something that describes or is associated with the content. Here the content is a pixel. The pixel metacontent is for your exclusive use and is accessed with these MagickCore API methods:

  SetImageMetacontentExtent()
  GetImageMetacontentExtent()
  GetVirtualMetacontent()
  GetAuthenticMetacontent()
  GetCacheViewAuthenticMetacontent()
  GetCacheViewVirtualMetacontent()

Alpha

We support alpha now, previously opacity. With alpha, a value of 0 means that the pixel does not have any coverage information and is transparent; i.e. there was no color contribution from any geometry because the geometry did not overlap this pixel. A value of QuantumRange means that the pixel is opaque because the geometry completely overlapped the pixel. As a consequence, in version 7, the PixelInfo structure member alpha has replaced the previous opacity member.

Grayscale

Previously, grayscale images consumed 4 channels: red, green, blue, and alpha. With version 7, grayscale consumes only 1 channel consuming far less resources as a result. However, there may be unintended consequences. With 1 channel, all image processing algorithms write to this one channel. Drawing yellow text on a grayscale image will produce gray lettering. To get the expected results, simply modify the colorspace to RGB (e.g. -colorspace rgb).

MagickCore API Changes

Almost all image processing algorithms are now channel aware.

MagickCore, version 7, adds an ExceptionInfo argument to those methods that lacked it in version 6, e.g. NegateImage(image,MagickTrue,exception);

All method channel analogs have been removed (e.g. BlurImageChannel()), they are no longer necessary, use pixel traits instead.

Public and private API calls are now declared with the GCC visibility attribute. The MagickCore and MagickWand dynamic libraries now only export public struct and function declarations.

The InterpolatePixelMethod enum is now PixelInterpolateMethod.

The IntegerPixel storage type is removed (use LongPixel instead) and LongLongPixel is added

Image signatures have changed to account for variable pixel channels.

All color packet structures, PixelPacket, LongPacket, and DoublePacket, are consolidated to a single color structure, PixelInfo.

Header Files

Prior versions of ImageMagick (4-6) reference the ImageMagick header files as magick/ and wand/. ImageMagick 7 instead uses MagickCore/ and MagickWand/ respectively. For example,

#include <MagickCore/MagickCore.h>
#include <MagickWand/MagickWand.h>

Deprecated Features Removed

All deprecated features from ImageMagick version 6 are removed in version 7. These include the Magick-config and Wand-config configuration utilities. Instead use:

  MagickCore-config
  MagickWand-config

The FilterImage() method has been removed. Use ConvolveImage() instead.

In addition, all deprecated MagickCore and MagickWand methods are no longer available in version 7.

Command-line Interface

By default, most algorithms update the red, green, blue, black (for CMYK), and alpha channels. If appropriate, alpha is blended with red, green, blue, and black. For some algorithms, it only makes sense to copy alpha. For these cases, use the -channel option (e.g. convert castle.gif -channel RGB -negate castle.png).

The -convolve option no longer normalizes the kernel argument and accepts non-square user defined kernels.

Version 7 Change Summary

Changes from ImageMagick version 6 to version 7 are summarized here:

Pixels

Pixels are no longer addressed with PixelPacket structure members (e.g. red, green, blue, opacity) but as an array of channels (e.g. pixel[PixelRedChannel]).

Use convenience macros to access pixel channels (e.g. GetPixelRed(), SetPixelRed()).

The black channel for the CMYK colorspace is no longer stored in the index channel, previously accessed with GetAuthenticIndexQueue() and GetCacheViewAuthenticIndexQueue((). Instead it is now a pixel channel and accessed with the convenience pixel macros GetPixelBlack() and SetPixelBlack().

The index channel for colormapped images is no longer stored in the index channel, previously accessed with GetAuthenticIndexQueue() and GetCacheViewAuthenticIndexQueue((). Instead it is now a pixel channel and accessed with the convenience pixel macros GetPixelIndex() and SetPixelIndex().

Use GetPixelChannels() to advance to the next set of pixel channels.

Use the metacontent channel to associate metacontent with each pixel.

All color packet structures, PixelPacket, LongPacket, and DoublePacket, are consolidated to a single color structure, PixelInfo.

Alpha

We support alpha rather than opacity (0 transparent; QuantumRange opaque).

Use GetPixelAlpha() or SetPixelAlpha() to get or set the alpha pixel channel value.

Grayscale

Grayscale images consume one pixel channel in ImageMagick version 7. To process RGB, set the colorspace to RGB (e.g. -colorspace rgb).

MagickCore API Changes

Almost all image processing algorithms are now channel aware.

MagickCore, version 7, adds an ExceptionInfo argument to those methods that lacked it in version 6, e.g. NegateImage(image,MagickTrue,exception);

All method channel analogs have been removed (e.g. BlurImageChannel()), they are no longer necessary, use pixel traits instead.

Public and private API calls are now declared with the GCC visibility attribute. The MagickCore and MagickWand dynamic libraries now only export public struct and function declarations.

The InterpolatePixelMethod enum is now PixelInterpolateMethod.

To account for variable pixel channels, images may now return a different signature.

Deprecated Methods

All ImageMagick version 6 MagickCore and MagickWand deprecated methods are removed and no longer available in ImageMagick version 7.

All MagickCore channel method analogs are removed (e.g. NegateImageChannels()). For version 7, use pixel traits instead.

The FilterImage() method has been removed. Use ConvolveImage() instead.

Command-line Interface

If you don't want alpha negated, use the -channel option, e.g. convert castle.gif -channel RGB -negate castle.png.

The -convolve option no longer normalizes the kernel argument and accepts non-square user defined kernels.