ImageMagick Version 7 Porting Guide

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.

The shell API (command line) of ImageMagick version 7 is also undergoing a major overhaul, with specific emphasis on the ability to read 'options' not only from the command line, but also from scripts, and file streams. This allows for the use of 'co-processing' programming techniques or performing image handling using 'deamon/server backends', and even multi-machine distributed processing.

With shell API overhaul other improvements are being made, including: better reporting of which option failed, the consolidation and depreciation of options, and more global use of 'image properities' (more commonly known as 'percent escapes' in option arguments.

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.

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) || (GetPixelMask(image,q) != 0))
          continue
        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()

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.

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).

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.

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>

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.

As mentioned the primary focus of the changes to the Shell API or Command Line Interface is the the abstration so that not only can 'options' be read from command line arguments, but also from a file (script) or from a file stream (interactive commands, or co-processing).

To do this the CLI parser needed to be re-written, so as to always perform all options, in a strict, do-it-as-you-see it order. Previously in IMv6 options were performed in groups (known as 'FireOptions), this awkwardness is now gone. However the strict order means that you can no long give operations before providing an image for the operations to work on. To do so will now produce an error.

Error reporting is now reporting exactly which option (by argument count on command line, or line,column in scripts) caused the 'exception'. This is not complete as yet but getting better. Also not complete is 'regard-warnings' handling or its replacement, which will allow you to ignore reported errors and continue processing (as appropriate due to error) in co-processes or interactive usage.

The parenthesis options used to 'push' the current image list, and image settings (EG: '(' and ')' ) on to a stack now has a completely seperate image settings stack. That is parenthesis 'push/pull' image lists, and curly braces (EG: '{' and '}' ) will 'push/pull' image settings.

Of course due to the previously reported changes to the underlying channel handling will result be many side effects to almost all options. Here are some specific

Most algorithms update the red, green, blue, black (for CMYK), and alpha channels. Most operators will blend alpha the other color channels, but other operators (and situations) may require this blending to be disabled, and is currently done by removing alpha from teh active channels via -channel option. (e.g. convert castle.gif -channel RGB -negate castle.png).

Reading gray-scale images will generate a image with only one channel. If that image is to then accept color the -colorspace setting needs to be applied to expand the one channel into separate RGB (or other) channels.

Command Changes

magick

The "magick" command is the new primary command of the Shell API, replacing the old "convert" command. This allows you to create a 'magick script' of the form "#!/path/to/command/magick -script", or pipe options into a command "magick -script -, as abackground process.

magick-script

This the same as "magick", (only command name is different) but which has an implict "-script" option. This allows you to use it in an "env" style script form. That is a magick script starts with the 'she-bang' line of "#!/usr/bin/env magick-script" allowing the script interpreter to be found anywhere on the users command "PATH". This is required to get around a "one argument she-bang bug" that is common on most UNIX systems (including Linux, but not MacOSX).

New Options

ImageMagick version 7 supports these new options, though most are limited to the "magick" command, or to use in "magick" scripts.

{ ... }

Save (and restore) the current image settings (internally known as the "image_info" structure). This is automatically done with parenthesis (EG: '(' and ')') is "-regard-parenthesis" has been set, just as in IMv6. Caution is advised to prevent un-balanced braces errors.

--

End of options, to be used in IMv7 "mogrify" command to explictally separate the operations to be applied and the images that are to be processed 'in-place'. (not yet implemented). However if not provided, "-read" can still be used to differentiate secondary image reads (for use in things like alpha composition) from the 'in-place' image being processed.

In other commands (such as "magick") it is equivelent to a explict "-read" (see below) of the next option as a image (as it was in IMv6).

-channel-fx expression

exchange, extract, or copy one or more image channels.

The expression consists of one or more channels, either mnemonic or numeric (e.g. red, 1), separated by certain operation symbols as follows:

  <=>     exchange two channels (e.g. red<=>blue)
  =>      copy one channel to another channel (e.g. red=>green)
  =       assign a constant value to a channel (e.g. red=50%)
  ,       write new image with channels in the specified order (e.g. red, green)
  ;       add a new output image for the next set of channel operations
  |       move to the next input image for the source of channel data

For example, to create 3 grayscale images from the red, green, and blue channels of an image, use:

  -channel-fx "red; green; blue"

A channel without an operation symbol implies separate (i.e, semicolon).

-exit

Stop processing at this point. No further options will be processed after this option. Can be used in a script to force the "magick" command to exit, without actually closing the pipeline that it is processing options from.

May also be used as a 'final' option on the "magick" command line, instead of a implicit output image, to completely prevent any image write. ASIDE: even the "NULL:" coder requires at least one image, for it to 'not write'! This option does not require any images at all.

-read {image}

Explicit read of an image, rather than an implicit read. This allows you to read from filenames that start with an 'option' character, and which otherwise could be mistaken as an option (unknown or otherwise). This will eventually be used in "mogrify" to allow the reading of secondary images, and allow the use of image list operations within that command.

-region

not yet implemented in "magick". (very soon)

-script {file}

In "magick", stop the processing of command line arguments as image operations, and read all further options from the given file or pipeline.

Changed Options

These options are known to have changed, in some way.

-negate

currently negates all channels, including alpha if present. As such you may need to use the -channel option to prevent alpha negation.

-convolve

option no longer normalizes the kernel argument (unless some type of masking is involved, such as alpha blending) and accepts named kernels (see IM Examples, Convolve ) as well as non-square user defined kernels. Use -define convolve:normalize=true on the command-line to force normalization.

Depreciated warning given, but will work (for now)

-affine

Replaced by -draw "affine ...". (see transform)

-average

Replaced by -evaluate-sequence Mean.

-box

Replaced by -undercolor.

-deconstruct

Replaced by -layers CompareAny.

-gaussian

Replaced by -gaussian-blur.

-/+map

Replaced by -/+remap.

-/+matte

Replaced by -alpha Set/Off.

-transform

Replaced by -distort Affine "...".

Depreciated warning given, and ignored (for now)

Almost 'plus' (+) option that did not do anything has been marked as depreciated, and does nothing. It does not even have associated code. For example "+annotate", "+resize", "+clut", and "+draw" .

-affinity

Replaced by

-affinity.

-maximum

Replaced by -evaluate-sequence Max.

-median

Replaced by -evaluate-sequence Median.

-minimum

Replaced by -evaluate-sequence Min.

-recolor

Replaced by

-color-matrix.

Removed / Replaced Options ("no such option" error and abort)

-origin

old option, unknown meaning.

-pen

Replaced by

-fill.

-passphrase

old option, unknown meaning

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

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.