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The citizens of Oz were quite content with their benefactor, the all-powerful Wizard. They accepted his wisdom and benevolence without ever questioning the who, why, and where of his power. Like the citizens of Oz, if you feel comfortable that ImageMagick can help you convert, edit, or compose your images without knowing what goes on behind the curtain, feel free to skip this section. However, if you want to know more about the software and algorithms behind ImageMagick, read on. To fully benefit from this discussion, you should be comfortable with image nomenclature and be familiar with computer programming. An image typically consists of a rectangular region of pixels and metadata. To convert, edit, or compose an image in an efficient manner we need convenient access to any pixel anywhere within the region (and sometimes outside the region). And in the case of an image sequence, we need access to any pixel of any region of any image in the sequence. However, there are hundreds of image formats such JPEG, TIFF, PNG, GIF, etc., that makes it difficult to access pixels on demand. Within these formats we find differences in:
In addition, some image pixels may require attenuation, some formats permit more than one frame, and some formats contain vector graphics that must first be rasterized (converted from vector to pixels). An efficient implementation of an image processing algorithm may require we get or put:
In addition, some images include a clip mask that define which pixels are eligible to be updated. Pixels outside the area defined by the clip mask remain untouched. Given the varied image formats and image processing requirements, we implemented the ImageMagick pixel cache to provide convenient access to any pixel on demand anywhere inside the image region and from any image in a sequence. In addition, the pixel cache permits access to pixels outside the boundaries defined by the image (we call these virtual pixels). In addition to pixels, images have a plethora of image properties and profiles. Properties include the well known items such as width, height, depth, and colorspace. An image may have optional properties which might include the image author, a comment, a create date, and others. Some images also include profiles for color management, or EXIF, IPTC, 8BIM, or XMP informational profiles. ImageMagick provides command line options and programming methods to get, set, or view image properties or profiles or apply profiles. ImageMagick has more than 325,000 lines of C code and optionally depends on several million lines of code in dependent libraries (e.g. JPEG, PNG, TIFF libraries). Given that, one might expect a huge architecture document. However, a great majority of image processing is simply accessing pixels and its metadata and our simple and elegant implementation makes this easy for the ImageMagick user. We discuss the implementation of the pixel cache and getting and setting image properties and profiles in the next few sections. Next, we discuss image coders to read or write a particular image format followed by a few words on creating a filter to access or update pixels based on your custom requirements. In the final section, we discuss using ImageMagick within a thread of execution. The ImageMagick pixel cache is a repository for image pixels with up to 5 channels. The first 4 channels are stored contiguously and an optional second area follows with 1 channel. The channels are at the depth specified when ImageMagick was built. The channel depths are 8 bits-per-pixel for the Q8 version of ImageMagick, 16 bits-per-pixel for the the Q16 version, and 32 bits-per-pixel for the Q32 version. By default pixel components are unsigned quantities, however, if you use the high dynamic-range version of ImageMagick, the components are 32-bit floating point. The primary 4 channels can hold any value but typically contain red, green, blue, and alpha intensities or cyan, magenta, yellow, and alpha intensities. The optional fifth channel contains the colormap indexes for colormapped images or the black channel for CMYK images. The pixel cache storage may be anonymous memory mapped memory, heap memory, disk-backed memory mapped, or on disk. The pixel cache is reference-counted. Only the cache properties are copied when the cache is cloned. The cache pixels are subsequently copied when you signal your intention to update any of the pixels. Create the Pixel CacheThe pixel cache is associated with an image when it is created and it is initialized when you try to get or put pixels. Here are three common methods to associate a pixel cache with an image:
In our discussion of the pixel cache we use the MagickCore API to illustrate our points, however, the principles are the same for other program interfaces to ImageMagick. When the pixel cache is initialized, pixels are scaled from whatever bit depth they originated from to that required by the pixel cache. For example, a 1-channel 1-bit monochrome PBM image is scaled to a 4 channel 8-bit RGBA image, if you are using the Q8 version of ImageMagick, and 16-bit RGBA for the Q16 version. You can determine which version you have with this command: -> identify -version Version: ImageMagick 6.3.4 11/01/07 Q16 http://www.imagemagick.org As you can see, the convenience of the pixel cache sometimes comes with a tradeoff in storage and speed. Access the Pixel CacheOnce the pixel cache is associated with an image, you typically want to get, update, or put pixels into it. Use these methods to access the pixels in the cache:
Here is a typical MagickCore code snippet for manipulating pixels in the pixel cache. In our example we copy pixels from the input image to the output image and decrease the intensity by 10%: When we first create the destination image by cloning the source image, the pixel cache pixels are not copied. They are only copied when you signal your intentions to modify the pixel cache by calling GetImagePixels() or SetImagePixels(). Use SyncImagePixels() to ensure any pixel changes are pushed to the pixel cache. Recall how we mentioned that the indexes of a colormapped image or the black channel of a CMYK image are stored separately. We use GetIndexes() to gain access to this channel. For example, to print the colormap indexes, use:
IndexPacket
*indexes;
for (y=0; y < (long) source->rows; y++)
{
p=AcquireImagePixels(source,0,y,source->columns,1);
if (p == (const PixelPacket *) NULL)
break;
indexes=GetIndexes(source);
for (x=0; x < (long) source->columns; x++)
(void) printf("%d\n",indexes[x];
}
if (y < (long) source->rows)
/* an exception was thrown */
The pixel cache manager decides whether to give you direct or indirect access to the image pixels. In some cases the pixels are staged to an intermediate buffer-- and that is why you must call SyncImagePixels() to ensure this buffer is pushed out to the pixel cache to guarantee the corresponding pixels in the cache are updated. For this reason we recommend that you only acquire, get, or set a scanline or a few scanlines of pixels at a time. However, you can get any rectangular region of pixels you want. GetImagePixels() requires the region you get is within the bounds of the image area. For a 640 by 480 image, you can get a scanline of 640 pixels but if you ask for 641 pixels, an error is returned. AcquireImagePixels() does not have this constraint. For example, p=AcquireImagePixels(source,-3,3,source->columns+7,7); gives you the pixels you asked for without complaint, even though some are not within the confines of the image region. Virtual PixelsWe call any pixels outside the image region virtual pixels. Their value is defined by the SetImageVirtualPixelMethod() from the MagickCore API or -virtual-pixel option from the command line. The methods include: background: The area surrounding the image is the background color. dither: Use a dithered pattern to choose a pixel in a 32x32 neighborhood. edge: Extend the edge pixel toward infinity. mirror: Mirror the image. random: Choose a random pixel from the image. tile: Tile the image (default). transparent: The area surrounding the image is transparent blackness. There are a plethora of image processing algorithms that require a neighborhood of pixels about a pixel of interest. There is typically a caveat concerning how to handle pixels around the image boundaries, known as edge pixels. With virtual pixels, you do not need to concern yourself about special edge processing other than choosing which virtual pixel method is most appropriate for your algorithm. Cache Storage and Resource RequirementsWe mentioned previously that this simple and elegant design of the ImageMagick pixel cache comes at a price in terms of storage and processing speed. The pixel cache storage requirements scales with the area of the image and the bit depth of the pixel components. For example, if we have a 640 by 480 image and we're using the Q16 version of ImageMagick, the pixel cache consumes image width * height * bit-depth / 8 * channels bytes or approximately 2.3 megabytes (i.e. 640 * 480 * 2 * 4). Not too bad, but what if your image is 25000 by 25000 pixels? The pixel cache requires approximately 4.7 gigabytes of storage. Ouch. ImageMagick accounts for possible huge storage requirements by caching large images to disk rather than memory. Typically the pixel cache is stored in memory using anonymous memory mapping. If anonymous memory is exhausted, pixels are stored in heap memory; if heap memory is exhausted, we create the pixel cache on disk and attempt to memory-map it; and if memory-map memory is exhausted, we simply use standard disk I/O. Disk is cheap but it is also very slow, upwards of 1000 times slower than memory. We can get some speed improvements, up to 5 times, if we use memory mapping to the disk-based cache. These decisions about storage are made automagically by the pixel cache manager negotiating with the operating system. However, you can influence how the pixel cache manager allocates the pixel cache with cache resource limits. The limits include:
To determine the current setting of these limits, use this command: -> identify -list resource File Area Memory Map Disk ------------------------------------------------ 768 2gb 1.5gb 8gb 4eb You can set these limits either with environment variables, the -limit command line option, or the SetMagickResourceLimit() MagickCore API method. As an example, our online web interface to ImageMagick, ImageMagick Studio, has an area limit of 64 megabytes, a memory limit of 128 megabytes and a map limit of 256 megabytes and a disk limit of 1 gigabytes. Since we process multiple simultaneous sessions, we don't want any one session consuming all the available memory. Instead large images are cached to disk. If the image is too large and exceeds the pixel cache disk limit, the program exits. Note, the cache limits are global, meaning if you create several images, the combined resource requirements are compared to the limit to determine the pixel cache storage disposition. Cache ViewsAcquireImagePixels(), GetImagePixels(), SetImagePixels(), and SyncImagePixels() from the MagickCore API can only deal with one pixel cache area per image at a time. Suppose you want to access the first and last scanline from the same image at the same time? The solution is to use a cache view. A cache view permits you to access as many areas simultaneously in the pixel cache as you require. The cache view methods behave like the previous methods except you must first open a view and close it when you are finished with it. Here is a snippet of MagickCore that permits us to access two areas of an image simultaneously:
ViewInfo
*view_1,
*view_2;
view_1=OpenCacheView(source);
view_2=OpenCacheView(source);
for (y=0; y < (long) source->rows; y++)
{
u=AcquireCacheView(view_1,0,y,source->columns,1);
v=AcquireCacheView(view_2,0,source->rows-y-1,source->columns,1);
if ((u == (const PixelPacket *) NULL) || (v == (const PixelPacket *) NULL))
break;
for (x=0; x < (long) source->columns; x++)
{
/* do something with u & v here */
}
}
view_1=CloseCacheView(view_1);
view_2=CloseCacheView(view_2);
if (y < (long) source->rows)
{ /* an exception was thrown */ }
Magick Persistent Cache FormatRecall that each image format is decoded by ImageMagick and the pixels are deposited in the pixel cache. If you write an image, the pixels are read from the pixel cache and encoded as required by the format you are writing (e.g. GIF, PNG, etc.). The Magick Persistent Cache (MPC) format is designed to eliminate the overhead of decoding and encoding pixels to and from an image format. MPC writes two files. One, with the extension .mpc, retains all the properties associated with the image or image sequence (e.g. width, height, colorspace, etc.) and the second, with the extension .cache, is the pixel cache in the native format. When reading an MPC image file, ImageMagick reads the image properties and memory maps the pixel cache on disk eliminating the need for decoding the image pixels. The tradeoff is in disk space. MPC is generally larger in file size than most other image formats. Best PracticesAlthough you can request any pixel from the pixel cache, any block of pixels, any scanline, multiple scanlines, any row, or multiple rows with the AcquireImagePixels(), GetImagePixels(), SetImagePixels, AcquireCacheView(), GetCacheView(), and SetCacheView() methods, ImageMagick is optimized to return a few pixels or a few pixels rows at at time. There is additional optimizations if you request a single scanline or a few scanlines at a time. These methods also permit random access to the pixel cache, however, ImageMagick is optimized for sequential access. If you are updating pixels obtained with GetImagePixels() or GetCacheView(), don't forget to call SyncImageCache() or SyncCacheView() respectively after you update the pixels to ensure any updates are synchronized with the pixel cache. Use SetImagePixels() or SetCacheView() if you are setting an initial pixel value. The GetImagePixels() or GetCacheView() method reads pixels from the cache and if you are setting an initial pixel value, this read is unnecessary. Don't forget to call SyncImagePixels() or SyncCacheView() respectively to push your updates to the pixel cache. AcquireImagePixels(), GetImagePixels(), SetImagePixels(), and SyncImagePixels() are slightly more efficient than their cache view counter-parts. However, cache views are required if you need access to more than one region of the image simultaneously or if more than one thread of execution is accessing the image. You can request pixels outside the bounds of the image with AcquireImagePixels() or AcquireCacheView(), however, it is more efficient to request pixels within the confines of the image region. Although you can force the pixel cache to disk using appropriate resource limits, disk access can be upwards of 1000 times slower than memory access. For fast, efficient, access to the pixel cache, try to keep the pixel cache in anonymous memory mapped or heap memory. The ImageMagick Q16 version of ImageMagick permits you to read and write 16 bit images without scaling but the pixel cache consumes twice as much resources as the Q8 version. If your system has constrained memory or disk resources, you might want to use the Q8 version of ImageMagick. The Q8 version is typically faster than the Q16 version. A great majority of image formats and algorithms restrict themselves to a fixed range of pixel values from 0 to some maximum value, for example, the Q16 version of ImageMagick permit intensities from 0 to 65535. High dynamic-range imaging (HDRI), however, permits a far greater dynamic range of exposures (i.e. a large difference between light and dark areas) than standard digital imaging techniques. HDRI accurately represents the wide range of intensity levels found in real scenes ranging from the brightest direct sunlight to the deepest darkest shadows. Enable HDRI at ImageMagick build time to deal with high dynamic-range images, but be mindful that each pixel component is 32-bit floating point. If you are dealing with large images, make sure the pixel cache is written to a disk area with plenty of free space. Under Unix, this is typically /tmp and for Windows, c:/temp. You can tell ImageMagick to write the pixel cache to an alternate location with the MAGICK_TMPDIR environment variable. For example, export MAGICK_TMPDIR=/data/magick If you plan on processing the same image many times, consider the MPC format. Reading a MPC image has near-zero overhead because its in the native pixel cache format eliminating the need for decoding the image pixels. Here is an example: convert bigassimage.tif bigassimage.mpc convert bigassimage.mpc -crop 100x100+0+0 +repage 1.png convert bigassimage.mpc -crop 100x100+100+0 +repage 2.png convert bigassimage.mpc -crop 100x100+200+0 +repage 3.png ... MPC is ideal for web sites. It reduces the overhead of reading and writing an image. We use it exclusively at our online image studio. ImageMagick provides for streaming pixels as they are read from or written to an image. This has several advantages over the pixel cache. The time and resources consumed by the pixel cache scale with the area of an image, whereas the pixel stream resources scale with the width of an image. The disadvantage is the pixels must be consumed as they are streamed so there is no persistence. Use ReadStream() or WriteStream() with an appropriate callback method in your MagickCore program to consume the pixels as they are streaming. Here's an abbreviated example of using ReadStream:
static size_t StreamHandler(const Image *image,const void *pixels,
const size_t columns)
{
/* process pixels here */
return(columns);
}
...
/* invoke the pixel stream here */
image=ReadStream(image_info,&StreamHandler,exception);
We also provide a lightweight tool, stream, to stream one or more pixel components of the image or portion of the image to your choice of storage formats. It writes the pixel components as they are read from the input image a row at a time making stream desirable when working with large images or when you require raw pixel components. Images have metadata associated with them in the form of properties (e.g. width, height, description, etc.) and profiles (e.g. EXIF, IPTC, color management). ImageMagick provides convenient methods to get, set, or update image properties and get, set, update, or apply profiles. Some of the more popular image properties are associated with the Image structure in the MagickCore API. For example:
(void) printf("image width: %lu, height: %lu\n",image->columns,image->rows);
For a great majority of image properties, such as an image comment or description, we use the GetImageProperty() and SetImageProperty() methods. Here we set an property and fetch it right back:
const char
*comment;
(void) SetImageProperty(image,"comment","This space for rent");
comment=GetImageProperty(image,"comment");
if (comment == (const char *) NULL)
(void) printf("Image comment: %s\n",comment);
Image profiles are handled with GetImageProfile(), SetImageProfile(), and ProfileImage() methods. Here we set a profile and fetch it right back:
StringInfo
*profile;
profile=AcquireStringInfo(length);
SetStringInfoDatum(profile,my_exif_profile);
(void) SetImageProfile(image,"EXIF,profile);
DestroyStringInfo(profile);
profile=GetImageProfile(image,"EXIF");
if (profile != (StringInfo *) NULL)
(void) PrintStringInfo(stdout,"EXIF",profile);
An image coder is responsible for registering, optionally classifying, optionally reading, optionally writing, and unregistering one image format (e.g. PNG, GIF, JPEG, etc.). Registering an image coder alerts ImageMagick a particular format is available to read or write. While unregistering tells ImageMagick the format is no longer available. The classifying method looks at the first few bytes of an image and decides if the image is in the expected format. The reader sets the image size, colorspace, and other properties and loads the pixel cache with the pixels. The reader returns a null image and an exception if an error occurs or returns a single image or an image sequence if the format supports multiple images per file. The writer does the reverse. It takes the image properties and unloads the pixel cache and writes them as required by the image format. The modular design of the image coder makes it relatively easy to support new formats. Here is a complete image coder with the exception of the classification method: ImageMagick provides a convenient mechanism for adding your own custom image processing algorithms. We call these image filters and they are invoked from the command line with the -process option or from the MagickCore API method ExecuteModuleProcess(). ImageMagick includes a sample custom filter listed here. It computes a few statistics such as the brightness and saturation mean and standard-deviation. To invoke the custom filter from the command line, use this command:
-> convert logo: -process Analyze -verbose info:
Image: logo:
Format: LOGO (ImageMagick Logo)
Class: PseudoClass
Geometry: 640x480
...
Filter:brightness:mean: 61191
Filter:brightness:standard-deviation: 11906
Filter:saturation:mean: 6030
Filter:saturation:standard-deviation: 16647
ImageMagick is thread safe with the exception of the MagickCore AcquireImagePixels(), GetImagePixels(), SetImagePixels(), and SyncImagePixels() pixel cache methods. These methods are intended for one thread of execution only. To access the pixel cache with more than one thread of execution, use a cache view. We do this for the CompositeImage() method, for example. Suppose we want to composite a single image over a different image in each thread of execution. If we use AcquireImagePixels(), the results could be corrupt since multiple threads might be asking for different areas of the pixel cache simultaneously. Instead we use AcquireCacheView() which creates a unique view for each thread of execution ensuring our program behaves properly regardless of how many threads are invoked. The other program interfaces, such as the MagickWand API, are completely thread safe so there are no special precautions for threads of execution. |