How to use RMagick

## Table of contents

- Basic concepts
- Reading, writing, and creating images
- Displaying images
- Examining and modifying images
- Drawing on and adding text to images
- Where to go from here
- Performance issues

## Basic concepts

RMagick defines a single module, the Magick module. Defined within the Magick module are 3 major classes, ImageList, Image, and Draw. The Draw class is explained in Drawing on and adding text to images, below. This section describes the ImageList and Image classes.

An Image object (an image) describes a single image or one frame in a multi-frame image. You can create a image object from an image file such as a GIF, PNG, or JPEG, or you can create a blank image from scratch. You can write an image to disk, display it on a screen, enlarge or shrink it, convert it to another format, or otherwise modify it using one of the scores of Image methods. In almost all cases, an Image method does not modify the image to which it is sent. Instead, the method returns a new image, suitably modified. For example, the resize method returns a new image in a different size. The receiver image is unaltered. (Following the Ruby convention, when an Image method alters the receiver object, the method name ends with "!". For example, the erase! method "erases" the image to which it is sent.)

An ImageList object (an imagelist) contains one or more images and a scene number. The scene number is an index into the ImageList object that indicates the current image. If the imagelist contains only one image, the scene number is always 0. If the imagelist contains no images, the scene number is `nil`. With a very few exceptions, any method defined in the Image class can be sent to an ImageList object. Since Image methods always operate on a single image, when an Image method is sent to an imagelist, the ImageList class sends the method to the current image. Like images, imagelists can be read, written, and displayed. Unlike single images, imagelists can be used to produce animations. The ImageList class defines a handful of methods that operate on all the images in an imagelist, however in most cases you must modify each image individually. You can get and set the scene number with the scene and scene= methods, respectively. The cur_image method returns the current image. You can change the images in the imagelist with the usual Array methods, such as `<<` to add images to the list, etc.

Let's look at the RMagick equivalent of "Hello, world".

    1. #! /usr/local/bin/ruby -w
    2.
    3. require 'RMagick'
    4.
    5. cat = Magick::ImageList.new("Cheetah.jpg")
    6. cat.display
    7. exit

Line 1 is the usual "shebang" statement that identifies this file as a Ruby program file.

Line 3 requires the RMagick.rb file, which defines the Magick module. This statement must appear in all RMagick programs before any references to RMagick classes.

Line 5 is where the magic starts. This statement creates an imagelist object and initializes it by reading the `Cheetah.jpg` file1.

The statement on line 6 sends the display method to `cat`. When sent to an imagelist, the display method displays all the images in the imagelist to the default X Window screen2 . In this case, the display method makes a picture of a cheetah appear on your monitor.

Type this program in and try running it now. The `Cheetah.jpg` file is in the `doc/ex/images` subdirectory where you installed RMagick.

Let's make one modification.

    1. #! /usr/local/bin/ruby -w
    2.
    3. require 'RMagick'
    4.
    5. cat = Magick::ImageList.new("Cheetah.jpg")
    6. smallcat = cat.minify
    7. smallcat.display
    8. exit

The difference is the statement on line 6. This statement sends the minify method to `cat`. The minify method is an Image method that reduces the size of an image to half its original size. Since minify is an Image method, the ImageList class sends minify to the current (and only) image. The return value is a new image, half the size of the original. The image in the `cat` imagelist is unchanged.

The Image class has its own display method, which displays the single image on the X Window screen. Like Image#display, Image#display makes a picture of a (in this case, small) cheetah appear on your monitor.

One more example.

    1. #! /usr/local/bin/ruby -w
    2.
    3. require 'RMagick'
    4.
    5. cat = Magick::ImageList.new("Cheetah.jpg")
    6. smallcat = cat.minify
    7. smallcat.display
    8. smallcat.write("Small-Cheetah.gif")
    9. exit

The statement on line 8 sends the write method to `smallcat`. The write method, as you might reasonably expect, writes the image to a file. Notice that the filename extension is `gif`. When writing images, ImageMagick uses the filename extension3 to determine what image format to write. In this example, the `Small-Cheetah.gif` file will be in the GIF format. Notice how easy it is to covert an image from one format to another?

So why, in the previous example, did I create `cat` as an ImageList object containing just one image, instead of creating an Image object? No reason, really. When you only have one image to deal with, imagelists and images are pretty much interchangeable.

## Reading, writing, and creating images

You've already seen that you can create an imagelist and initialize it by specifying the name of an image file as the argument to ImageList.new. In fact, new can take any number of file name arguments. If the file contains a single image, new reads the file, creates an image, and adds it to the imagelist. If the file is a multi-image file (say, an animated GIF), new adds an image for each image in the file. Lastly, new changes the scene number to point to the last image in the imagelist. In the simplest case, new reads a single image from a file and sets the scene number to 0.

Underneath the covers, new4 calls the Image class's read method to read the image file. The read method is a class method. Given one or more file names, read reads the files and constructs a Image object for each image in the files. The return value is an array, even if there is only one image to be returned. Upon return, new simply appends the new images to the imagelist.

You can also create an Image from scratch by calling Image.new. This method takes 2 or 3 arguments. The first argument is the number of columns in the new image (its width). The second argument is the number of rows (its height). If present, the 3rd argument is a Fill object.5

To add a "scratch" image to the imagelist, call ImageList#new_image. This method calls Image.new, adds the new image to the imagelist, and sets the scene number to point to the new image.

Like many other methods in the Image and ImageList classes, Image.new accepts an optional block that can be used to set additional optional parameters. If the block is present, Image.new creates a special parameter object6 and yields to the block in the scope of that object. You set the parameters by calling attribute setter methods defined in the parameter object's class. For example, you can set the background color of a new image to red with the background_color= method, as shown here:

    #! /usr/local/bin/ruby -w
    require 'RMagick'
    # Create a 100x100 red image.
    f = Magick::Image.new(100,100) { self.background_color = "red" }
    f.display
    exit

(Likewise, ImageList#new_image also accepts an image block, passing it on to Image.new.)

In addition to new and read, there are 3 other ways to create a new image, constitute, from_blob, and ping. These methods are less generally useful.

Both the Image class and the ImageList class have write methods. Both accept a single argument, the name of the file to be written. Image#write simply writes the image to a file. Like the Image#read method, write yields to an optional block that you can use to set parameters that control how the image is written.

If an ImageList object contains only one image, then ImageList#write is the same as Image#write. However, if the imagelist contains multiple images and the file format (determined3 by the file name extension, as I mentioned earlier) supports multi-frame images, Image#write will automatically create a multi-frame image file.7

For example, the following program reads three GIF files and then uses ImageList#write to combine all the images in those files (remember, each input file can contain multiple images) into one animated GIF file.

    #! /usr/local/bin/ruby -w
    require 'RMagick'
    anim = ImageList.new("start.gif", "middle.gif", "finish.gif")
    anim.write("animated.gif")
    exit

## Displaying images

RMagick defines 3 methods for displaying images and imagelists. Both the Image class and the ImageList class have a display method. The Image#display method displays the image on the default X Window screen. For imagelists with just one image, ImageList#display is identical to Image#display. However, if the imagelist contains multiple images, ImageList#display displays each of the images in turn. With both methods, right-clicking the display window will produce a menu of other options.

The Image#animate method repeatedly cycles through all the images in an imagelist, displaying each one in turn. You can control the speed of the animation with the delay= method.

## Examining and modifying images

Once you've created an image or imagelist, what can you do with it? The Image and ImageList classes define over 100 methods for examining and modifying images, both individually and in groups. Remember, unless the ImageList class defines a method with the same name, you can send any method defined in the Image class to an instance of the ImageList class. The ImageList class8 sends the method to the current image and returns the result.

The methods can be classified into the following broad groups.9 Image class methods are shown in blue.

Utility methods
<=> | Compare images
<=> | Compare imagelists
== | Compare images
== | Compare imagelists
[] | Reference an image property
[]= | Set an image property
changed? | Has the image been changed?
copy | Deep copy the image
copy | Deep copy the imagelist
difference | Compute the difference between two images
get_pixels | Copy a region of pixels from the image
gray? | Are all pixels gray?
monochrome? | Are all pixels black or white?
opaque? | Are all pixels opaque?
palette? | Is the image PseudoClass type and does it have 256 colors or less?
properties | Return all image properties
signature | Compute the image's 64-byte message digest
Methods to reduce the number of colors in an image or group of images
compress_colormap! | Remove duplicate or unused entries in the colormap
map | Replace the colors of an image with the closest color from a reference image
map | Replace the colors of all the images in an imagelist with the closest color from a reference image
ordered_dither | Reduce color images to monochrome using the ordered dithering technique
quantize | Analyze the colors within an image and choose a fixed number of colors to represent the image
quantize | Analyze the colors within all the images in an imagelist and choose a fixed number of colors to represent the images
Methods to resize an image
magnify | Increase the size of an image by 2
minify | Decrease the size of an image by 2
resize | Change the size of an image using the specified filter
sample | Change the size of an image with pixel sampling
scale | Change the size of an image
Methods to transform an image or group of images
append | Append all the images in the imagelist vertically or horizontally
average | Average all the images in the imagelist into a single image
chop | Remove a region of an image and collapses the image to occupy the removed portion
coalesce | Composite the images in the imagelist
composite | Composite an image onto another image
composite_affine | Composite an image onto another image as dictated by the affine transform
crop | Extract a region of the image
deconstruct | Compare each image with the next and returns the maximum bounding region of any pixel differences it discovers
flatten_images | Merge the images in the imagelist
flip | Create a vertical mirror image of the image
flop | Create a horizontal mirror image of the image
mosaic | Inlay the images in the imagelist into a single image
profile! | Add or remove a ICM, IPTC, or generic profile from a image
roll | Offset an image
shave | Shave regions from the edges of the image
Methods to shear or rotate an image by an arbitrary angle
affine_transform | Transform an image as dictated by an affine matrix
rotate | Rotate an image by the specified angle
shear | Shear an image along the X or Y axis, creating a parallelogram
Methods to enhance an image
contrast | Enhance the intensity differences between the lighter and darker elements of the image
equalize | Apply a histogram equalization to the image
gamma_correct | Gamma-correct an image
level | Adjust the levels of an image
modulate | Change the brightness, saturation, and hue of a image
negate | Negate the colors in the image
normalize | Enhance the contrast of a color image by adjusting the pixels' color to span the entire range of colors available
Methods to add effects to an image
add_noise | Add random noise
blur_image | Blur the image
charcoal | Add a charcoal effect
colorize | Blend the fill color with each pixel in the image
convolve | Apply a custom convolution kernel to the image
despeckle | Reduce the speckle noise in the image
edge | Find edges in the image
emboss | Return a grayscale image with a three-dimensional effect
enhance | Apply a digital filter that improves the quality of a noisy image
gaussian_blur | Blur the image
implode | Implode the pixels in the image
median_filter | Applie a digital filter that improves the quality of a noisy image
morph | Transform each image in the imagelist to the next in sequence by creating intermediate images
motion_blur | Simulate a motion blur
oil_paint | Add an oil paint effect
reduce_noise | Smooth the contours of an image while still preserving edge information
shade | Shine a distant light on an image to create a three-dimensional effect
sharpen | Sharpen the image
solarize | Apply a special effect to the image, similar to the effect achieved in a photo darkroom by selectively exposing areas of photo sensitive paper to light
spread | Randomly displace each pixel in a block
stegano | Hide a digital watermark within the image
stereo | Combine two images and produces a single image that is the composite of a left and right image of a stereo pair
swirl | Swirl the pixels about the center of the image
threshold | Change the value of individual pixels based on the intensity of each pixel compared to threshold. The result is a high-contrast, two color image.
unsharp_mask | Sharpen the image
wave | Add a "ripple" effect
Methods to decorate an image
border | Surround the image with a border
frame | Add a simulated three-dimensional border around the image
raise | Create a simulated three-dimensional button-like effect by lightening and darkening the edges of the image
Methods to change the colors or opacity of an image
color_fill_to_border | Change the color value of any neighbor pixel that is not the border color.
color_floodfill | Change the color value of any neighbor pixel that matches the color of the target pixel
colormap | Get or set the color in the colormap at the specified index
color_point | Set the color of a single pixel.
color_reset! | Set all the pixels in the image to the specified color.
cycle_colormap | Displace an image's colormap by a given number of positions
erase! | Set all the pixels in the image to the background color
matte_fill_to_border | Make all neighbor pixels that are not the border color transparent
matte_floodfill | Make all the neighbor pixels that are the same color as the target pixel transparent
matte_point | Make the target pixel transparent
matte_replace | Make all the neighbor pixels that are the same color as the target pixel transparent
matte_reset! | Make all pixels transparent
opaque | Change all pixels having the target color to the replacement color
pixel_color | Get or set the color of a single pixel
store_pixels | Replace a region of the image with an arbitrary set of pixels
texture_fill_to_border | Replace the neighbor pixels that are not the border color with a texture
texture_floodfill | Replace the neighbor pixels having the specified color with a texture
transparent | Change the opacity value of the pixels having the specified color
Methods that don't really fit into any of the other categories
channel | Extract a color channel
montage | Tile image thumbnails across a canvas
segment | Segment an image by analyzing the histograms of the color components and identifying units that are homogeneous with the fuzzy c-means technique
Methods to convert an image to/from a Binary Large OBject
to_blob | Construct a BLOB from an image
to_blob | Construct a BLOB from all the images in an imagelist
from_blob | Create an image from a BLOB
from_blob | Create an imagelist from one or more BLOBs

## Drawing on and adding text to images

The Draw class is the third major class in the Magick module. This class defines two kinds of methods. The first kind are methods that support 2D vector and raster drawing on images. The second kind are methods that support writing text on an image, known as annotation.

### Drawing

ImageMagick supports a set of 2D drawing commands that are very similar to the commands and elements defined by the W3C's Scalable Vector Graphics (SVG) 1.0 Specification. In RMagick, each command (called a primitive) is implemented as a method in the Draw class. To draw on an image, simply

1. Create an instance of the Draw class.
2. Call one or more primitive methods with the appropriate arguments.
3. Invoke the draw method, specifying an image to draw on.

The primitive methods do not draw anything directly. When you call a primitive method, you are simply adding the primitive and its arguments to a list of primitives stored in the draw object. To "execute" the primitive list, call draw. Drawing the primitives does not destroy them. You can draw on another image by calling draw again, specifying a different image "canvas." Of course you can also draw on an image with multiple draw objects, too. The canvas can be any image or imagelist, created by reading an image file or from scratch using ImageList#new_image or Image.new. (If you pass an imagelist object to draw, it draws on the current image.)

Here's an illustration of the default drawing coordinate system. The origin is in the top left corner. The x axis extends to the right. The y axis extends downward. The units are pixels. 0° is at 3 o'clock and rotation is clockwise. The units of rotation are usually degrees.10

You can change the default coordinate system by specifying a scaling, rotation, or translation transformation.

(Click the image to see the Ruby program that created it.)

RMagick's primitive methods include methods for drawing points, lines, Bezier curves, shapes such as ellipses and rectangles, and text. Shapes and lines have a fill color and a stroke color. Shapes are filled with the fill color unless the fill opacity is 0. Similarly, shapes are stroked with the stroke color unless the stroke opacity is 0. Text is considered a shape and is stroked and filled. Other rendering properties you can set include the stroke width, antialiasing, stroke patterns, and fill patterns. (Yes, you can even fill text with a pattern!)

As an example, here's the section of the Ruby program that created the circle in the center of the above image.

     1. !# /usr/local/bin/ruby -w
     2. require 'RMagick'
     3.
     4. canvas = Magick::ImageList.new
     5. canvas.new_image(250, 250, Magick::HatchFill.new('white', 'gray90'))
     6.
     7. circle = Magick::Draw.new
     8. circle.stroke('tomato')
     9. circle.fill_opacity(0)
    10. circle.stroke_opacity(.75)
    11. circle.stroke_width(6)
    12. circle.stroke_linecap('round')
    13. circle.stroke_linejoin('round')
    14. circle.ellipse(canvas.rows/2,canvas.columns/2, 80, 80, 0, 315)
    15. circle.polyline(180,70, 173,78, 190,78, 191,62)
    16. circle.draw(canvas)

The statements on lines 4 and 5 create the "canvas" image with a single 250x250 image. The HatchFill object fills the image with light-gray lines 10 pixels apart. The statement on line 7 creates a Draw object. The method calls on lines 8-15 construct a list of primitives that are "executed" by the draw method call on line 16.

The stroke method sets the stroke color, as seen on line 8. Normally, shapes are filled, but the call to fill_opacity on line 9 sets the opacity to 0, so the background will show through the circle. The tomato-colored stroke line itself is set slightly transparent by the call to stroke_opacity on line 10. The method calls on lines 11 through 13 set the stroke width and specify the appearance of the line ends and corners.

The ellipse method call on line 14 describes an circle in the center of the canvas with a radius of 80 pixels. The ellipse occupies 315° of a circle, starting at 0° (that is, 3 o'clock). The polyline call on line 15 adds the arrowhead to the circle. The arguments (always an even number) are the x- and y-coordinates of the points the line passes through.

Finally, the draw method on line 16 identifies the canvas to be drawn on and executes the stored primitives.

### Annotation

The Draw#annotate method draws text on an image. In its simplest form, annotate requires only arguments that describe where to draw the text and the text string. Of course, nothing is ever quite that simple!

Most of the time, you'll want to specify text properties such as the font, its size, font styles such as italic, font weights such as bold, the fill and stroke color, etc. The Draw class defines attribute writers for this purpose. You can set the desired text properties by calling the attribute writers before calling annotate, or you can call them in an image block associated with the annotate call.

The following example shows how to use annotate to produce this image.

     1.   #! /usr/local/bin/ruby -w
     2.   require 'RMagick'
     3.
     4.   # Demonstrate the annotate method
     5.
     6.   Text = 'RMagick'
     7.
     8.   granite = Magick::ImageList.new('granite:')
     9.   canvas = Magick::ImageList.new
    10.   canvas.new_image(300, 100, Magick::TextureFill.new(granite))
    11.
    12.   text = Magick::Draw.new
    13.   text.font_family = 'helvetica'
    14.   text.pointsize = 52
    15.   text.gravity = Magick::CenterGravity
    16.
    17.   text.annotate(canvas, 0,0,2,2, Text) {
    18.      self.fill = 'gray83'
    19.   }
    20.
    21.   text.annotate(canvas, 0,0,-1.5,-1.5, Text) {
    22.      self.fill = 'gray40'
    23.   }
    24.
    25.   text.annotate(canvas, 0,0,0,0, Text) {
    26.      self.fill = 'darkred'
    27.   }
    28.
    29.   canvas.write('rubyname.gif')
    30.   exit

This program uses three calls to annotate to produce the "etched" appearance. All three calls have some parameters in common but the fill color and location are different.

First, the statements in lines 8-10 create the background. See class Fill for information about the TextureFill class. The "granite:" image format is one of ImageMagick's built-in image formats. See "Built-in image formats" for more information. The statement on line 12 creates the Draw object that does the annotation. The next 3 lines set the values of the attributes that are common to all 3 annotate calls.

The first annotate argument is the image on which the text will be drawn. Arguments 2-5, `width`, `height`, `x`, and `y`, describe a rectangle about which the text is drawn. This rectangle, combined with the value of gravity, define the position of the text. When the gravity value is `CenterGravity` the values of `width` and `height` are unused.

The first call to annotate, on lines 17-19, draws the text 2 pixels to the right and down from the center. The `self.fill = 'gray83'` statement sets the text color to light gray. The second call to annotate, on lines 21-22, draws dark gray text 1.5 pixels to the left and up from the center. The last call, on lines 25-27, draws the text a third time, in dark red, exactly in the center of the image.

## Where to go from here

The next section, "ImageMagick Conventions," describes some common ImageMagick conventions that you need to know, such as how ImageMagick determines the graphic format of an image file, etc. The ImageMagick web site (from which much of the information in these pages has been taken) offers a lot of detail about ImageMagick. While the ImageMagick web site doesn't describe RMagick, you can often use the documentation to learn more about a RMagick method by reading about the ImageMagick functions the method calls. (In the Reference section of this document, most of the method descriptions include the name of the ImageMagick function that the method calls.) Check out the example programs. Almost every one of the RMagick methods is demonstrated in one of the examples.

Good luck!

## Performance issues

There's an old saying that there are only 4 computer resources: CPU, memory, I/O, and time. Image processing requires a lot of each. Here's a few recommendations for getting good performance from RMagick.

1. Use a fast computer with lots of memory. If you don't have lots of CPU and memory, you'll need lots of time.
2. If you're cramped for RAM or want to process very large images, build ImageMagick using an 8-bit pixel quantum. (The default is 16 bits.) See the README.txt file in the ImageMagick distribution.
3. Scale down first. If the image you're producing is smaller than the image you're starting with, scale down the image before doing any other processing. That way you won't be manipulating pixels you'll throw away later.
4. While we're talking about scaling, RMagick provides 4 ways to make an image smaller. (resize, scale, minify, and sample.) Some take longer but produce a better-quality image. Similarly, some of the FilterType arguments to resize can take a very long time to run. Experiment to find a method or filter that best suits your needs.
5. Use quantize to reduce the number of colors if possible.
6. Some methods take more time than others. Because of the overhead of converting between ImageMagick's PixelPacket data type and RMagick's `Pixel` class objects, the get_pixel and set_pixel methods can be slow if you're processing a lot of pixels. The same goes for dispatch and constitute.
7. Use RMagick methods whenever you can. Since almost all these methods are implemented in C, they'll be very fast compared to doing the same work with Ruby image. RMagick provides over 100 methods for image manipulation, so it's quite likely that there's a method that will do what you need.

#### Footnotes

1in the current directory.

2usually your display.

3among other things. See Image formats and filenames.

4well, the Image#initialize method, to be precise

5about which no more will be said in this section. See Image.new and class Fill.

6An Image::Info class object

7So automatic, in fact, that you have to go to some trouble to defeat it. See Image#write for details.

8Using method_missing

9Most of the content in this table is taken from the ImageMagick documentation.

10The rotation attributes `rx` and `ry` in the `AffineMatrix` class use radians instead of degrees.