###### RMagick 2.9.0 User's Guide and Reference

# How to use RMagick

## Basic concepts

Let's look at the RMagick equivalent of "Hello, world". This program reads an image file named "Cheetah.jpg" and displays1 it on your monitor

    1. require 'RMagick'
    2. include Magick
    3.
    4. cat = ImageList.new("Cheetah.jpg")
    5. cat.display
    6. exit

MS Windows users note: The display method does not work on native MS Windows. You must use an external viewer to view the images you create.

Line 1 requires 2 the RMagick.rb file, which defines the Magick module. The Magick module contains 3 major classes, ImageList, Image, and Draw. This section - Basic Concepts - describes the ImageList and Image classes. The Draw class is explained in the Drawing on and adding text to images section, below.

The statement on line 5 creates an imagelist object and initializes it by reading the `Cheetah.jpg` file in the current directory. Line 6 sends the `display` method to the object. When you send `display` to an imagelist, it causes all the images in the imagelist to be displayed on the default X Window screen. 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 `ex/images` subdirectory where you installed the RMagick documentation.

The Image and ImageList classes are closely related. An Image object describes one image or one frame in an image with multiple frames. (An animated GIF or a Photoshop image with multiple layers are examples of images with multiple frames.) You can create a image object from an image file such as a GIF, PNG, or JPEG. You can create a image from scratch by specifying its dimensions. You can write an image to disk, display it on a screen, change its size or orientation, convert it to another format, or otherwise modify it using one of over 100 methods.

An ImageList object is a list of images. It contains zero or more images and a scene number. The scene number indicates the current image. The ImageList class includes methods that operate on all the images in the list. Also, with a very few exceptions, any method defined in the Image class can be used as well. 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, that is, the image specified by the scene number.

The ImageList class is a subclass of the Array class, so you can use most Array methods to change the images in the imagelist. For example, you can use the `<<` method to add an image to the list.

Going back to the example, let's make one modification.

    1. require 'RMagick'
    2. include Magick
    3.
    4. cat = ImageList.new("Cheetah.jpg")
    5. smallcat = cat.minify
    6. smallcat.display
    7. exit

The difference is the statement on line 5. This statement sends the `minify` method to the imagelist. The `minify` method is an Image method that reduces the size of an image to half its original size. Remember, 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.

Line 6 demonstrates the Image class's `display` method, which displays a single image on the X Window screen. `Image#display` makes a picture of a (in this case, small) cheetah appear on your monitor.

Here's how to write the small cheetah to a file in GIF format.

    1. require 'RMagick'
    2. include Magick
    3.
    4. cat = ImageList.new("Cheetah.jpg")
    5. smallcat = cat.minify
    6. smallcat.display
    7. smallcat.write("Small-Cheetah.gif")
    8. exit

The statement on line 7 writes the image to a file. Notice that the filename extension is `gif`. When writing images, ImageMagick uses the filename extension 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? (For more details, see Image formats and filenames.)

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.

Note: In most 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, sized as specified. The receiver image is unaltered. (Following the Ruby convention, when a method alters the receiver object, the method name ends with "!". For example, the `resize!` method resizes the receiver in place.)

## 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-frame image file, `new` adds an image for each frame or layer in the file. Lastly, `new` changes the scene number to point to the last image in the imagelist. In the simple case, `new` reads a single image from a file and sets the scene number to 0.

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. To add a "scratch" image to an 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. Scratch images are good for drawing on or creating images by compositing.

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 parameter object 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:

    require 'RMagick'
    include Magick
    # Create a 100x100 red image.
    f = Image.new(100,100) { self.background_color = "red" }
    f.display
    exit

Within the parameter block you must use `self` so that Ruby knows that this statement is a method call, not an assignment to a variable.

You can create an image by capturing it from the XWindow screen using `Image.capture`. This method can capture the root window, a window identified by name or ID number, or perform an interactive capture whereby you designate the desired window by clicking it or by drawing a rectangle on the screen with your mouse.

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 (determined by the file name extension, as I mentioned earlier) supports multi-frame images, `Image#write` will automatically create a multi-frame image file.

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 `ImageList#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 `ImageList#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 class sends the method to the current image and returns the result.

The methods can be classified into the following broad groups. ImageList method descriptions look like this. Some of the listed methods are not available in some releases of ImageMagick. See the method documentation for details.

#### Utility methods

<=>
  Compare 2 images
<=>
  Compare 2 imagelists
[ ]
  Reference an image property
[ ]=
  Set an image property
add_profile
  Add an ICC, IPTC, or generic profile
changed?
  Has the image been changed?
channel
  Extract a color channel from the image
compare_channel
  Compare one or more channels between two images
channel_depth
  Return the depth of the specified channel or channels
channel_extrema
  Return the maximum and minimum intensity values for one or more channels in the image
check_destroyed
  Raise an exception if the image has been destroyed
clone
  Return a shallow copy of the image
clone
  Return a shallow copy of the imagelist
color_histogram
  Count the number of times each unique color appears in the image
combine
  Combines the grayscale value of the pixels in each image to form a new image.
copy
  Return a deep copy of the image
copy
  Return a deep copy of the imagelist
delete_profile
  Delete an ICC, IPTC, or generic profile
destroy!
  Return all memory associated with the image to the system
destroyed?
  Has the image has been destroyed?
difference
  Compute the difference between two images
distortion_channel
  Compare one or more channels to a reconstructed image
dup
  Return a shallow copy of the image
dup
  Return a shallow copy of the imagelist
each_pixel
  Iterate over all the pixels in the image
each_profile
  Iterate over all the profiles associated with the image
export_pixels
  Extract pixel data from the image into an array
export_pixels_to_str
  Extract pixel data from the image into a string
find_similar_region
  Search for a rectangle matching the target
get_exif_by_entry, get_exif_by_number
  Get one or more EXIF property values for the image
get_pixels
  Copy a region of pixels from the image
gray?
  Are all the pixels in the image gray?
histogram?
  Does the image have <= 1024 unique colors??
import_pixels
  Replace pixels in the image with pixel data from an array
monochrome?
  Are all the pixels in the image either black or white?
opaque?
  Are all the pixels in the image opaque?
palette?
  Is the image a PseudoClass type image with 256 colors or less?
preview
  Show the effect of a transformation method on the image
profile!
  Add or remove an ICM, IPTC, or generic profile from the image
properties
  Iterate over all the properties associated with the image
separate
  Construct a grayscale image for each channel specified
set_channel_depth
  Set the specified channel's depth
signature
  Compute the 64-byte message signature for the image
store_pixels
  Replace a region of pixels in the image
strip!
  Strip the image of all comments and profiles
sync_profiles
  Synchronize the image properties with the image profiles
unique_colors
  Construct an image from the unique colors
view
  Access pixels by their coordinates.

#### Reduce the number of colors

compress_colormap!
  Remove duplicate or unused entries in the image's colormap
ordered_dither
  Dither the image to a predefined pattern
posterize
  Reduce the number of colors for a "poster" effect
quantize
  Choose a fixed number of colors to represent the image
quantize
  Choose a fixed number of colors to represent all the images in the imagelist
remap
  Change the colors in the image to the colors in a reference image.
remap
  Change the images in the imagelist to use a common color map.

#### Resize

change_geometry
  Compute a new constrained size for the image
liquid_rescale
  Rescale image with seam-carving
magnify
  Double the size of the image
minify
  Halve the size of the image
resample
  Resample the image to the specified horizontal and vertical resolution
resize
  Resize the image using a filter
resize_to_fill
  Resize and crop while retaining the aspect ratio
resize_to_fit
  Resize the image retaining the aspect ratio
sample
  Resize the image using pixel sampling
scale
  Resize the image
thumbnail
  Quickly create a thumbnail of the image

#### Change colors or opacity

color_fill_to_border
  Change the color of neighboring pixels. Stop at the image's border color.
color_floodfill
  Change the color of neighboring pixels that are the same color
colormap
  Get or set a color in the image's colormap
color_point
  Change the color of a single pixel in the image
color_reset!
  Set the entire image to a single color
cycle_colormap
  Displace the image's colormap
erase!
  Set the entire image to a single color
matte_fill_to_border
  Make transparent neighboring pixels. Stop at the image's border color.
matte_floodfill
  Make transparent neighboring pixels that are the same color
matte_point
  Make a single pixel transparent
matte_reset!
  Make the entire image transparent
opaque, opaque_channel
  Change all pixels from the specified color to a new color
pixel_color
  Get or set the color of a single pixel
splice
  Splice a solid color into the image.
texture_fill_to_border
  Replace neighbor pixels with pixels from a texture image. Stop at the image's border color.
texture_floodfill
  Replace neighboring pixels that are the same color with pixels from a texture image
paint_transparent, transparent
  Change the opacity value of pixels having the specified color
transparent_chroma
  Change the opacity value of pixels within the specified range

#### Rotate, flip, or shear

affine_transform
  Transform the image as dictated by an affine matrix
auto_orient
  Rotate or flip the image using the EXIF orientation tag
deskew
  Straighten the image
flip
  Create a vertical mirror image
flop
  Create a horizontal mirror image
rotate
  Rotate the image by the specified angle
shear
  Shear the image along the X or Y axis, creating a parallelogram
transpose
  Create a horizontal mirror image
transverse
  Create a vertical mirror image

#### Composite

add_compose_mask
  Add a mask to the background image in a composite operation
average
  Average all the images in the imagelist into a single image
blend
  Blend two images together
composite
  Composite the image onto another image
composite_affine
  Composite the image onto another image as dictated by an affine matrix
composite_layers
  Composite two imagelists
composite_tiled
  Composite multiple copies of an image over another image
delete_compose_mask
  Delete a compose mask
displace
  Distort the image using a displacement map
dissolve
  Dissolve two images into each other
extent
  Extend or crop the image over the background color.
watermark
  Composite a watermark onto the image

#### Transform

append
  Append all the images in the imagelist into a single image
chop
  Chop a region from the image
coalesce
  Merge successive images in the imagelist into a new imagelist
crop
  Extract a region from the image
decipher
  Convert enciphered pixels to plain pixels
deconstruct
  Construct a new imagelist containing images that include only the changed pixels between each image and its successor
distort
  Distort the image
encipher
  Encipher plain pixels
excerpt
  Excerpt a rectangle from the image
flatten_images
  Merge all the images in the imagelist into a single image
mosaic
  Inlay all the images in the imagelist into a single image
optimize_layers
  Optimize or compare image layers
resize_to_fill
  Resize and crop while retaining the aspect ratio
roll
  Offset the image
shave
  Shave regions from the edges of the image
trim
  Remove borders from the edges of the image

#### Enhance

clut_channel
  Replace the channel values in the image with a lookup of its replacement value in an LUT gradient image.
contrast
  Enhance the intensity differences between the lighter and darker elements in the image
contrast_stretch_channel
  Improve the contrast in the image by stretching the range of intensity values
despeckle
  Reduce the speckle noise in the image
enhance
  Apply a digital filter that improves the quality of a noisy image
equalize, equalize_channel
  Apply a histogram equalization to the image
gamma_correct, gamma_channel
  Gamma correct the image
level, level_channel, level_colors, levelize_channel
  Adjust the levels of one or more channels in the image
linear_stretch
  Stretch with saturation the image contrast
median_filter
  Apply a digital filter that improves the quality of a noisy image
modulate
  Change the brightness, saturation, or hue of the image
negate, negate_channel
  Negate the colors of the image
normalize, normalize_channel
  Enhance the contrast of the image
reduce_noise
  Smooth the contours of an image while still preserving edge information
adaptive_sharpen, adaptive_sharpen_channel, sharpen, sharpen_channel, unsharp_mask, unsharp_mask_channel
  Sharpen the image
sigmoidal_contrast_channel
  Adjusts the contrast of an image channel with a non-linear sigmoidal contrast algorithm

#### Add effects

adaptive_threshold
  Threshold an image whose global intensity histogram doesn't contain distinctive peaks
add_noise, add_noise_channel
  Add random noise
bilevel_channel
  Change the value of individual image pixels based on the intensity of each pixel channel
black_threshold
  Force all pixels below the threshold into black
adaptive_blur, adaptive_blur_channel, blur_image, blur_channel, gaussian_blur, gaussian_blur_channel, motion_blur, radial_blur, radial_blur_channel
  Blur the image
colorize
  Blend the fill color with each pixel in the image
convolve, convolve_channel
  Apply a custom convolution kernel to the image
fx
  apply a mathematical expression to an image
segment
  Segment an image by analyzing the histograms of the color components and identifying units that are homogeneous with the fuzzy c-means technique
random_threshold_channel
  Change the value of individual pixels based on the intensity of each pixel compared to a random threshold.
recolor
  translate, scale, shear, or rotate the image colors
threshold
  Change the value of individual pixels based on the intensity of each pixel compared to threshold
white_threshold
  Force all pixels above the threshold into white

#### Add special effects

charcoal
  Add a charcoal effect
edge
  Find edges in the image
emboss
  Add a three-dimensional effect
implode
  Implode or explode the center pixels in the image
morph
  Transform each image in the imagelist to the next in sequence by creating intermediate images
oil_paint
  Add an oil paint effect
polaroid
  Simulate a Polaroid® instant picture
sepiatone
  Applies an effect similar to the effect achieved in a photo darkroom by sepia toning.
shade
  Shine a distant light on an image to create a three-dimensional effect
shadow
  Add a shadow to an image
sketch
  Simulate a pencil sketch
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 into 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
vignette
  Soften the edges of the image to create a vignette
wave
  Add a ripple effect to the image
wet_floor
  Create a "wet floor" reflection of the image

#### Decorate

border
  Surround the image with a solid-colored 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

#### Create thumbnail montages

montage
  Tile image thumbnails across a canvas

#### Create image blobs

from_blob
  Create an image from a BLOB
from_blob
  Create an imagelist from one or more BLOBs
to_blob
  Construct a BLOB from an image
to_blob
  Construct a BLOB from all the images in the imagelist

## Marshaling images

ImageList, Image, Draw, and Pixel objects are marshaled using custom serialization methods. Images are marshaled with ImageMagick's Binary Large OBject functions `ImageToBlob` (for dumping) and `BlobToImage` (for loading).

#### Notes

1. Some image formats cannot be dumped. The only way to be sure it will work is to try it.
2. Images in lossy formats, such as JPEG, will have a different signature after being reconstituted and therefore will not compare equal (using <=>) to the original image.
3. Image metadata may be changed by marshaling.

## 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, drawing methods and annotation methods.

### 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) 1.1 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. Call the `draw` method.

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 as the "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.3

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.

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(0.75)
    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 drawing canvas 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 (opacity = 1.0), but the call to `fill_opacity` on line 9 sets the opacity to 0, so the background will show through the circle. Similarly, the stroke lines are normally opaque, but the tomato-colored stroke line in this example is made 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 `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.

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

Good luck!

##### Footnotes

1The `display` and `animate` methods do not work on MS Windows. You will have to write the image to a file and view it with a separate image viewer.

2If you installed RMagick using Rubygems you must set up the RubyGems environment before using RMagick. You can do one of

1. add the `require 'rubygems'` statement to your program
2. use the -rubygems command line option
3. add `rubygems` to the RUBYOPT environment variable

See the RubyGems doc for more information.

3The rotation attributes `rx` and `ry` in the `AffineMatrix` class use radians instead of degrees.