Adding Text to an Image

Credit: Antonio Cangiano

Problem

You want to add some text to an imageperhaps a caption or a copyright statement.

Solution

Create an RMagick Draw object and call its annotate method, passing in your image and the text.

The following code adds the copyright string © NPS to the bottom-right corner of the canyon.png image. It also specifies the font, the text color and size, and other features of the text:

	require 
ubygems
	require RMagick

	img = Magick::Image.read(canyon.png).first
	my_text = "251 NPS"

	copyright = Magick::Draw.new
	copyright.annotate(img, 0, 0, 3, 18, my_text) do
	 self.font = Helvetica
	 self.pointsize = 12
	 self.font_weight = Magick::BoldWeight
	 self.fill = white
	 self.gravity = Magick::SouthEastGravity
	end
	img.write( 
canyoncopyrighted.png)

The resulting image looks like Figure 12-1.

Figure 12-1. With a copyright message in the bottom-right corner


Discussion

The annotate method takes a code block that sets properties on the Magick::Draw object, describing how the annotation should be done. You can also set the properties on the Draw object before calling annotate. This code works the same as the code given in the Solution:

	require 
ubygems
	require RMagick

	img = Magick::Image.read("canyon.png").first
	my_ 
text = \251 NPS

	copyright = Magick::Draw.new
	copyright.font = Helvetica
	copyright.pointsize = 12
	copyright.font_weight = Magick::BoldWeight
	copyright.fill = white
	copyright.gravity = Magick::SouthEastGravity
	copyright.annotate(img, 0, 0, 3, 18, my_text)
	img.write(canyoncopyrighted.png)

What do these attributes do?

  • The font attribute selects the font type from among those installed on your system. You can also specify the path to a specific font that is in a nonstandard location (e.g., "/home/antonio/Arial.ttf").
  • pointsize is the font size in points (the default is 12). By default, there is one pixel per point, so you can just specify the font size in pixels.
  • font_weight accepts a WeightType constant. This can be a number (100, 200, 300,…900), BoldWeight (equivalent to 700), or the default of NormalWeight (equivalent to 400).
  • If you need your text to be italicized, you can set the font_style attribute to Magick::ItalicStyle.
  • fill defines the text color. The default is "black". You can use X or SVG color names (such as "white", "red", "gray85", and "salmon"), or you can express the color in terms of RGB values (such as "#fff" or "#ffffff"two of the most common formats)
  • gravity controls which part of the image will contain the annotated text, subject to the arguments passed in to annotate. SouthEastGravity means that offsets will be calculated from the bottom-right corner of the image.

Draw#annotate itself takes six arguments:

  • The Image object, or else an ImageList containing the images you want to annotate.
  • The width and height of the rectangle in which the text is to be positioned.
  • The x and y offsets of the text, relative to that rectangle and to the gravity of the Draw object.
  • The text to be written.

In the Solution I wrote:

	 
copyright.annotate(img, 0, 0, 3, 15, my_text)

The width and height are zeros, which indicates that annotate should use the whole image as its annotation rectangle. Earlier I gave the Draw object a gravity attribute of SouthEastGravity. This means that annotate will position the text at the bottom-right corner of the rectangle: that is, at the bottom-right corner of the image itself. The offsets of 3 and 18 indicate that the text should start vertically 18 pixels from the bottom of the box, and end horizontally 3 pixels from the right border of the box.

To position the text in the center of the image, I just change the gravity:

	copyright.gravity = Magick::CenterGravity
	copyright.annotate(img, 0, 0, 0, 0, my_text)

Note that I didn have to specify any offsets: CenterGravity orients the text to be is in the exact center of the image (Figure 12-2). Specifying offsets would only move the text off-center.

The Magick library does substitutions for various special characters: for instance, the string "%t" will be replaced with the filename of the image. For more information about special characters, GravityType constants, and other annotate attributes that can let you fully customize the text appearance, refer to the RMagick documentation.

Figure 12-2. With a copyright message in the center of the image


See Also

  • RMagick Documentation (http://studio.imagemagick.org/RMagick/doc/)
  • On converting points to pixels (http://redux.imagemagick.org/RMagick/doc/draw.html#get_type_metrics)
  • SVG color keywords list (http://www.w3.org/TR/SVG/types.html#ColorKeywords)
  • This chapters introduction gives instructions on installing RMagick


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