19. Editing Images

When you take an image file--such as one containing a digitized photograph or a picture drawn with a graphics program--and you make changes to it, you are editing an image.

This chapter contains recipes for editing and modifying images, including how to convert between image file formats. It also gives an overview of other image applications you might find useful, including the featuresome GIMP image editor.

19.1 Transforming Images		Transforming images from the command line.
19.2 Converting Images between Formats		Converting image formats.
19.3 Editing Images with the GIMP		The famous GIMP image processor.
19.4 Interactive Image Editors and Tools		A list of good image editors to try.

19.1 Transforming Images

@sf{Debian}: `imagemagick'
@sf{WWW}: ftp://ftp.wizards.dupont.com/pub/ImageMagick/

mogrify always takes the name of the file to work on as an argument, and it writes its changes to that file. Use a hyphen (`-') to specify the standard input, in which case mogrify writes its output to the standard output.

I'll use the image `phoenix.jpeg' in the examples that follow to give you an understanding of how to use mogrify:

NOTE: You can also perform many of the image transformations described in the following sections interactively with the GIMP (see section Editing Images with the GIMP).

19.1.1 Changing the Size of an Image		Changing the size of an image.
19.1.2 Rotating an Image		Rotating an image.
19.1.3 Adjusting the Colors of an Image		Reducing the colors in an image.
19.1.4 Annotating an Image		Annotating an image.
19.1.5 Adding Borders to an Image		Putting a border around an image.
19.1.6 Making an Image Montage		Making a montage of images.
19.1.7 Combining Images		Combining images.
19.1.8 Morphing Two Images Together		Morphing two images.

19.1.1 Changing the Size of an Image

To resize an image with mogrify, use the `-geometry' option with the width and height values, in pixels, as an argument.

• To resize `phoenix.jpeg' to 480x320 pixels, type:

  $ mogrify -geometry 480x320 phoenix.jpeg RET

This transforms the original `phoenix.jpeg' file to:

NOTE: Images scaled to a larger size will appear blocky or fuzzy.

When mogrify resizes an image, it maintains the image's aspect ratio, so that the ratio between the width and height stays the same. To force a conversion to a particular image size without necessarily preserving its aspect ratio, append the geometry with an exclamation point.

• To resize `phoenix.jpeg' to exactly 480x320 pixels, regardless of aspect ratio, type:

  $ mogrify -geometry 640x480! phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

You can also specify the width or height by percentage. To decrease by a percentage, give the value followed by a percent sign (`%'). To increase by a percentage, give the value plus 100 followed by a percent sign. For example, to increase by 25 percent, give `125%'.

• To increase the height of `phoenix.jpeg' by 25 percent and decrease its width by 50 percent, type:

  $ mogrify -geometry 125%x50% phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

NOTE: To view an image at a particular scale without modifying it, use display; when you resize its window, you resize the image on the screen only (see section Resizing a Window).

19.1.2 Rotating an Image

To rotate an image, use mogrify with the `-rotate' option followed by the number of degrees to rotate by. If the image width exceeds its height, follow this number with a `>', and if the height exceeds its width, follow it with a `<'. (Since both `<' and `>' are shell redirection operators, enclose this argument in quotes, omitting either if the image height and width are the same.)

• To rotate `phoenix.jpeg', whose height exceeds its width, by 90 degrees, type:

  $ mogrify -rotate '90<' phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

NOTE: After this command, the width of `phoenix.jpeg' now exceeds its height, so to rotate it again use `>' instead of `<'.

19.1.3 Adjusting the Colors of an Image

You can use mogrify to make a number of adjustments in the color of an image. To reduce the number of colors in an image, use the `-colors' option, followed by the number of colors to use.

• To reduce the colors in `phoenix.jpeg' to two, type:

  $ mogrify -colors 2 phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

Use the `-dither' option to reduce the colors with Floyd-Steinberg error diffusion, a popular algorithm for improving image quality during color reduction.

• To reduce the colors in `phoenix.jpeg' to four and apply Floyd-Steinberg error diffusion, type:

  $ mogrify -colors 4 -dither phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

Use the `-map' option with a second file name as an argument to read the color map, or the set of colors, from the second image and use them in the first image.

• To change the colors in the file `rainbow.jpeg' to those used in the file `prism.jpeg', type:

  $ mogrify -map prism.jpeg rainbow.jpeg RET

Use the `-monochrome' option to make a color image black and white.

• To make the color image `rainbow.jpeg' black and white, type:

  $ mogrify -monochrome rainbow.jpeg RET

If you have a PPM file, use ppmquant to quantize, or reduce to a specified quantity the colors in the image--see the ppmquant man page for details (see section Reading a Page from the System Manual).

Because of differences in display hardware, the brightness of an image may vary from one computer system to another. For example, images created on a Macintosh usually appear darker on other systems. When you adjust the brightness of an image it is called gamma correction.

To adjust the brightness of an image, give the numeric level of correction to apply as an argument to the `-gamma' option. Most PC displays have a gamma value of 2.5, while Macintosh displays have a lower gamma value of 1.4.

• To set the gamma correction of the image `rainbow.jpeg' to .8, type:

  $ mogrify -gamma .8 rainbow.jpeg RET

19.1.4 Annotating an Image

@sf{Debian}: `libjpeg-progs'
@sf{WWW}: http://www.ijg.org/

• To annotate the image file `phoenix.jpeg', type (all on one line):

  $ mogrify -comment "If you can read this, you're too close!" phoenix.jpeg RET

You won't see the annotation when you view the image; it is added to the image header in the file. You can, however, read image annotations with tools that display information about an image file, such as display or the GIMP. To read annotations in JPEG files, you can also use the rdjpgcom tool--it outputs any comments in the JPEG file whose file name is given as an argument.

• To read any comments made in the image file `phoenix.jpeg', type:

  $ rdjpgcom phoenix.jpeg RET If you can read this, you're too close! $

NOTE: Another method for writing comments in JPEG files is to use wrjpgcom, which is distributed with rdjpgcom in the `libjpeg-progs' package.

19.1.5 Adding Borders to an Image

To draw a border around an image, use mogrify with the `-border' option followed by the width and height, in pixels, of the border to use.

• To add a border two pixels wide and four pixels high to `phoenix.jpeg', type:

  $ mogrify -border 2x4 phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

NOTE: The border is added to the outside of the existing image; the image is not cropped or reduced in size to add the border.

The `-frame' option works like `-border', but it adds a more decorative border to an image.

• To add a decorative frame eight pixels wide and eight pixels high to `phoenix.jpeg', type:

  $ mogrify -frame 8x8 phoenix.jpeg RET

This transforms the original `phoenix.jpeg' to:

19.1.6 Making an Image Montage

To make a montage image of other images, use montage. It takes as arguments the names of the images to use followed by the name of the output file to write the montage image to.

The montage image is made by scaling all of the input images to fit the largest size possible up to 120x120 pixels, and tiling these images in rows of five and columns of four.

• To create a montage from the files `owl.jpeg', `thrush.jpeg', and `warbler.jpeg' and write it to `endangered-birds.png', type:

$ montage owl.jpeg thrush.jpeg warbler.jpeg endangered-birds.png RET

NOTE: In this example, three JPEGs were read and output to a PNG file; to specify the format to use in the output, give the appropriate file extension in the output file name.

19.1.7 Combining Images

Use combine to combine two images into one new image--give the names of the two source image files and the new file to write to as arguments. Without any options, it makes a new image file by overlaying the smaller of the two images over the larger, starting in the top left corner; if both images are the same size, only the second image is visible.

• To combine two images, `ashes.jpeg' and `phoenix.jpeg', into a new file `picture.jpeg', type:

  $ combine ashes.jpeg phoenix.jpeg picture.jpeg RET

You can specify the percentage to blend two images together with the `-blend' option. Give the amount to blend the second image into the first (as a percentage) as an argument to the option.

• To combine the image files `phoenix.jpeg' and `ashes.jpeg' so that the blended image contains 70 percent of the second image, type:

  $ combine -blend 70 ashes.jpeg phoenix.jpeg picture.jpeg RET

This command combines the two images and writes a new image file, `picture.jpeg', whose contents contain 70 percent of the first image.

NOTE: Use `-blend 50' to blend the two source files equally.

19.1.8 Morphing Two Images Together

Morphing is a method of computer imaging for finding the difference between the shapes in two images; it's often used in special effects to transform aspects of two creatures, such as the faces of a human and some other animal.

You can use combine to get a morph-like effect by giving the difference argument to the `-compose' option. When specified with two input images and an output file, this command takes the difference between corresponding pixels in the two images; the effect is like a "morphed" image.

• To make a morphed image of the files `ashes.jpeg' and `phoenix.jpeg', and write it to `picture.jpeg', type:

$ combine -compose difference ashes.jpeg phoenix.jpeg picture.jpeg RET

The result in file `picture.jpeg' is:

NOTE: `xmorph' is a tool for morphing images; see Interactive Image Editors and Tools.

19.2 Converting Images between Formats

@sf{Debian}: `imagemagick'
@sf{WWW}: ftp://ftp.wizards.dupont.com/pub/ImageMagick/

To specify the file type to convert to, use that file type's standard file extension in the file name of the converted file.

• To convert the JPEG file `phoenix.jpeg' to a PNG image, type:

  $ convert phoenix.jpeg phoenix.png RET

This command converts the JPEG image `phoenix.jpeg' to PNG format and writes it to a new file, `phoenix.png'.

The following table lists the file extensions to use and describes their format. (The convention is to give extensions in all lowercase letters.)

When converting a file to JPEG format, be sure to use the `-interlace NONE' option to make sure the resultant JPEG image is non-interlaced--unless, of course, you want an interlaced image; an interlaced image is drawn in multiple passes, and is often used on the Web where a reader may view the low-resolution image consisting of early passes before the entire image is downloaded. A non-interlaced image is drawn in one single pass.

For example, use convert to convert a PNM file to non-interlaced JPEG, while sharpening it, adding a border, and adding a copyright statement.

• To convert the PNM file `pike.pnm' to non-interlaced JPEG while sharpening the image by 50 percent and adding both a 2x2 border and a copyright comment, type:

  $ convert -interlace NONE -sharpen 50 -border 2x2 -comment 'copyright 1999 MS' pike.pnm pike.jpeg RET

This command writes its output to a file `pike.jpeg'. Notice that the options `-border' and `-comment' were previously described for the `mogrify' tool. Some ImageMagick tools share common options, which is useful if you are making multiple changes to an image file at once; only one tool is needed for the job.

NOTE: Some image formats are "lossy," in that some image information is lost when you convert to it. For example, the JPEG format is a lossy format that is usually used for photographic images. If you convert a file from its source PNM format to JPEG and then back to PNM, the resultant PNM will not be identical to the original source PNM.

To convert image files interactively, use the GIMP to open the image, and then choose `Save as' from the File menu, and select the file type to use; see Editing Images with the GIMP.

19.3 Editing Images with the GIMP

@sf{Debian}: `gimp'
@sf{Debian}: `gimp-manual'
@sf{WWW}: http://www.gimp.org/

The GIMP comes with hundreds of tools, filters, fonts, and other goodies installed. Here is a partial list of its features:

• Contains a full suite of painting tools, including Brush, Pencil, Airbrush, and Clone.

• Supports custom brushes and patterns.

• Includes a full suite of image selection, transformation, and manipulation tools, including a gradient editor, color blending, and special effects.

• Includes animation support.

• Permits the use of layers and channels.

• Allows for large images, with their size being limited only by available disk space.

• Provides high-quality anti-aliasing.

• Offers full alpha-channel support.

• Supports command scripting.

• Permits multiple undo and redo, limited only by available disk space.

• Allows multiple images to be open simultaneously.

• Supports all popular file formats, including GIF, JPEG, PNG, XPM, TIFF, TGA, MPEG, PS, PDF, PCX, and BMP.

• Allows the easy addition of more than 100 plug-ins for new file formats and new effect filters.

NOTE: To learn the basics of the GIMP, consult The GIMP User's Manual and the other documentation and resources on the Web at http://www.gimp.org/. You can also install the manual on your system; it comes in the Debian `gimp-manual' package.

19.4 Interactive Image Editors and Tools

There are all kinds of image-editing software applications available for Linux--and there are as many way to make and edit an image as there are tools to do it with.

The following table lists some other popular tools and applications for making and editing images--including CAD engineering software--that you may want to explore. It is not exhaustive.