### Learn common ways to view and modify images with this lightweight image editor.
Image by :opensource.com
In a recent article about[lightweight image viewers][8], author Scott Nesbitt mentioned display, one of the components in[ImageMagick][9]. ImageMagick is not merely an image viewer—it offers a large number of utilities and options for image editing. This tutorial will explain more about using the**display**command and other command-line utilities in ImageMagick.
With a number of excellent image editors available, you may be wondering why someone would choose a mainlynon-GUI, command-line based program like ImageMagick. For one thing, it is rock-solid dependable. But an even bigger benefit is that it allows you to set up methods to edit a large number of images in a particular way.
This introduction to common ImageMagick commands should get you started.
### The display command
Let's start with the command Scott mentioned:**display**. Say you have a directory with a lot of images you want to look at. Start**display**with the following command:
```
cd Pictures
display *.JPG
```
This will load your JPG files sequentially in alphanumeric order, one at a time in a simple window. Left-clicking on an image brings upa simple, standalone menu (the only GUI feature you'll see in ImageMagick).
***File**contains the options_Open, Next, Former, Select, Save, Print, Delete, New, Visual Directory_ , and_Quit_ ._Select_ picks a specific image file to display,_Visual Directory_shows all of the files (not just the images) in the current working directory. If you want to scroll through all the selected images, you can use_Next_and_Former_ , but it's easier to use their keyboard shortcuts (Spacebar for the next image and Backspace for the previous).
***Edit**offers_Undo, Redo, Cut, Copy_ , and_Paste_ , which are just auxiliary commands to more specific editing process._Undo_ is especially useful when you're playing around with different edits to see what they do.
***View**has_Half Size, Original Size, Double Size, Resize, Apply, Refresh_ , and_Restore_ . These are mostly self-explanatory and, unless you save the image after applying one of them, the image file isn't changed._Resize_brings up a dialog to name a specific size either in pixels, with or without constrained dimensions, or a percentage. I'm not sure what_Apply_ does.
***Transform**shows_Crop, Chop, Flop, Flip, Rotate Right, Rotate Left, Rotate, Shear, Roll_ , and_Trim Edges_ ._Chop_ uses a click-drag operation to cut out a vertical or horizontal section of the image, pasting the edges together. The best way to learn how these features work is to play with them, rather than reading about them.
***Enhance**provides_Hue, Saturation, Brightness, Gamma, Spiff, Dull, Contrast Stretch, Sigmoidal Contrast, Normalize, Equalize, Negate, Grayscale, Map_ , and_Quantize_ . These are operations for color manipulation and adjusting brightness and contrast.
***Effects**has_Despeckle, Emboss, Reduce Noise, Add Noise, Sharpen, Blur, Threshold, Edge Detect, Spread, Shade, Raise_ , and_Segment_ . These are fairly standard image editing effects.
***F/X**options are_Solarize, Sepia Tone, Swirl, Implode, Vignette, Wave, Oil Paint_ , and_Charcoal Draw_ , also very common effects in image editors.
***Image Edit**contains_Annotate, Draw, Color, Matte, Composite, Add Border, Add Frame, Comment, Launch_ , and_Region of Interest_ ._Launch_ will open the current image in GIMP (in my Fedora at least)._Region of Interest_allows you to select an area to apply editing; press Esc to deselect the region.
***Miscellany**offers_Image Info, Zoom Image, Show Preview, Show Histogram, Show Matte, Background, Slide Show_ , and_Preferences_ ._Show Preview_ seems interesting, but I struggled to get it to work.
***Help**shows_Overview, Browse Documentation_ , and_About Display_ ._Overview_ gives a lot of basic information about display and includes a large number of built-in keyboard equivalents for various commands and operations. In my Fedora,_Browse Documentation_took me nowhere.
Although**display**'s GUI interface provides a reasonably competent image editor, ImageMagick also provides 89 command-line options, many of which correspond to the menu items above. For example, if I'm displaying a directory of digital images that are larger than my screen size, rather than resizing them individually after they appear on my screen, I can specify:
```
display -resize 50% *.JPG
```
Many of the operations in the menus above can also be done by adding an option in the command line. But there are others that aren't available from the menu, including**‑monochrome**, which converts the image to black and white (not grayscale), and**‑colors**, where you can specify how many colors to use in the image. For example, try these out:
```
display -resize 50% -monochrome *.JPG
```
```
display -resize 50% -colors 8 *.JPG
```
These operations create interesting images. Try enhancing colors or making other edits after reducing colors. Remember, unless you save and overwrite them, the original files remain unchanged.
### The convert command
The**convert**command has 237 options—yes 237—that provide a wide range of things you can do (some of which display can also do). I'll only cover a few of them, mostly sticking with image manipulation. Twosimple things you can do with**convert**would be:
```
convert DSC_0001.JPG dsc0001.png
```
```
convert *.bmp *.png
```
The first command would convert a single file (DSC_0001) from JPG to PNG format without changing the original. The second would do this operation on all the BMP images in a directory.
If you want to see the formats ImageMagick can work with, type:
```
identify -list format
```
Let's pick through a few interesting ways we can use the**convert**command to manipulate images. Here is the general format for this command:
```
convert inputfilename [options] outputfilename
```
You can have multiple options, and they are done in the order they are arranged, from left to right.
The**‑monochrome**option has no associated setting, but the**‑charcoal**variable needs an associated factor. In my experience, it needs to be a small number (even less than 1) to achieve something that resembles a charcoal drawing, otherwise you get pretty heavy blobs of black. Even so, the sharp edges in an image are quite distinct, unlike in a charcoal drawing.
The original image is in the upper left. In the first command, I applied an**‑edge**option with a setting of 3 (see the upper-right image)—anything less than that was too subtle for my liking. In the second command (the lower-left image), we have reduced the number of colors to four, which doesn't look much different from the original. But look what happens when we combine these two in the third command (lower-right image)! Perhaps it's a bit garish, but who would have expected this result from the original image or either option on its own?
The**‑canny**command provided another surprise. This is another kind of edge detector, called a "multi-stage algorithm." Using**‑canny**alone produces a mostly black image and some white lines. I followed that with a**‑negate**command:
It's a bit minimalist, but I think it resembles a pen-and-ink drawing, a rather remarkable difference from the original photos. It doesn't work well with all images; generally, it works best with images with sharp lines. Elements that are out of focus are likely to disappear; notice how the background sandbar in the egret picture doesn't show up because it is blurred. Also notice in the ship picture, while most edges show up very well, without colors we lose the gestalt of the picture, so perhaps this could be the basis for some digital coloration or even coloring after printing.
### The montage command
Finally, I want to talk about the**montage**command. I've already shown examples of it above, where I have combined single images into composites.
Here's how I generated the charcoal example (note that it would all be on one line):
The**-label**option labels each image with its filename (**%f**) underneath. Without the**‑geometry**option, all the images would be thumbnail size (120 pixels wide), and**+10+10**manages the border size. Next, I resized the entire final composite (**‑resize 25%**)and added a shadow (with no settings, so it's the default), and finally created a**title**for the montage.
You can place all the image names at the end, with the last image name the file where the montage is saved. This might be useful to create an alias for the command and all its options, then I can simply type the alias followed by the appropriate filenames. I've done this on occasion to reduce the typing needed to create a series of montages.
In the**‑canny**examples, I had four images in the montage. I added the**‑tile**option, specifically**‑tile 2x**, which created a montage of two columns. I could have specified a**matrix**,**‑tile 2x2**, or**‑tile x2**to produce the same result.
There is a lot more to learn about ImageMagick, so I plan to write more about it, maybe even about using[Perl][10]to script ImageMagick commands. ImageMagick has extensive[documentation][11], although the site is short on examples or showing results, and I think the best way to learn is by experimenting and changing various settings andoptions.
Greg Pittman - Greg is a retired neurologist in Louisville, Kentucky, with a long-standing interest in computers and programming, beginning with Fortran IV in the 1960s. When Linux and open source software came along, it kindled a commitment to learning more, and eventually contributing. He is a member of the Scribus Team.
