Work the Shell - Resizing Images, Sort Of
This might be a peculiarity of how I work with the Web, taking screenshots and then wanting to scale them to fit my page (especially when they're full-screen images), but I find that I spend a lot of time calculating how to reduce and scale images down evenly.
For example, I might take a full-size screen capture of the window within which I'm writing this particular column just to find that it's 722 x 719 pixels across and down, respectively. But if I were to include it on my Weblog, I would want to reduce it down to no more than 600 pixels so that it doesn't break my site layout.
I actually could reduce the image within the screen capture application or use a secondary graphical app, but it turns out that Web browsers can scale images up or down based on explicit “height” and “width” attributes. For example, let's say that the doc window is called edit.png. Then, I could include the image on a Web page with:
<img src="edit.png" alt="editing a file" />
and it would work fine. To scale, it's easy, simply add those height and width parameters. To make it match the image itself, I'd use:
<img src="edit.png" alt="editing" height="719" width="722" />
However, as I said, it turns out that you actually can calculate different values, and the browser will scale it to match. To reduce the image 50%, for example, I would tweak it to read:
<img src="edit.png" alt="editing" height="359" width="361" />
So that's what I do on my site, and frankly, it's a pain.
Instead, what I'd really like is a utility that can figure out the current height and width of an image and then automatically scale it to the new value I desire based on a scaling factor. That's what we'll dig into for this column.
There are some terrific image manipulation packages available in Linux, most notably ImageMagick, but we don't need anything that fancy because the pedestrian, old, undersung file command can do the job for us. I'm going to be looking at only PNG (progressive network graphic) files, as those are very much the best for most Web uses, but it's worth noting that many Linux file commands have a harder time calculating image dimensions for JPEG images.
Here's an example:
$ file edit.png edit.png: PNG image data, 722 x 719, 8-bit/color RGB, non-interlaced
That's quite a bit of information actually, including the key elements—the dimensions of the image file itself. In this case, it's width x height, so 722 is the width, in pixels, and 719 is the height. These can be extracted from the output in a variety of ways, but the easiest is to use cut:
width="$(file $filename | cut -f5 -d\ )" height="$(file $filename | cut -f7 -d\ )"
If you try this, however, you'll find that the height is wrong. It has a trailing comma because cut is using spaces as the delimiter (which is what the weird-looking -d\ is specifying. The backslash escapes the shell interpreting the space as an arg delimiter. When you type this in, you'll want a space after the backslash and before the closing parenthesis for just that reason. It's fixable though, by using sed:
Now that we have numeric values, how do we scale them automatically? I like using the bc binary calculator, even though its interface is so crufty. Multiplying 722 by 0.50 (which is, of course, 50%), is done like this:
echo 722 * 0.50 | bc
except that the \* will be expanded. So, in fact, some judicious use of quotes addresses the problem neatly:
width="$(echo "$width * $multiplier" | bc)"
That's certainly more shell-scripty, and it works fine, except I found that with some implementations of bc, even adding scale=0, which theoretically should remove the trailing fractional element that results from the multiplication, didn't give us an integer return value. Again, a simple fix gives us the final script line:
width="$(echo "$width * $multiplier" | bc | cut -d. -f1)"
The same thing gives us the newly calculated “height”, and if the user specifies a multiplier that's less than one, it scales down. If you specify a greater value, you just as easily can scale up.
Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at www.DaveTaylorOnline.com.
|Using Salt Stack and Vagrant for Drupal Development||May 20, 2013|
|Making Linux and Android Get Along (It's Not as Hard as It Sounds)||May 16, 2013|
|Drupal Is a Framework: Why Everyone Needs to Understand This||May 15, 2013|
|Home, My Backup Data Center||May 13, 2013|
|Non-Linux FOSS: Seashore||May 10, 2013|
|Trying to Tame the Tablet||May 08, 2013|
- Using Salt Stack and Vagrant for Drupal Development
- Making Linux and Android Get Along (It's Not as Hard as It Sounds)
- New Products
- Validate an E-Mail Address with PHP, the Right Way
- Drupal Is a Framework: Why Everyone Needs to Understand This
- A Topic for Discussion - Open Source Feature-Richness?
- New Products
- New Products
- The Pari Package On Linux
- Dart: a New Web Programming Experience
- This is the easiest tutorial
2 hours 28 min ago
- Ahh, the Koolaid.
8 hours 6 min ago
- git-annex assistant
14 hours 6 min ago
- direct cable connection
14 hours 29 min ago
- Agreed on AirDroid. With my
14 hours 39 min ago
- I just learned this
14 hours 43 min ago
15 hours 13 min ago
- not living upto the mobile revolution
18 hours 4 min ago
- Deceptive Advertising and
18 hours 40 min ago
- Let\'s declare that you have
18 hours 41 min ago
Enter to Win an Adafruit Prototyping Pi Plate Kit for Raspberry Pi
It's Raspberry Pi month at Linux Journal. Each week in May, Adafruit will be giving away a Pi-related prize to a lucky, randomly drawn LJ reader. Winners will be announced weekly.
Fill out the fields below to enter to win this week's prize-- a Prototyping Pi Plate Kit for Raspberry Pi.
Congratulations to our winners so far:
- 5-8-13, Pi Starter Pack: Jack Davis
- 5-15-13, Pi Model B 512MB RAM: Patrick Dunn
- Next winner announced on 5-21-13!
Free Webinar: Linux Backup and Recovery
Most companies incorporate backup procedures for critical data, which can be restored quickly if a loss occurs. However, fewer companies are prepared for catastrophic system failures, in which they lose all data, the entire operating system, applications, settings, patches and more, reducing their system(s) to “bare metal.” After all, before data can be restored to a system, there must be a system to restore it to.
In this one hour webinar, learn how to enhance your existing backup strategies for better disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible bare-metal recovery solution for UNIX and Linux systems.