Sharing a Printer from Mavericks to Windows

January 14, 2015

Every few years I have to figure out a new way to share a Mac’s printer with Windows PCs. The printer is connected to a Mac with USB and shared using System Preferences>Sharing>Printer Sharing.

Apple provides Bonjour Print Services for Windows as a free download. It starts a service that discovers printers using the Zeroconf protocol, of which Apple’s implementation is called Bonjour (formerly Rendezvous before they lost a trademark dispute). It works great on Windows XP. But on Windows 7, Apple’s Bonjour service (as of version 2.0.2, which hasn’t been updated in years) is flaky, spewing tons of error messages to the event log and frequently losing the connection to the printer. I had to give up on it (and that’s without a Windows firewall getting involved).

When the Mac with the printer was running Snow Leopard (10.6), I could add the printer on Windows 7 using Control Panel>Hardware and Sound>Devices and Printers>Add a printer>Add a local printer>Create a new port>LPR Port and then entering the hostname or IP address of the Mac and the CUPS printer queue name (which the lpq command in Terminal shows). Snow Leopard shares the printer using the Unix LPD (also called LPR) protocol, among others.

When I upgraded the Mac to Mavericks (10.9), I found that it wasn’t listening on the LPD port. After a lot of research, I discovered that support for LPD printer sharing is included in Mavericks but disabled by default. Here is how to enable it.

Open a Terminal window.

sudo launchctl load -w /System/Library/LaunchDaemons/org.cups.cups-lpd.plist

Type your password. Your Mac should now be listening on the LPD port. If you have its firewall enabled in System Preferences, you might need to open the LPD printer port (TCP port 515) in the firewall. Now printing to it from Windows should work.

The load -w option forces the service to be enabled. If you want to more clearly mark it as enabled, you can:

sudo bash

Edit /System/Library/LaunchDaemons/org.cups.cups-lpd.plist using a text editor like vi or nano.

Under the Disabled key, change the “true” to “false”, then save the file and exit the editor.

All of this probably applies to Yosemite (10.10) too, but I haven’t tested that, or Windows 8 or Vista.


Fixing Photo Color Casts and Contrast

November 4, 2014

Before and after restoration of a faded old photo. Notice in the top histogram, the peaks of the red, green, and blue channels aren’t lined up and don’t reach to the left and right edges. That results in a color cast and reduced contrast. If you have software that has individual R,G,B levels or curves controls, it’s pretty simple to fix, and followed by fine-tuning of exposure looks like the bottom picture. I’ve used both Lightroom and Photoshop Elements to fix hundreds of old photos like this. In Lightroom, it’s “Edit Point Curve” in the Tone Curve section. In Photoshop Elements, a Levels Adjustment Layer. Photoshop has a Curves Adjustment Layer as well, which is more flexible but can be harder to use.

I start by moving the endpoints of all three channels to the edges, maximizing the contrast and luminosity range. If the results still have a color cast (often not the original one), I move midpoints until it looks the way I want. Sometimes I fine-tune it afterward with conventional white-balance controls (color temperature and tint).

Histogram+levels demo

From Novice to Master, and Back Again

January 14, 2013

In 1985, I was a freshman at St. Olaf College in Minnesota. The college had a VAX 11/780 running 4.2BSD and a PDP-11/70 running v7 with some Berkeley and local code hacked in. It was my first experience with multi-user systems other than dialing into an MS-DOS BBS or two.

The college’s Academic Computing Center had printouts of the 4.2BSD manuals, plus some home-grown documentation, available for sale so students could learn how to use UNIX. One week I sat in the Science Center terminal room and started going through the alphabetical list of the commands available on the VAX, trying each one and reading its man page to learn what it did.

Eventually I got to “su”. “Become the super-user”? What’s that? Does it involve wearing a cape? Sounds interesting, so I tried it. To my disappointment, it just asked for a password, and wouldn’t do anything.

Shortly thereafter, someone came running into the room and asked, “Are you David MacKenzie? Did you just run ‘su’?” “Yeah… what does it do?” “Uh, don’t do that.” My failed “su” attempt had been logged on the system console and one of the sysadmins was worried about an attempted breakin.

Within a year, I did have root access on the VAX, as I learned enough to be hired as a student system programmer. I contributed to upgrading the machine to 4.3BSD when that was released.

Recently I was working on a CentOS Linux virtual machine and needed to look up the command-line options to “su”. I had worked for the past several years mostly on Macs where “sudo” is preferred, so my “su” skills were rusty. I ran “man su” and got the information I needed. Then at the bottom of the screen I sheepishly read “Written by David MacKenzie.”

In the 1990s, while filling in gaps in the GNU toolset, I wrote the GNU “su”, and I had forgotten about it. It’s still what Red Hat and other distributions are shipping.

At least I know what it does now.