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.

Migrating Firefox

June 18, 2012

This is the procedure I use when migrating someone who uses Firefox to a new computer or rebuilding their computer. This works on both Windows and Mac.

  1. Backup their bookmarks. There used to be a bookmarks.html file you could copy from the Firefox profile, but now they’re in a database so you need to export them.
    1. From the Bookmarks menu, select Show All Bookmarks. This pops up a Library window.
    2. In the Library window, from the rightmost pulldown menu (with a star on it), select Backup and put the file on a network drive or removable drive. This backs up the bookmarks to a “JSON” file.
  2. Backup their saved login/passwords.

    1. From the Tools menu, select Add-ons.
    2. On the left, click Get Add-ons. In the “Search all add-ons” box in the upper-right, type “password exporter”.
    3. Find Password Exporter in the list of search results and click the Install button next to it.
    4. Go to Preferences (Mac) or Tools>Options (Windows), Security tab.
    5. Click the “Import/Export Passwords” button added by the Password Exporter add-on.
    6. Click the “Export Passwords” button and put the file on a network drive or removable drive. This backs up their logins and passwords to an XML file.
  3. Then, on their new computer, use similar procedures, except select Restore in the Library window menu and “Import Passwords” in the Import/Export window.


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