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.

How Not to Build a Fence

April 1, 2012

Three years ago, we hired Capital Fence to replace the deteriorating old fence around our yard. They’re a popular fence company here in the DC/MD/VA area, so I assumed they’d do a reasonable job. It wasn’t until they were building the fence that I learned that

  1. They just stick the posts in the ground, with no concrete or other means of keeping them from heaving seasonally, and
  2. They use nail guns, not screws, to put the fence together.

The result? Within a year, I was having to make repairs to our new fence because of warping boards pulling out the nails and heaving gate posts pulling the gate so far out of alignment that the lock wouldn’t open. Buyer beware: ask detailed questions before signing a contract. Even if they are a big name. I couldn’t imagine that professionals would use such poor construction methods, but by the time I found out it was too late.

Here are some closeups of our three year old fence. I’ve got some work ahead of me driving in deck screws to pull it back together.

Here are some examples of warping boards pulling the nails out.
Warping board pulling out on top
Warping top board

Boards pulling apart

Boards warping at the bottom

Here is a closeup of the gate lock showing the original top screw hole above the lock. I had to drill new holes because the post sank so much that the lock no longer lined up.
Gate lock misalignment due to sinking

Here’s their sign on this shoddy work.
Logo of fence company to avoid