Thursday I broke an old rule. I went to see a documentary, Peter Jackson’s “They Shall Not Grow Old.” And I’m not sorry I went.

Jackson is the renowned director of “The Lord of the Rings” movies. He has a reputation as someone who knows how to make use of computer technology in filmmaking. And he is also, it turns out, a World War I buff.

We’ve just had the 100th anniversary of the end of that struggle. In July of 1914, Germany, the Austrian empire and Turkey went to war against Great Britain, France, Italy, imperial Russia and eventually the United States. Weapons included poisonous gas, airplanes and tanks.

So did “trench warfare.” Large numbers of combatants spent months in and around vast networks of trenches, shelling each other and taking pot shots, sometimes rushing each other over thoroughly churned up and blasted countryside, and then spending week after week hunkered down, hoping to avoid rats, trench foot and dysentery.

Seventy-million soldiers and sailors were involved. We believe 16 million people died, exclusive of the victims of the influenza epidemic which spread at the very end of the war.

Kiwi Jackson had relations in the conflict. He collected publications, uniforms and even weapons from it. And apparently someone at London’s Imperial War Museum knew about his fascination.

With the centennial approaching, administrators there asked him to make a documentary using their large collection of motion picture footage.

Working without pay, Jackson spent a year looking at the film. He got the BBC’s 600 hours of audio tape of veterans’ war recollections, and began imagining a way he could put the two things together.

But first he and his technicians had to work on the aging and either fading or darkening film stock.

The movie they have made does a solid job of suggesting the ugliness of trench warfare. It is not about the whole war, but about the experiences of ordinary infantry soldiers serving in the huge German, French and British imperial armies in Belgium and northern France.

Their lives were miserable. But the soldiers apparently enjoyed great comradery and, like young people everywhere including university students, they found ways to have fun.

These occasional references to distractions from the horrors of war are welcome. And they ring true.

Some of the technical things Jackson’s company has done to the footage is remarkable. They have recorded sounds, including the sounds of machinery and artillery operations, to go along with the moving pictures.

They have even lip-read soldiers and provided region-correct audio to give those previously silent speakers voice again.

The movie features a little music, and it is tactfully handled. The closing credits include some inoffensive verses of “Mademoiselle from Armentières” (“Hinky-dinky parlevous”) sung by British diplomats (so that the accents are generally correct) in New Zealand during the last days of the film’s preparation.

The one correction Jackson has made that one might quibble about is the colorization. From the time soldiers reach the area of battle until the end of the war, the footage — which was all black and white — has been given color through computer processes.

The result doesn’t look like new color footage, however. It looks like color applied later to tin type images, or to the color sometimes given in mortuaries to the dead.

So the exercise draws attention to the color rather than letting us see what the historic footage looked like.

Following the fairly brief movie proper is an added half-hour during which Jackson explains some of the technical choices made during the refurbishment and editing. It is fascinating.

As is “They Shall Not Grow Old” in general. It is a dignified but not forbidding retrospective that will help us old guys to remember what our grandfathers lived through a hundred years ago and will inform younger generations who now know probably too little about World War I.

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