2014年经济学人 计量测定学 识别人们的行踪

Science and technology

Biometrics

Clocking people's clocks

Facial-recognition systems are getting better

WITH some pride, the FBI trumpeted¹ the news last month that thanks to the agency's facial-recognition system Neil Stammer², wanted for sexual assault and kidnapping, had been apprehended³ in Nepal after being on the run for 14 years. The truth was slightly more prosaic⁴. A State Department official had used the FBI's “Wanted” posters in a test for passport fraud. The system then matched Mr Stammer's face with an American calling himself Kevin Hodges who regularly visited the US embassy in Kathmandu to renew his visa. Still, Mr Stammer's arrest illuminates⁵ the growing importance of facial-recognition technology.

The two main techniques used to recognise faces electronically are principal-component analysis (PCA) and linear-discriminant analysis (LDA). Both compare a picture of someone's phizog with a reference image taken in a controlled environment. Passport photos and mugshots, then, are about as ideal as it gets.

Basic PCA and LDA are good for skin colour, hair colour and the like. Advanced systems, such as that used with British biometric passports, may look at cheek bones, the bridge of the nose, jaw⁶ lines and eyes.

All of which is fine when someone is sitting or standing⁷ in front of a camera, but is less useful in the world beyond the studio. That requires a technique called Elastic⁸ Bunch Graph Matching (EBGM), which tries to create a three-dimensional (3D) model from two-dimensional images. This model can, thereafter, be used to match any subsequent image, or part thereof.

EBGM considers the head as a union of two ellipsoids: one whose main axis⁹ is vertical¹⁰, and runs from forehead to chin; the other whose main axis is horizontal, and runs from tip of the nose to the back of the cranium. This basic scheme is overlaid with “fiducial” points which act as anchors for the modelling. These can be as few as half a dozen (the pupils of the eyes, the corners of the mouth, and so on), or as many, in one system, as 40,000.

EBGM allows the construction of a three-dimensional representation of a face from poorly lit images taken at odd angles, such as a closed-circuit television camera might provide. Once it recognises enough fiducial points it can work out what aspect of a face it is viewing. It then extrapolates the expected positions of other fiducial points. As more data come in from the camera, the model's shape is updated. Given enough horsepower, says a British official, such a system can build a model from as few as 80 pixels located between a subject's eyes—and only two images are needed for a 3D reconstruction¹¹.

Governments are not the only ones interested. Earlier this year, Facebook's DeepFace system was asked whether thousands of pairs of photos were of the same person. It answered correctly 97.25% of the time, a shade behind humans at 97.53%. Although DeepFace is only a research project, and is aided by the fact that many Facebook photos are tagged with the names of people in the images, which lets the system learn those faces in different poses and lighting¹² conditions, it is still an impressive feat¹³.

As DeepFace shows, access to an accurate gallery of images is crucial. Passport photos, or those on national identity cards, can act as such galleries, as they can be rendered by EBGM into usable 3D models. Add in the increasing ubiquity of closed-circuit television, and the idea that anyone will be able hide for long in Nepal, or anywhere else, looks quaint¹⁴.

1 trumpeted   

大声说出或宣告（trumpet的过去式与过去分词形式）

参考例句：

• Soldiers trumpeted and bugled. 士兵们吹喇叭鸣号角。
• The radio trumpeted the presidential campaign across the country. 电台在全国范围大力宣传总统竞选运动。

2 stammer   

n.结巴，口吃；v.结结巴巴地说

参考例句：

• He's got a bad stammer.他口吃非常严重。
• We must not try to play off the boy troubled with a stammer.我们不可以取笑这个有口吃病的男孩。

3 apprehended   

逮捕，拘押( apprehend的过去式和过去分词 )； 理解

参考例句：

• She apprehended the complicated law very quickly. 她很快理解了复杂的法律。
• The police apprehended the criminal. 警察逮捕了罪犯。

4 prosaic   

adj.单调的，无趣的

参考例句：

• The truth is more prosaic.真相更加乏味。
• It was a prosaic description of the scene.这是对场景没有想象力的一个描述。

5 illuminates   

v.使明亮( illuminate的第三人称单数 )；照亮；装饰；说明

参考例句：

• The light shines on from over there and illuminates the stage. 灯光从那边照进来，照亮了舞台。 来自《简明英汉词典》
• The sun illuminates the sky. 太阳照亮了天空。 来自《简明英汉词典》

6 jaw   

n.颚，颌，说教，流言蜚语；v.喋喋不休，教训

参考例句：

• He delivered a right hook to his opponent's jaw.他给了对方下巴一记右钩拳。
• A strong square jaw is a sign of firm character.强健的方下巴是刚毅性格的标志。

7 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

8 elastic   

n.橡皮圈，松紧带；adj.有弹性的；灵活的

参考例句：

• Rubber is an elastic material.橡胶是一种弹性材料。
• These regulations are elastic.这些规定是有弹性的。

9 axis   

n.轴，轴线，中心线；坐标轴，基准线

参考例句：

• The earth's axis is the line between the North and South Poles.地轴是南北极之间的线。
• The axis of a circle is its diameter.圆的轴线是其直径。

10 vertical   

adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置

参考例句：

• The northern side of the mountain is almost vertical.这座山的北坡几乎是垂直的。
• Vertical air motions are not measured by this system.垂直气流的运动不用这种系统来测量。

11 reconstruction   

n.重建，再现，复原

参考例句：

• The country faces a huge task of national reconstruction following the war.战后，该国面临着重建家园的艰巨任务。
• In the period of reconstruction,technique decides everything.在重建时期，技术决定一切。

12 lighting   

n.照明，光线的明暗，舞台灯光

参考例句：

• The gas lamp gradually lost ground to electric lighting.煤气灯逐渐为电灯所代替。
• The lighting in that restaurant is soft and romantic.那个餐馆照明柔和而且浪漫。

13 feat   

n.功绩；武艺，技艺；adj.灵巧的，漂亮的，合适的

参考例句：

• Man's first landing on the moon was a feat of great daring.人类首次登月是一个勇敢的壮举。
• He received a medal for his heroic feat.他因其英雄业绩而获得一枚勋章。

14 quaint   

adj.古雅的，离奇有趣的，奇怪的

参考例句：

• There were many small lanes in the quaint village.在这古香古色的村庄里，有很多小巷。
• They still keep some quaint old customs.他们仍然保留着一些稀奇古怪的旧风俗。