問54
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スキャナで写真や絵などを読み込むときの解像度を表す単位はどれか。
平成25年度 春期 ITパスポート試験 問54
テクノロジ系情報メディア › マルチメディア応用
This question's explanation and your answers so far.
解説
問 54
- ア
正解 The correct answer is ア, “dpi”. dpi stands for dots per inch, a unit that expresses how many dots are lined up within one inch (25.4 mm). A scanner is a device that reads the surface of a photo or a picture as a collection of tiny dots and converts it into a digital image, so how finely it can read — the level of detail — is expressed by how many dots can be packed into one inch. This is the resolution of a scanner; 600 dpi, for example, means reading the original as 600 dots per inch. The larger the value, the more detail can be reproduced, and the larger the data size becomes. dpi is used with the same meaning for printers as well, for the number of ink dots that can be placed per inch, that is, the fineness of printing. For the pixel density of displays and image data, the unit ppi (pixels per inch) is also used, but both express resolution as a density of “how many per inch”, so the underlying idea is the same.
Incorrectイ
fps stands for frames per second, the unit for how many frames (still images) a video displays or records in one second — what is called the frame rate. It is an indicator of a quantity per unit of time, where a larger value means smoother motion, and it describes a completely different property from the fineness of an image. A scanner is a device that reads a still photo or picture as a single image, and the concept of frames per second has nothing to do with it, so it cannot be a unit of resolution.
Incorrectウ
pixel is the word for the smallest unit that makes up a digital image itself; it is not a unit that expresses the fineness, that is, the density, of dots. Giving an image size as 1920 × 1080 pixels, for example, states the total number of pixels lined up vertically and horizontally, and it does not tell you how finely that image was actually read off the paper. Even for the same 1000 pixels, reading one inch versus reading ten inches differs in fineness by a factor of ten. Resolution has to be expressed as a density of “how many per inch”, so pixel, which merely counts a number of items, cannot express resolution.
Incorrectエ
ppm stands for pages per minute, the unit for how many pages a printer can print in one minute, that is, an indicator of printing speed (how fast the work is done). Like dpi, it often appears in printer catalogs, so the two are easy to confuse, but what ppm shows is the speed of the work, not the fineness of the image that is output. Nor is it a unit used when expressing the resolution at which a scanner reads photos or pictures.
ここがポイント
- dpi (dots per inch) is a unit for the number of dots per inch, and it is the resolution indicator for how finely a scanner can read an original and how finely a printer can print. The larger the value, the finer the result, and the larger the image data becomes as well. A closely related unit is ppi (pixels per inch), which expresses the pixel density of displays and image data as the number of pixels per inch. Both follow the same idea of expressing resolution as a density of “how many per inch”.
- Units are easier to remember if you organize them by “what is counted, and per what”. dpi and ppi express resolution (fineness) as the number of dots or pixels per inch; fps expresses the smoothness of video as the number of frames per second; ppm expresses a printer's printing speed as the number of pages per minute; and bps expresses communication speed as the number of bits per second. Note as well that pixel is not a unit but a word for the element that makes up an image itself.
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