디지털영상처리-3

[HHa!ha!]

 

CHAPTER 3. The Digital Image

◆ digital image created by computer

◆ most common image : ⇒ Optical image(X-ray, infrared, radar, and acoustic images)

                         ⇒ Analog image

▷▶ Creating a Digital Image ◀◁

◆ continuous image ⇒ digital image

 (digitization - sampling and quantization)

<Fig. 3.1>An image is converted from a continuous-tone form to a digital from through the processes fo sampling and quantization.

 ① monochrome or color

 ② gray tone, brightness or gray level :  계조도

 ③ pixel (picture element) : 화소

 ④ image resolution : image quality

        brightness resolution, spatial resolution ← still picture

        frame rate(temporal resolution) ← moving picture

        

<Fig. 3.2> (a) The discrete pixel numbering convention

(b) A 640 pixel×480 line digital image showing the location of pixel (200,150)

▷▶ Spatial Resolution ◀◁

◆ scanning, sampling 에 의한 spatial density와 imaging system의 photosensor에 의한 optical resolution에 의해 결정됨.

◆ spatial frequency : low or high spatial frequency

<Fig 3.3> Several distinct spatial frequency patterns. The wider ones are low-frequency patterns, and the tighter ones are high-frequency patterns.

<Fig 3.4>(a) A scene of varying spatial frequency detail

<Fig 3.4>(b) The brightness plot taken along the highlighted line.

◆ sampling theorem : Nyquist rate

        fs ≥ 2fm, where, fs : sampling frequency

                        fm : highest frequency of signal

        ⇒ spatial resolution을 결정

                              640 × 480                              320 × 240

              160 × 120                                      80 × 60

              40 × 30                              20 × 15

<Fig 3.5 a, b, c, d, e, f>

* blocking effect

* resolution 응용분야에 따라 다르게 요구됨.

* spatial resolution에 따른 image data 량.

<Table 3.1> Image Size Versus Spatial Resolution

NUMBER OF PIXELS	NUMBER OF LINES	TOTAL PIXELS IN IMAGE
20	15	300
40	30	1,200
80	60	4,800
160	120	19,200
320	240	76,800
640	480	307,200

▷▶ Viewing Geometry ◀◁

◆ 거리에 따른 spatial resolution

<Fig 3.6> The display-observer viewing geometry

▷▶ Aspect Ratio ◀◁

◆ commercial NTSC TV

        ① image aspect ratio ⇒ 4 : 3 (현장감 부족)

        ② pixel aspect ratio ⇒ 1 : 1 (square pixel)