image acquisition 1

8/7/2019 Image Acquisition 1

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How the camera sees the Image

A presentation on


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A typical analog camcorder contains two basic parts:

A camera section, consisting of a CCD, lens and motors to

handle the zoom, focus and aperture .

AVCR section, in which a typical TV VCR is shrunk down to fit

in a much smaller space.

The camera component's function is to receive visual information and

interpret it as an electronic video signal. The VCR component is

exactly like the VCR connected to your television. It receives anelectronic video signal and records it on video tape as magnetic

patterns.

These two sections are easily seen in the following photos:


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Camcorder with the

outer shell removed


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The camcorder's VCR unit


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The camcorder's camera unit


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The camcorder's Charge

Coupled Device (CCD)


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Like a film camera, a camcorder "sees" the world through lens.

In a film camera, the lens serves to focus the light from a scene

onto film treated with chemicals that have a controlled reaction

to light. In this way, camera film records the scene in front ofit: It picks up greater amounts of light from brighter parts of

the scene, and lower amounts of light from darker parts of the

scene. The lens in a camcorder also serves to focus light, but

instead of focusing it onto film, it shines the light onto a small

semiconductor image sensor. This sensor, a charge-coupleddevice (CCD), measures light with a half-inch (about 1 cm)

panel of 300,000 to 500,000 tiny light-sensitive diodes called

photo sites.

The CCD


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Each photo site measures the amount of light (photons) that

hits a particular point, and translates this information into

electrons (electrical charges):A brighter image is represented

by a higher electrical charge, and a darker image isrepresented by a lower electrical charge. Just as an artist

sketches a scene by contrasting dark areas with light areas, a

CCD creates a video picture by recording light intensity.

During playback, this information directs the intensity of a

television's electron beam as it passes over the screen.


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Of course, measuring light intensity only gives us a

black-and-white image. To create a color image, a

camcorder has to detect not only the total light levels,

but also the levels of each color of light. Since you

can produce the full spectrum of colors by combiningthe three colors red, green and blue, a camcorder

actually only needs to measure the levels of these

three colors to be able to reproduce a full-color

picture.


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In some high-end camcorders, a beam splitter separates a signal into

three different versions of the same image -- one showing the level of red

light, one showing the level of green light and one showing the level of

blue light. Each of these images is captured by its own chip -- the chips

operate as described above, but each measures the intensity of only onecolor of light. The camera then overlays these three images and the

intensities of the different primary colors blend to produce a full-color

image. A camcorder that uses this method is often referred to as a three-

chip camcorder.


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This simple method produces a rich, high-resolution picture.

CCDs are expensive and eat lots of power, however, so using

three of them adds considerably to the manufacturing costs of a

camcorder. Most camcorders get by with only one CCD by

fitting permanent color filters to individual photosites. A certainpercentage of photosites measures only levels of red light,

another percentage measures only green light and the rest of the

photosites measure only blue light. The color designations are

spread out in a sort of grid (the Bayer filter is a common

configuration), so that the video camera computer can get a

sense of the color levels in all parts of the screen. This methodrequires the computer to interpolate the true color of light

arriving at each photosite by analyzing the information received

by the other photosites in the vicinity.


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Amore economical and practical way to record the primary colors

is to permanently place a filter called a color filter array over

each individual photosite. By breaking up the sensor into a variety

of red, blue and green pixels, it is possible to get enough

information in the general vicinity of each sensor to make very

accurate guesses about the true color at that location. This process

of looking at the other pixels in the neighborhood of a sensor and

making an educated guess is called interpolation.


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The most common pattern of filters is the Bayer filter pattern.This pattern alternates a row of redand green filters with a row of blue and green filters. The pixels

are not evenly divided -- there areas many green pixels as there areblue and red combined. This isbecause the human eye is notequally sensitive to all threecolors. It's necessary to includemore information from the green

pixels in order to create an imagethat the eye will perceive as a"true color."


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The advantages of this method are that only one sensor is

required, and all the color information (red, green and blue) is

recorded at the same moment. That means the camera can be

smaller, cheaper, and useful in a wider variety of situations. The

raw output from a sensor with a Bayer filter is a mosaic of red,

green and blue pixels of different intensity.


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As camcorders produce moving images, their CCDs have some additional

pieces that arent found in digital camera CCDs. To create a video signal, a

camcorder CCD must take many pictures every second, which the camera then

combines to give the impression of movement.

As we know a television "paints" images in horizontal lines across a screen, starting

at the top and working downward. TVs actually paint every other line in one pass

(this is called a "field") and then paint the alternate lines in the next pass. To create a

video signal, a camcorder captures a frame of video from the CCD and records it as

the two fields. The CCD actually has another sensor layer behind the image sensor.

For every field of video, the CCD transfers all the photosite charges to this second

layer, which then transmits the electric charges at each photosite, one by one. In ananalog camcorder, this signal goes to the VCR, which records the electric charges

(along with color information) as a magnetic pattern on videotape. While the second

layer is transmitting the video signal, the first layer has refreshed itself and is

capturing another image.


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A digital camcorder works in basically the same way, except that at this

last stage an analog-to-digital converter samples the analog signal and

turns the information into bytes of data (1s and 0s). The camcorder records

these bytes on a storage medium, which could be, among other things, a

tape, a hard disk or a DVD. Most of the digital camcorders in the markettoday actually use tapes (because they are less expensive), so they have a

VCR component much like an analog camcorder's VCR. Instead of

recording analog magnetic patterns, however, the tape head records binary

code. Interlaced digital camcorders record each frame as two fields, just as

analog camcorders do. Progressive digital camcorders record video as an

entire still frame, which they then break up into two fields when you output

the video as an analog signal.


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SCANNINGSCANNING


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SCANNINGSCANNING

The act of systematically moving a finely focused beam of light or electrons over aThe act of systematically moving a finely focused beam of light or electrons over asurface in order to produce an image of it for analysis or transmission.surface in order to produce an image of it for analysis or transmission.

The process of translating photographs into a digital form that can be recognized by aThe process of translating photographs into a digital form that can be recognized by acomputer .computer .

Pictures displayed on television screens sold today, are made up of a series ofPictures displayed on television screens sold today, are made up of a series ofhorizontal lines. These lines are comprised of tiny dots, called pixels. The number ofhorizontal lines. These lines are comprised of tiny dots, called pixels. The number ofthese lines and dots determine the "resolution" of the picture we see. The higher thethese lines and dots determine the "resolution" of the picture we see. The higher the

resolution, the sharper the picture.resolution, the sharper the picture.Television pictures are thus created by a sequence of lines of resolution scannedTelevision pictures are thus created by a sequence of lines of resolution scanned

and displayed in one of two different ways. One is calledand displayed in one of two different ways. One is called progressive scanningprogressive scanning andandthe other isthe other is interlaced scanninginterlaced scanning..

Progressive scanning creates an image by scanning horizontal lines made up ofProgressive scanning creates an image by scanning horizontal lines made up oftiny dots. These lines are scanned starting with the first line then the second, third,tiny dots. These lines are scanned starting with the first line then the second, third,fourth, fifth, and so on until the entire frame is scanned. Thirty of these frames arefourth, fifth, and so on until the entire frame is scanned. Thirty of these frames arescanned in one second. These frames, like the frames of a motion picture, whenscanned in one second. These frames, like the frames of a motion picture, when

played back one after the other create the look of motion. Whereas in Interlacedplayed back one after the other create the look of motion. Whereas in Interlacedscanning the display alternates between drawing the evenscanning the display alternates between drawing the even--numbered lines and thenumbered lines and theoddodd--numbered lines of each frame. It is a method of storing or transmitting video innumbered lines of each frame. It is a method of storing or transmitting video inwhich the odd and even fields are separated.which the odd and even fields are separated.

Look at how scanning is done through these slides


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INTERLACED SCANNINGINTERLACED SCANNING

It is aIt is a scanningscanning technique in which all oddtechnique in which all odd--numberednumbered scanningscanning lines are first tracedlines are first traced

in succession, followed by the tracing of the evenin succession, followed by the tracing of the even--numberednumbered scanningscanning lines inlines in

succession, each of which is traced between a pair of oddsuccession, each of which is traced between a pair of odd--numberednumbered scanningscanning lines.lines.

The pattern created by tracing the oddThe pattern created by tracing the odd--numberednumbered scanningscanning lines is called thelines is called the odd fieldodd field,,

and the pattern created by tracing the evenand the pattern created by tracing the even--numberednumbered scanningscanning lines is called thelines is called the eveneven

fieldfield. Each field contains half the information content,. Each field contains half the information content, i.e.,i.e.,pixels, of the complete videopixels, of the complete video

frame. Because of persistence of vision pairs of fields are perceived at the same time,frame. Because of persistence of vision pairs of fields are perceived at the same time,

giving the appearance of a full framegiving the appearance of a full frame

Our current analog interlaced picture is achieved by scanning 525 horizontal lines. The

Our current analog interlaced picture is achieved by scanning 525 horizontal lines. Thetotal number of lines scanned to produce an image is the resolution of the system, i.e.total number of lines scanned to produce an image is the resolution of the system, i.e.

525i meaning, 525 lines scanned using the interlaced method. A single frame of an525i meaning, 525 lines scanned using the interlaced method. A single frame of aninterlaced picture is made up by scanning the 525 lines, but not at the same time. First,interlaced picture is made up by scanning the 525 lines, but not at the same time. First,the 262.5 odd numbered lines are scanned.. Then the 262.5 even numbered lines arethe 262.5 odd numbered lines are scanned.. Then the 262.5 even numbered lines arescannedscanned..

An advantage of interlaced scanning is that Image flicker is less apparent in anAn advantage of interlaced scanning is that Image flicker is less apparent in aninterlacedinterlaced display than in a noninterlaced display, because the rate at which successivedisplay than in a noninterlaced display, because the rate at which successive

fields occur in anfields occur in an interlacedinterlaced display is twice that at which successive frames woulddisplay is twice that at which successive frames wouldoccur in a noninterlaced display containing the same number ofoccur in a noninterlaced display containing the same number ofscanningscanning lines andlines andhaving the same frame refresh rate.having the same frame refresh rate.


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Scanning one frame of Video

Click for scanning animation


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In anIn an interlacedinterlaced system, lines are drawn at a very slight diagonal slope such that thesystem, lines are drawn at a very slight diagonal slope such that theright end of each line is two lines lower than the left end. The offset between the tworight end of each line is two lines lower than the left end. The offset between the twofields is then produced by having both an odd number of overall lines and verticalfields is then produced by having both an odd number of overall lines and verticalflyback between the odd and even fields occur halfway through one line. For example, inflyback between the odd and even fields occur halfway through one line. For example, inPAL, the blanking period starts after 292.5 lines of the odd field have been transmitted,PAL, the blanking period starts after 292.5 lines of the odd field have been transmitted,and lasts for 20 lines. Whenand lasts for 20 lines. When scanningscanningbegins again at the top of the screen, thebegins again at the top of the screen, the scanningscanningbeam is still halfway across the picture. Because of the slant, the centre top of the picturebeam is still halfway across the picture. Because of the slant, the centre top of the pictureis one line above the line begun at the top left corner.is one line above the line begun at the top left corner.

Interlacing is used by all the analogue TV broadcast systems in current use:Interlacing is used by all the analogue TV broadcast systems in current use:

PALPAL: 50 fields per second, 625 lines, even field drawn first: 50 fields per second, 625 lines, even field drawn first

SECAMSECAM: 50 fields per second, 625 lines: 50 fields per second, 625 lines

NTSCNTSC: 59.94 fields per second, 525 lines, even field drawn first: 59.94 fields per second, 525 lines, even field drawn first

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