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1.6 Computer Graphics All the Lines at Once from Top to Bottom

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1.6 Computer Graphics All the Lines at Once from Top to Bottom 1.6 Computer Graphics all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. In the figure, the odd-numbered lines represent scanning one half of the screen and the even-numbered lines represent scanning of the other half Fig. 1.2. Interlaced refresh procedure cathode ray tube A CRT is similar to a big glass bottle. It contains three electron guns that emits beam of electrons, deflection apparatus (magnetic or electrostatic) which deflects these beams both up and down and sidewise, and a phosphor-coated screen over which these beams impinge. The vacuum is necessary to let those electron beams travel across the tube without running into air molecules that could absorb or scatter them. The primary component in an electron gun is a negatively charged cathode encapsulated by a metal cylinder known as the control grid. A heating element inside the cathode causes the cathode to be heated up as the current is passed and as a result electrons emit from the cathode surface. These electrons are accelerated towards the CRT screen by a high positive voltage applied near the screen or by an accelerating anode. If allowed to continue uninterrupted, the naturally diverging electrons would simply flood the entire screen. The cloud of electrons is forced to converge to a small spot on the CRT screen by a focusing system using an electrostatic or magnetic field. Fig. 1.3. Cathode Ray Tube Introduction 1.7 A pair of deflection coils mounted outside/inside the CRT envelope deflects the concentrated electron beam sidewise. Horizontal deflection is obtained by one pair of coils and vertical deflection is obtained by other pair. The deflection amount is controlled by adjusting the current passing through the coils. Fig. 1.4. Deflection of electron beam in CRT When the electron beam converges on to a point on the phosphor-coated face of the CRT screen, the phosphor dots absorb some of the kinetic energy from the electrons. This causes the electrons in the phosphor atoms to jump to higher energy state. After a short time these excited electrons drop back to their earlier stable state, releasing their extra energy in the form of light energy. As long as these electrons return back to their stable state phosphor continues to glow gradually losing brightness which is known as phosphorescence. The duration of phosphoresce exhibit by phosphor is known as persistence. The brightness of the light emitted by a phosphor depends on the intensity with which the electron beam strikes the phosphor. The intensity of the beam can be regulated by applying measured negative voltage at the control grid. Corresponding to zero value in the frame buffer a high negative voltage is applied in the control grid which in turn will shut off the electron flow by repelling the electrons and stopping them from coming out of the gun and hitting the screen. The corresponding points on the screen will remain black. Similarly, a bright white spot can be created at a particular point by minimizing the negative voltage at the control grid of the three electron guns when they are directed to that point by the deflection mechanism. color monitor In a monochrome CRT there is only one electron gun, whereas in color CRT there are three electron guns each controlling the display of red, green and blue light respectively. Unlike the screen of a monochrome CRT which has a uniform coating of phosphor, the color CRT has three color phosphor-dots (dot triad) – red, green and blue. When struck by electron beam the red dot emits red light, the green dot emits green light and the blue dots emit blue light. Each triad is arranged in a triangular pattern, as are the three electron guns. The beam deflection arrangement allows all 1.8 Computer Graphics the three beams to be deflected at the same time to form a raster scan pattern. There are separate video stream for each RGB color component which drive the electron guns to create different intensities of RGB colors at each point on the screen. To ensure that the electron beam emitted from individual electron guns strikes only the correct phosphor dots, a shadow mask is used just before the phosphor screen. The mask is a fine metal sheet with a regular array of holes punched on it. The mask is so aligned that as the set of three beams sweeps across the shadow mask they converge and intersect at the holes and then hits the correct phosphor dot; the beams are prevented or masked from intersecting other two dots of the triad. Fig. 1.5. Shadow Masking of CRT Fig. 1.6. RGB and CMY Color Models Cutaway rendering of a color CRT: 1. Three Electron guns (for red, green, and blue phosphor dots) 2. Electron beams 3. Focusing coils 4. Deflection coils 5. Anode connection 6. Mask for separating beams for red, green, and blue part of displayed image 7. Phosphor layer with red, green, and blue zones 8. Close-up of the phosphor-coated inner side of the screen Introduction 1.9 color moDel A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three values or color components. The purpose of a color model is to facilitate the specification of colors in some standard generally accepted way. In computer graphics we generally use two color models RGB and CMY. rgB color model In the RGB model, each color appears as a combination of red, green, and blue. This model is called additive, and the colors are called primary colors. The primary colors can be added to produce the secondary colors. Each primary color can take on an intensity value ranging from 0 to 1. Mixing these three primary colors at different intensity levels produces a variety of colors. The collection of all the colors obtainable by such a linear combination of red, green and blue forms the cube shaped RGB color space. The corner of the RGB color cube that is at the origin of the coordinate system corresponds to black, whereas the corner of the cube that is diagonally opposite to the origin represents white. The diagonal line connecting black and white corresponds to all the gray colors between black and white. It is called gray axis. Color specification using the RGB model is an additive process. We begin with black and add on the appropriate primary components to yield a desired color. On the other hand, there is a complementary color model, called the CMY color model that defines colors using a subtracting process. Fig. 1.7. Primary and Secondary Colors for RGB and CMY Models The importance of the RGB color model is that it relates very closely to the way that the human eye perceives color. RGB is a basic color model for computer graphics because color displays use red, green, and blue to create the desired color. Therefore, the choice of the RGB color space simplifies the architecture and design of the system. However, RGB is not very efficient when dealing with real-world images. To generate any color within the RGB color cube, all three RGB components need to be of equal pixel depth and display resolution. Also, any modification of the image requires modification of all three planes. 1.10 Computer Graphics cmy color model The CMY color model is a subset of the RGB model and is primarily used in color print production. CMY is an acronym for cyan, magenta, and yellow. In the CMY model we begin with white and take away the appropriate primary components to yield a desired color. The corner of the CMY color cube that is at the origin corresponds to white, whereas the corner just opposite to origin represents black. Fig. 1.8. A single bit plane black & white frame buffer The following formulae summarize the conversion between the two color models. R 1 C C 1 R G = 1 – M M = 1 – G B 1 Y Y 1 B Differences between rgB and cmy • RGB is based on projecting. Red light plus Green light plus Blue light all projected together create white. Black is encoded as the absence of any color. • CMY is based on ink. Superimpose Cyan ink plus Magenta ink plus Yellow ink, and you get black. White is encoded by the absence of any color. • Prism uses RGB internally. Exporting in RGB will give you results very close to what you see on screen. • Even though it uses one more number to encode a color, the CMY scheme encodes a smaller “color space” than does RGB. • When a color is converted from RGB to CMY, the appearance may change. Most noticeably, bright colors in RGB will look duller and darker in CMYK. Bit planes, color Depth The appearance and color of a pixel of an image is a result of intersection of three primary colors (red, green, blue) at different intensities. When the intensities of all three electron beams are set to the highest level the result is a white pixel; when all are set to zero, the pixel is black. And for many different combinations Introduction 1.11 of intermediate intensity levels, several million color pixels can be generated. For a black and white monitor using a single electron gun, the phosphor material can glow with varies intensities depending on the intensity of the electron beam. As a result a pixel can be black or white or have different shades of gray. The number of different colors that can be displayed depends upon the intensities of the primary color that can be generated.
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