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Color CRT Monitors

• Color pictures usually produced in CRT monitors by using a combination of phosphors that emit different-colored light

• A wide range of colors can be produced by combining the emitted light of different phosphors

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• The two main methods for generating color display with a CRT are:

Beam-Penetration methodShadow-mask method

• The Beam-Penetration Method:

It is used usually with random-scan systems. The CRT screen coated from inside with two layers of phosphor, usually Red and Green.

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the production of colors depends on how far the electron beam penetrates into the phosphor layers.

Red is produced by a slow electron beam, Green color is generated by a very fast electrons penetrates through the red layer and excites the inner green layer. Orange and Yellow are generated by an intermediate beam speeds, as a combination of red and green.

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• Advantages:Inexpensive way to produce color in random-scan displays

• Disadvantages:Only four colors could be produced.The quality of pictures is not as good as with other methods

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Shadow-Mask Method • is used usually in raster-scan displays.

• it is capable of generating wider range of colors than the beam-penetration method.

• Each pixel of the screen is coated with three phosphor color dots, one for emitting red, another for emitting green, and the third for emitting blue.

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• This method used three electron guns, each gun is dedicated to activate one of the three phosphor color dots.

• A shadow-mask grid usually placed behind the phosphor-coated screen.

• the delta-delta shadow mask method is used commonly in color CRT monitors.

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• The shadow mask has a series of holes aligned with the phosphor-dot patterns, to enable the three electron beams to pass through to activate a dot triangle, which appears as a small color spot on the screen.

• The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.

• By varying the intensity levels of the three electron beams we obtain a wide range of colors.

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• The color depends on the amount of excitation of R, G, B phosphors. High quality raster graphics systems have 24 bits per pixel in the frame buffer, allowing 256 voltage for each electron gun and nearly 17 million color choices for each pixel.

• In-line arrangement of the three electron guns. the red-green-blue color dots on the screen, are aligned along one scan line. It is easier to keep the electron guns aligned and it is used in high-resolution color CRTs.

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Examples:

Delta electron gun arrangement In-line electron gun arrangement

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Examples:

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Flat-Panel DisplaysAdvantages:

Small Volume.Light Weight.Low Power Consumption.Thin.

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FPDs are divided into two main categories:1. Emissive Displays: convert electrical energy into light.

Examples:Plasma panelsThin-Film Electroluminescent DisplaysLight Emitting DiodesFlat CRTs

2. Non-Emissive Displays: Use optical effects to convert sunlight or light from some other source into graphics patterns. Example: Liquid Crystal Display (LCD)

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Plasma Panels• Constructed by filling the region between two

glass plates with a mixture of gases that usually includes neon.

• A series of vertical conducting ribbons is placed on one glass panel, and a set of horizontal conducting ribbons is built into the other glass panel.

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• Firing voltages applied to an intersecting pair of horizontal and vertical conductors cause the gas at the intersection of the two conductors to break down into a glowing plasma of electrons and ions.

• Picture definition is stored in a refresh buffer, and the firing voltages are applied to refresh the pixel positions (at the intersection of the conductors) 60 times per second.

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Thin-Film Electroluminescent Displays

• Similar in construction to plasma panels. But the region between the two glass plates is filled with a phosphor, such as zinc sulfide doped with manganese.

• When a high voltage is applied to a pair of crossing electrodes, the phosphor becomes a conductor in the area of the intersection of the two electrodes.

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• Electrical energy is absorbed by the manganese atoms, which then release the energy as a spot of light.

• Electroluminescent displays requires more power than plasma panels, and good color displays are harder to achieve.

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Light Emitting Diode (LED)

• A matrix of diodes is arranged to form the pixel positions in the display, and picture definition is stored in a refresh buffer.

• Information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

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Liquid Crystal Displays (LCDs)Used in small systems such as laptops and calculators.

Produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crystal material that can be aligned to either block or transmit the light.

Liquid-crystal: these compounds have a crystalline arrangement of molecules, they flow like a liquid.

FPDs use nematic (threadlike) Liquid-crystal compounds that keep the long axes of the rod-shaped molecules aligned.

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Passive-matrix LCD

• Two glass plates, each containing a light polarizer that is aligned at a right angle to the other plate, sandwich the liquid-crystal material.

• Rows of horizontal, transparent conductors are built into one glass plate, and columns of vertical conductors are put into the other plate.

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• The intersection of the two defines a pixel position. Polarized light passing through the material is twisted so that it will pass through the opposite polarizer. The light is then reflected back to the viewer.

• To turn off the pixel, we apply a voltage to the two intersecting conductors to align the molecules so that the light is not twisted.

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Active-matrix LCD

• This type of LCD is constructing by placing a transistor at each pixel location, using thin-film transistor technology.

• The transistors are used to control the voltage at pixel locations and to prevent charge from gradually leaking out of the liquid-crystal cells.

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Liquid Crystal Displays (LCDs)

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Raster-Scan Systems

• Interactive raster graphics systems naturally use several processing units, as well as the CPU, a special-purpose processor, called the video controller or display controller is used to control the operation of the display device.

• Frame buffer can be anywhere in the system memory.

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Raster-Scan Systems

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Raster-Scan Systems• Video controller (Display controller)

accesses the frame buffer to refresh the screen.

• Sophisticated raster systems use additional processors as coprocessors and accelerators to implement various graphics operations.

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Architecture of a raster system with a fixed portion of the system memory reserved for the frame buffer

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Basic Video-controller Refresh operations

1- Two registers are used to store the coordinates of the screen pixels.* Initially x register is set initially to 0* Initially y register is set initially to ymax.

2- Retrieve Intensity value stored in the frame (refresh) buffer for the above pixel, and using it to set the intensity of the CRT electron beam.

3- Incrementing x register by 1.4- If (the end of present scan line reached) and (y ≠0) then {horizontal retrace}

* Reset x register to 0.* Decrement y register by 1.* GoTo 2.

5- If (x > xmax) and (y = 0) then {vertical retrace}* Reset x register to 0.* Reset y register to ymax.* GoTo 2.

6- If (x ≤ xmax) then* GoTo 2

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Basic Video-controller Refresh operations

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Video Controller • The above refresh procedure could not be handle by

some RAM chips if the cycle time is too slow. To speed up pixel processing, video controllers can retrieve multiple pixel values from the refresh buffer on each pass.

• The multiple pixel intensities are then stored in a separate register and used to control the CRT beam intensity for a group of adjacent pixels. When that group of pixels has been processed, the next block of pixel values is retrieved from frame buffer.

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Video Controller• In some systems, for example, multiple frame buffers

are often provided, so that one buffer can be used for refreshing while pixel values are being loaded into the other buffers. Then the current refresh buffer can switch roles with one of the other buffers.

• This provides a fast mechanism for generating real-time animations, for example, since different views of moving objects can be successively loaded into a buffer without interrupting a refresh cycle.

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Video Controller• Another video controller task is to carry out some

transformations like scaling, translating, and rotating part of the screen from one location to another during the refresh cycles.

• The video controller often contains a Lookup Table, so that pixel values in the frame buffer are used to access the lookup table instead of controlling the CRT beam intensity directly. This provides a fast method for changing screen intensity values.

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Video Controller

• Some systems are designed to allow the video controller to mix the refresh-buffer image with an input image from a video camera or other input device.

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Raster-Scan Display Processor

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Raster-Scan Display Processor

• Referred to as display processor, a graphics controller or a display coprocessor.

• The purpose of the display processor is to free the CPU from the graphics chores.

• a separate display-processor memory area can be supplied.

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Run-length encoding technique• Is used to reduce memory requirements in raster systems.

• organizes the frame buffer as a linked list and encoding the color information.

• store each scan line as a set of number pairs. The first number each pair can be a reference to a color value, and the second number can specify the number of adjacent pixels on the scan line that are to be displayed in that color.

• This technique can result in a considerable saving in storage space if a picture is to be constructed mostly with long runs of a single color each.

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Raster-Scan Display Processor

Display-processor operations:

1. Scan conversion:Digitizing a picture definition given in an application program into a set of pixel values for the storage in the frame buffer.

Graphics commands specifying straight lines and other geometric objects are scan converted into a set of discrete points, corresponding to screen pixel positions.

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2. Generating various lines styles (dashed, dotted, or solid).

3. Displaying color areas.

4. Performing certain transformations on objects.

5. Interface with interactive input devices, such as a mouse.

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Frame buffers• Frame buffer: is a memory area in which the

computer stores an image– On most computers, separate memory bank from

main memory.– Many different variations, motivated by cost of

memory

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Color and Gray Scale

Color information can be stored in the frame buffer in two ways:1. Directly:

RGB color codes can be stored directly in the frame buffer.

2. Using Color Lookup Tables (CLUTs):Put the color codes into a separate table and use the pixel locations to store index values referencing the color-table entries.

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Color Raster Images

• The color depth is the number of bits used to represent the color of each pixel. A color depth of 3 would allow one bit for each color component.

• A byte is often used to represent the pixel color yielding 256 different colors.

• True-color images have a color depth of 24 with each component using one byte.

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Color Codes (3 bits/pixel frame buffer)

white1 1 1Yellow1 1 0

Magenta1 0 1Red1 0 0

Cyan0 1 1Green0 1 0Blue0 0 1black0 0 0

Displayed ColorR G B

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FRAME BUFFERS & CLUT• Each entry in the frame buffer is an index into the

CLUT. • If n bits/pixel 2n entries in the CLUT • CLUT entry then determines the color sent to the

screen. • If each CLUT entry is p bits, then can display 2p

possible colors• (example if p=24 (16777216) ≈17 million

colors in the palette)• Can display only 2n colors simultaneously.

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CLUT: Example

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CLUT: Advantages

1. Do not require large frame buffers.

2. The number of different colors stored in CLUT are sufficient for a single picture.

3. Table entries can be changed at any time without changing the attribute setting for graphics data structures.

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Loading The Frame-Buffer• Since the scan conversion algorithms generate pixel

positions at successive unit intervals we use incremental methods to calculate frame buffer address for a pixel.

• Suppose the frame buffer array is addressed in row-major order:

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For a bilevel system (1 bit per pixel),the Frame-Buffer bit address for pixel position (x, y) is calculated as:1. addr(x, y) = addr (0,0) + y(xmax +1) + x

Example: Find the address of the pixel (3,4), where the address of (0,0) =100, xmax=10, and ymax = 8.Solution:

addr(x, y) = addr (0,0) + y(xmax +1) + xaddr(3, 4) = 100 + 4(10 +1) + 3 = 147

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2. addr(x + 1, y) = addr(x, y) + 1Example: Address(4, 4) = 147 + 1 = 148

3. addr(x + 1, y + 1) = addr(x, y) + xmax + 2Example: Address(4, 5) = 147 + 10 + 2 = 159