Keyboard Input Model

The system provides device-independent keyboard support for applications by installing a keyboard device driver appropriate for the current keyboard. The system provides language-independent keyboard support by using the language-specific keyboard layout currently selected by the user or the application. The keyboard device driver receives scan codes from the keyboard, which are sent to the keyboard layout where they are translated into messages and posted to the appropriate windows in your application.

Assigned to each key on a keyboard is a unique value called a scan code, a device-dependent identifier for the key on the keyboard. A keyboard generates two scan codes when the user types a key—one when the user presses the key and another when the user releases the key.

The keyboard device driver interprets a scan code and translates (maps) it to a virtual-key code, a device-independent value defined by the system that identifies the purpose of a key. After translating a scan code, the keyboard layout creates a message that includes the scan code, the virtual-key code, and other information about the keystroke, and then places the message in the system message queue. The system removes the message from the system message queue and posts it to the message queue of the appropriate thread. Eventually, the thread's message loop removes the message and passes it to the appropriate window procedure for processing. The following figure illustrates the keyboard input model.

Keyboard input processing model

Moniter

The size of an approximately rectangular display is usually given as the distance between two opposite screen corners, that is, the diagonal of the rectangle. One problem with this method is that it does take into account the fact that when a rectangle with a given length to its diagonal, becomes more rectangular, and less square (its aspect ratio increases), and at the same time its diagonal remains the same, then the area of the rectangle decreases. That is, given the same diagonal, the area of the display decreases if its aspect ratios increases. For example, a 4:3 21 in (53 cm) monitor has an area of about 211 sq in (1,360 cm²), while a 16:9 21-inch widescreen has about 188 sq in (1,210 cm²).

This method of measurement is inherited from the method used for the first generation of CRT television, when picture tubes with circular faces were in common use. Being circular, only their diameter was needed to describe their size. Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangle was equivalent to the diameter of the tube's face. This method continued even when cathode ray tubes were manufactured as rounded rectangles.

Another problematic practice was using the size of a monitor's imaging element, rather than the size of its viewable image, when describing its size in publicity and advertising materials. Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to overscan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.

Comparison

CRT

Pros:

• High dynamic range (up to around 15,000:1 [1],) excellent color, wide gamut and low black level.
• Can display natively in almost any resolution and refresh rate
• No input lag
• Sub-millisecond response times
• Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
• Usually much cheaper than LCD or Plasma screens.

Cons:

• Large size and weight, especially for bigger screens (a 20-inch unit weighs about 50 lb (23 kg))
• High power consumption
• Geometric distortion caused by variable beam travel distances
• Older CRTs are prone to screen burn-in
• Produces noticeable flicker at low refresh rates

LCD

Pros:

• Very compact and light
• Low power consumption
• No geometric distortion
• Rugged
• Little or no flicker depending on backlight technology

Cons:

• Limited viewing angle, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.
• Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.
• Slow response times, which cause smearing and ghosting artifacts. Modern LCDs have response times of 8 ms or less.
• Only one native resolution. Displaying resolutions either requires a video scaler, lowering perceptual quality, or display at 1:1 pixel mapping, in which images will be physically too large or won't fill the whole screen.
• Fixed bit depth, many cheaper LCDs are incapable of truecolor.
• Input lag
• Dead pixels may occur either during manufacturing or through use.