V. Read the text and render its contents

Each tiny dot on the screen of a computer is called a picture element, or pixel. Images and text are formed by combining a large number of pixels.

In a bit-mapped display, the dots displayed on the screen correspond, pixel by pixel, with bits in the main memory of the computer. The bits are held in an area of the memory called the «refresh buffer» and are stored in groups that represent the horizontal and vertical position of the pixels on the screen and whether the pixels are on or off.

On monochrome systems, one bit in this «map» represents one pixel on the screen and can be either «on» or «off» (black or white). On colour systems, each pixel is a certain combination of the three primary colours: red, green and blue. The total number of colours, which can be shown on the screen, is called the colour palette. The size of this palette depends on the graphics adaptor, a separate videocard that converts the bits into visual signals. A graphics adaptor with 1 bit per primary colour can generate up to 8, or 2³, colours. A graphics adaptor with 8 bits per primary colour can generate 16,7 million, or (2³)⁸ colours.

Unit V

INPUT DEVICE. POINT AND CLICK!

Typically, a mouse is a palm-sized device, slightly smaller than a pack of cards. On top of the mouse there are one or more buttons for communicating with the computer. A «tail» or wire extends from the mouse to a connection on the back of the computer.

The mouse is designed to slide around on your desktop. As it moves, it moves an image on the screen called a pointer or mouse cursor. The pointer usually looks like an arrow or I-bar, and it mimics the movements of the mouse on your desktop.

What makes the mouse especially useful is that it is a very quick way to move around on a screen. Move the desktop mouse half an inch and the screen cursor will leap four inches. Making the same movements with the arrow keys takes much longer. The mouse also issues instructions to the computer very quickly. Point to an available option with the cursor, click on the mouse, and the option has been chosen.

Mice are so widely used in graphics applications because they can do things that are difficult, if not impossible to do with keyboard keys. For example, the way you move an image with a mouse is to put the pointer on the object you want to move, press the mouse button and drag the image from one place on the screen to another. When you have the image where you want it, you release the mouse button and the image stays there. Similarly, the mouse is used to grab one corner of the image (say a square) and stretch it into another shape (say a rectangle). Both of these actions are so much more difficult to perform with a keyboard that most graphics programs require a mouse.

The buttons on the mouse are used to select items at which the mouse points. You position the pointer on an object on the screen, for example, on a menu or a tool in a point program, and then you press the mousebutton to «select» it. Mice are also used to load documents into a program: you put the pointer on the file name and double-click on the name -that is, you press a mouse button twice in rapid succession .

Поиск по сайту: