This digital resource follows the IGCSE ICT textbook Unit 6: Graphics.

Key features:

Graphic software can produce two types of images:

Resolution: The number of pixels in an image. Higher resolution means more pixels, which creates better image quality but larger file sizes.

中文说明：分辨率是图像中像素的数量。分辨率越高，图像越清晰，文件越大。

▲ Figure 16.2 Using a 256-pixel grid (right) allows you to represent a circle more accurately than using a 9-pixel grid (left)

Higher resolution images require more storage than lower resolution images because data has to be stored for a larger number of pixels. This means that higher resolution images have larger file sizes than lower resolution images.

Pixelation: If a bitmap graphic is enlarged, then each pixel is enlarged. This means that if the image is enlarged too much, individual pixels become visible. This is referred to as pixelation and can be seen in Figure 16.3.

中文说明：当位图被放大时，像素变得明显，这种现象叫像素化。

▲ Figure 16.3 An image will pixelate when enlarged, as the individual pixels are made more obvious

Vector graphics do not pixelate when enlarged because they are made up of points or co-ordinates and lines. These are recalculated each time the image is redrawn, either by a printer or on a screen. Although vector graphics can look unrealistic or stylised when compared with bitmap images, they create much smaller files because data for each pixel does not need to be stored.

中文说明：矢量图是用形状和数学坐标创建的图像。无论放大或缩小，都不会出现像素化。

Vector graphics can be used to create image styles that are difficult to achieve in a bitmap image. For example, they are often used to create cartoons, technical illustrations and landscapes used in computer games, all of which need to be redrawn at different sizes in some way for the purposes of animation and to give the appearance of depth and movement.

Figure 16.4 shows the key difference between vector graphics and bitmap images. Because a bitmap image is made up of thousands of pixels, it pixelates when the viewer zooms in too far. The vector graphic does not pixelate because it is made of a series of simple mathematical co-ordinates, which means that, when the viewer zooms in, the image is accurately reproduced at a larger size.

▲ Figure 16.4 The differences between vector graphics and bitmap images

When creating images, you must be able to combine basic shapes (形状) such as squares, rectangles, circles, ovals, triangles and arrows.

中文说明：基本图形，如矩形、圆形、三角形和箭头。

Although you can use specialist graphics applications, many of these shapes are also available in applications that are not obviously associated with graphics, such as Microsoft Word. For more information about the shapes available in word processing applications, see page 229. Often, you can choose from a range of basic shapes or create your own from lines. You also need to be able to incorporate lines (线条) and text, including text boxes (文本框).

Lines, shapes and text can have their fill colours altered, as shown in Figure 16.5.

Change the colour inside a shape. You can use solid colours or gradients.

The colour and thickness of the outside lines defining the shape can also be altered, as shown in Figure 16.6.

▲ Figure 16.6 The square's line colour and thickness has been altered

Figure 16.7 shows an example of the way in which basic shapes can be combined to produce an image. The basic shapes used to make the image on the left are shown on the right.

Layers are like transparent sheets stacked on top of each other. Each layer can contain different parts of an image. Layers help you organise complex images.

中文说明：图层就像透明的纸张叠在一起。每个图层可以包含图像的不同部分。图层帮助你组织复杂的图像。

Figure 16.8 is an example of a graphic produced using curved lines filled with colour.

▲ Figure 16.8 A graphic that uses curved lines, some of which are filled with colour; the layers show the individual elements used to create the image

In Figure 16.9, you can see the list of layers as displayed in LibreOffice Draw®. This shows the individual elements used to create the image and the order in which they are placed on the z axis. In Figure 16.9, you can see that the Outline layer is on top of the Color layer on the z axis.

▲ Figure 16.9 The layers used to create the image in Figure 16.8 (this is a close-up of the feature on the right of the screen in Figure 16.8)

Image editing tools allow you to change an image in a number of ways.

You can resize or rotate an image by clicking the image with your mouse to show its handles. These are the resize points and rotate points shown in Figure 16.10.

中文说明：改变图像的大小。围绕一个点旋转图像。

Resizing an image by dragging the handles in the corners of the image maintains the image's aspect ratio. However, resizing the image by dragging the handles on the sides of the image does not maintain its aspect ratio, meaning that the image appears distorted.

中文说明：图像宽度和高度之间的比例关系。图像被拉伸，原来的比例被破坏，称为变形。

▲ Figure 16.11 This image has been resized by dragging from the top or bottom, which means that it has lost its aspect ratio and has become distorted

Cropping an image means cutting off and deleting part or parts of it in order to make it a particular size or shape. For example, the image in Figure 16.12 has an original aspect ratio of 4:6. Figure 16.12 shows the image once it has been cropped to 5:7 and 8:10.

中文说明：裁剪是指删除图像中不需要的部分，使图像达到特定的大小或形状。

▲ Figure 16.12 Cropping allows images to fit different aspect ratios or to remove unwanted parts of an image

Skills: EXECUTIVE FUNCTION, COMMUNICATION, REASONING

Text can be added to an image. For example, you might add a caption to a photograph or add a slogan or strapline to a logo.

If there are areas of a bitmap image that you do not want, you can either erase them or recolour them so that they blend in with surrounding areas of the image. Figure 16.13 shows some of the tools available to do this in Microsoft Paint.

中文说明：擦除或覆盖图像中不需要的部分。可以使用颜色选择器工具选择附近区域的颜色，然后用画笔工具覆盖不需要的区域。

▲ Figure 16.13 Tools in Microsoft Paint

Most applications have a Colour Picker tool that allows you to select a colour from a nearby area of the image and then use a brush or pencil tool to cover the unwanted area with the selected colour. For example, you can remove the tower in the background of Figure 16.14 by picking the colours around it, as shown in Figure 16.15, and using a brush tool with this colour to paint over the tower, as shown in Figure 16.16. Doing this improves the image, as shown in Figure 16.17 and 16.18.

▲ Figure 16.14 The tower on top of a building on the right of this image is unwanted

▲ Figure 16.15 Using the Colour Picker to select a nearby colour

▲ Figure 16.16 Painting over the unwanted tower with the selected colour

▲ Figure 16.17 and 16.18 The improved image after removing the tower

Skills: EXECUTIVE FUNCTION