Computer Science 9618/11 — October/November 2023

Background Concept

Vector graphics store an image as a set of mathematically defined objects rather than as individual coloured pixels. Each shape in the image is described by data such as its type, position, size, colour and line thickness.

A drawing object is one individual component of the image, for example a circle, line or rectangle. It is created from formulas or coordinates.

A property is one attribute of that object, such as its fill colour, width, height, radius or position.

A drawing list is the full collection of data needed to recreate the entire image. It contains all the objects and the properties for each one.

Understanding the Question

This question gives three vector graphics terms and three descriptions. The task is simply to pair each term with the correct meaning.

The key is to think about scale:

• one characteristic of one component = property
• one whole component = drawing object
• all the information for the whole graphic = drawing list

Approach

Match each term by asking what level of information it represents:

1. Does it describe a single feature? That is a property.
2. Does it describe one shape/component? That is a drawing object.
3. Does it describe the complete set of instructions for the image? That is the drawing list.

Step-by-Step Reasoning

• A drawing object is a single item in the image, such as a shape. In vector graphics, shapes are generated using mathematical descriptions, so it matches "a component created using a formula".
• A property is an attribute of an object. For example, colour or line thickness is one characteristic, so it matches "defines one characteristic of a component".
• The drawing list must therefore be the remaining description. It is the set of data used to build the whole image, so it matches "data required to create all components in the graphic".

Key Takeaways

• Vector graphics are stored as objects, not pixels.
• A drawing object is one shape or component.
• A property is one characteristic of that shape.
• A drawing list is the complete set of information for the whole graphic.

Common Mistakes

• Confusing drawing object with drawing list: a drawing object is one component, while the drawing list is the full collection of all components.
• Thinking a property is the whole shape: it is only one feature of the shape.
• Mixing up bitmap ideas with vector ideas: vector graphics use formulas and object data, not a grid of pixels.

Things to Be Careful About

• Read each description precisely: "one characteristic" points to property.
• "All components in the graphic" shows that the answer must be drawing list, not drawing object.
• In vector graphics questions, words like formula, coordinates and object usually signal vector storage rather than bitmap storage.

Background Concept

A bitmap image is made from pixels arranged in a grid. Each pixel stores colour information. Bit depth tells us how many bits are available to represent the colour of one pixel.

Because bits are binary, the number of possible colours or shades depends on powers of 2:

• 1 bit gives 2 possibilities
• 8 bits gives 256 possibilities
• 24 bits gives over 16 million possibilities

So bit depth controls how accurately colours can be represented. More bits per pixel means a wider range of colours and smoother shading. It also means more data must be stored for the image.

Understanding the Question

This question has two parts hidden inside one sentence:

1. State what bit depth means.
2. Explain what happens when the bit depth changes.

So a complete answer must not stop at the definition. It must also link bit depth to the effect on the image itself.

Approach

Give the definition first in the standard exam wording: bits per pixel.

Then explain both effects of changing it:

• visual effect: more or fewer colours/shades, affecting realism/detail
• storage effect: more or fewer bits per pixel, affecting file size

That gives a full explanation rather than a one-point definition.

Step-by-Step Reasoning

• A bitmap stores each pixel separately.
• The bit depth is therefore about one pixel at a time.
• If each pixel has more bits available, it can represent more different colour values.
• More available colour values means the image can show finer colour changes, smoother gradients and more realistic detail.
• However, because every pixel now uses more bits, the total amount of data in the file increases.
• If the bit depth is reduced, the reverse happens: fewer colours can be stored, the image may look less realistic or more blocky/posterised, but the file size is smaller.

A strong answer normally mentions both quality and file size.

Key Takeaways

• Bit depth = number of bits used to represent one pixel.
• Higher bit depth means more colours/shades.
• More colours usually improve realism and detail.
• Higher bit depth also increases file size.

Common Mistakes

• Defining bit depth as number of pixels: that is resolution, not bit depth.
• Saying bit depth changes the physical size of the image: it changes colour detail, not image dimensions.
• Mentioning only quality or only file size: good exam answers usually link both.
• Confusing bit depth with resolution: resolution is how many pixels there are, while bit depth is how much data each pixel stores.

Things to Be Careful About

• Use the phrase "bits per pixel" in the definition.
• If you say "more colours", make sure you link it to increased bit depth.
• If you mention file size, explain why: each pixel uses more bits.
• Avoid vague statements like "the image gets better" without saying that it is because more colours or shades can be represented.

Background Concept

Compression reduces the amount of data needed to store a file. Images are often large, especially bitmap images, because every pixel must be stored. Compression helps make the file smaller.

There are two main types:

• lossless compression: reduces size without losing original data
• lossy compression: reduces size by discarding some data, usually data that has less visible effect on the image

For email, the main reason compression matters is practical transmission and storage.

Understanding the Question

The question asks why a bitmap image is often compressed before being attached to an email. This means the answer should focus on sending the file, not just on storage in general.

The important idea is that bitmap images can be large, and email systems have limits and transmission time considerations.

Approach

Start from the immediate effect of compression: smaller file size.

Then link that smaller size to email use:

• quicker to upload and send
• quicker for the receiver to download
• more likely to stay within attachment limits
• less bandwidth/storage used

For a short 2-mark answer, the most valuable linked points are file size and transmission/attachment limits.

Step-by-Step Reasoning

• A bitmap image stores colour data for many individual pixels.
• That can create a large file, especially if the image has high resolution or high bit depth.
• Compression removes some redundancy or reduces the amount of data stored.
• The result is a smaller file.
• Smaller files are easier to email because they upload more quickly from the sender's device.
• They also download more quickly for the recipient.
• Many email services limit the maximum attachment size, so compression helps keep the file within that limit.

So the full logic is: bitmap files can be large, compression makes them smaller, and smaller files are better suited to email transmission.

Key Takeaways

• Bitmap images are often large because they store pixel-by-pixel data.
• Compression reduces the amount of data in the file.
• Smaller files are faster to send and receive.
• Email attachment size limits make compression useful.

Common Mistakes

• Saying compression improves image quality: compression is mainly used to reduce size; lossy compression may reduce quality.
• Giving only "to save space" with no link to email: the question is specifically about attachment to an email.
• Confusing compression with encryption: compression reduces size, while encryption protects the data.

Things to Be Careful About

• Link your answer to email, not just to images in general.
• If you mention faster sending, remember this is because the file size is smaller.
• Do not claim that all compression keeps the image identical; only lossless compression does that.
• A concise explanation is enough, but it should include both reduced size and the benefit when emailing.