Bitmap images, vector graphics and sound
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 5 practice questions, 1 real past-paper question with the answer explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Pixels and samples
- Bitmap images
- File size of a bitmap
Read the transcript of the teaching part
Last time
Before we start: last time, in Binary, denary, hexadecimal and BCD, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Pixels and samples
A photo is a grid of tiny coloured dots. A song is a long list of measurements of air pressure. In both cases, the computer stores numbers, and the quality and the file size both depend on how many numbers, and how many bits for each.
Bitmap images
A bitmap image is made of pixels. Each pixel's colour is stored as a binary number. The resolution is the number of pixels: for example, 2048 wide by 1024 high. The colour depth, or bit depth, is the number of bits for each pixel. Eight bits gives 256 colours. More pixels or a greater colour depth mean a better image, but a bigger file, which takes longer to download and costs more to store.
File size of a bitmap
File size in bits equals width, times height, times colour depth. Divide by eight for bytes, then by 1024 for kibibytes, and by 1024 again for mebibytes. Do not forget to include metadata if the question says so. Otherwise it is an estimate.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Bitmap images: resolution, colour depth and file size.
The whole chapter: Information Representation.
Chapter 1: Information representation
- Binary, denary, hexadecimal, BCD and prefixes 6 min
- Binary addition, subtraction, overflow and two’s complement 7 min
- Character sets, run-length encoding and compression 6 min
- Bitmap images, vector graphics and sound 6 min
Also for A Level Computer Science 9618
Which topics come up most in A Level Computer Science 9618 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.