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CAIE 9618 · Paper 1 · Topic 1.1.5

Image
Representation

Pixels · Resolution · Colour Depth · File Size · Bitmap vs Vector

CSZone Cambridge International AS & A Level Computer Science 9618
Learning Objectives

By the end of this video you will be able to...

Define pixel, resolution, and colour depth
Calculate the file size of a bitmap image
Describe how RGB colour is encoded in binary
Compare bitmap and vector graphics and state advantages of each
Pixels & Resolution

The Building Blocks of Digital Images

PIXEL
Short for picture element
The smallest unit of a digital image
Each pixel stores a colour value
RESOLUTION
Number of pixels in an image
Expressed as width × height (e.g. 1920 × 1080)
Higher resolution → more detail, larger file
Total pixels = width × height · e.g. 1920 × 1080 = 2,073,600 pixels (~2 megapixels)
Colour Depth

How Many Colours Can We Represent?

Colour depth is the number of bits used per pixel. More bits = more possible colours = better quality image.
Colour DepthNumber of ColoursCommon Use
1 bit2 (black & white)Simple monochrome images
8 bits256GIF images, thumbnails
24 bits16,777,216 (~16M)True colour (standard photos)
32 bits4,294,967,296 (~4B)True colour + transparency (alpha)
24-bit RGB: 8 bits each for Red, Green, Blue → 256 × 256 × 256 = 16,777,216 colours
File Size Formula

Calculating Bitmap Image File Size

FILE SIZE FORMULA
File size (bits) = width × height × colour depth
Divide by 8 for bytes, 8000 for KB, 8,000,000 for MB
WORKED EXAMPLE
Image: 800 × 600 pixels, 24-bit colour
File size = 800 × 600 × 24 = 11,520,000 bits
÷ 8 = 1,440,000 bytes
÷ 1,000,000 = 1.44 MB
Bitmap vs Vector

Two Ways to Store Images

BITMAP (RASTER)
Stores colour of each pixel as binary
Loses quality when enlarged (pixelation)
Larger file size for high resolution
Best for: photographs, complex images
VECTOR
Stores mathematical descriptions (paths, shapes, coordinates)
Scales to any size without quality loss
Smaller file size for simple graphics
Best for: logos, diagrams, typography
Image Metadata

Data About the Image

An image file contains not just pixel data but also metadata — information about the image itself.
Image width and height (in pixels)
Colour depth per pixel
File format (BMP, JPEG, PNG, etc.)
Date and time of creation (EXIF data)
Camera settings: ISO, aperture, GPS location
⚠ Metadata increases actual file size beyond the raw pixel calculation
Exam Practice

Cambridge-style questions

Question 1
A bitmap image is 1200 pixels wide and 900 pixels high, with a colour depth of 16 bits. Calculate the file size of this image in megabytes. Show your working.
3 marks
✓ MARK SCHEME
1 mark
1200 × 900 × 16 = 17,280,000 bits
1 mark
÷ 8 = 2,160,000 bytes
1 mark
÷ 1,000,000 = 2.16 MB
Exam Practice
Question 2
Give two advantages of using a vector graphic instead of a bitmap image for a company logo.
2 marks
✓ MARK SCHEME (any two)
1 mark
Can be scaled/resized to any size without loss of quality / no pixelation
1 mark
Smaller file size for simple graphics (only stores mathematical descriptions)
1 mark
Individual elements/objects can be easily edited separately
Common Mistakes

Don't lose easy marks

1
Dividing by 8,000 for MB — divide by 8 to get bytes, then by 1,000,000 to get MB (Cambridge uses SI). Don't confuse bytes and bits in the formula.
2
Saying "vector images are always smaller" — this is only true for simple graphics. A vector image of a photograph would actually be larger than a bitmap because every colour variation needs a separate path.
3
Forgetting that metadata increases file size — the calculated file size from the formula is a minimum. The actual file will be larger due to metadata and file format overhead.
4
Confusing resolution with colour depth — increasing resolution adds more pixels (more detail). Increasing colour depth adds more bits per pixel (more colours). These are separate properties.
Topic Summary — 1.1.5

What You Need to Know

KEY DEFINITIONS
Pixel: smallest unit of a digital image
Resolution: width × height in pixels
Colour depth: bits per pixel
24-bit: 8 bits × RGB → 16.7M colours
FILE SIZE FORMULA
Bits = width × height × colour depth
Bytes = bits ÷ 8
KB = bytes ÷ 1,000
MB = bytes ÷ 1,000,000
Metadata adds to actual file size
BITMAP vs VECTOR
Bitmap: pixels, loses quality when scaled
Vector: mathematical paths, scales losslessly
Bitmap best for photos
Vector best for logos/diagrams
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1.1.6
Sound Representation
Sampling · Bit Depth · Nyquist · File Size
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