IGCSE · Practice questions · Representing images: resolution and colour depth

Count the pixels.

Six original Cambridge-style questions on representing images, with worked solutions.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Pixels times depth.

Resolution: number of pixels (width × height).
Colour depth: bits per pixel; n bits give 2ⁿ colours.
Higher either: better quality, bigger file.
Size: width × height × colour depth bits, then ÷ 8 and ÷ 1024 as needed.

Explain

[2 marks]

Explain how a computer represents an image in binary.

The image is made of a series of pixels. ✓
The colour of each pixel is stored as a binary number, using the number of bits set by the colour depth. ✓

Recall

[2 marks]

Define resolution and colour depth.

Resolution: the number of pixels in the image. ✓
Colour depth: the number of bits used to represent each colour (pixel). ✓

Calculate

[3 marks]

An image is 1024 pixels wide and 768 pixels high with a colour depth of 16 bits. Calculate the file size in KiB. Show your working.

1024 × 768 × 16 = 12 582 912 bits. ✓
12 582 912 ÷ 8 = 1 572 864 bytes. ✓
1 572 864 ÷ 1024 = 1536 KiB. ✓

Calculate

[1 marks]

State how many different colours can be represented with a colour depth of 12 bits.

2¹² = 4096 colours. ✓

Apply

[2 marks]

A 640 × 480 image has a colour depth of 1 bit. The colour depth is changed to 8 bits. State the effect on the file size and on the image.

The file size becomes 8 times larger. ✓
The image can now show 256 colours instead of 2, so it can look more realistic. ✓

Analysis

[2 marks]

A designer needs an image format that can show 300 different colours. A student suggests a colour depth of 8 bits. Explain whether this is enough and state the smallest suitable colour depth.

8 bits give only 2⁸ = 256 colours, which is fewer than 300, so it is not enough. ✓
9 bits give 512 colours, so 9 bits is the smallest suitable colour depth. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.