← IGCSE Computer Science Paper 1

Colour depth

The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

Part of: IGCSE Computer Science 0478 — 1.2 Text, Sound and Images — Module 18, IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 17, ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 37, ACARA Digital Technologies Years 9–10 — Data Compression — Module 7

Understanding Colour depth in IGCSE Computer Science Paper 1

What it does: 8 bits per pixel means 8 binary digits for every pixel, which gives 2^8 = 256 possible colours. 1 bit has two values, 0 and 1. So each pixel can be one of 2 colours, such as black or white. True colour uses 8 bits each for red, green and blue. 3 x 8 = 24 bits per pixel.

Common mistakes: The number of colours is 2 raised to the power of the colour depth, so it should be 2 ^ colourDepth. Multiplying gives 16 instead of 256. 24-bit true colour is three 8-bit colours added together (8 + 8 + 8 = 24 bits per pixel). Multiplying gives 512, which is wrong.

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Learning

Duo

What does colour depth measure?

The number of pixels in the imageThe number of bits used for each pixel's colour

Matches

colour depth = 800 x 600 pixels

↓

Colour depth is not the number of pixels. 800 x 600 is the image resolution. Colour depth is the bits used per pixel.

Applying

Meanings

Term

colour depth

Meaning

The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

Practice

colourDepth ← 8 + 8 + 8

Testing

Gaps

depth = # black and white

Check Answer

Spots

1colourDepth ← 8

2colours ← 2 * colourDepth

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Quick Questions

1. What does colour depth measure?

A. The number of bits used for each pixel's colour ✓

B. The number of pixels in the image

Colour depth is bits per pixel. The number of pixels is the image resolution, which is a different idea.

2. How many colours can a 1-bit colour depth give?

A. 2 colours (e.g. black and white) ✓

B. 1 colour

1 bit has two values, 0 and 1. So each pixel can be one of 2 colours, such as black or white.

3. An image uses 8 bits per pixel. How many different colours is that?

A. 8 colours

B. 256 colours ✓

With n bits you get 2^n values. 2^8 = 256, so 8 bits per pixel gives 256 colours.

4. 24-bit true colour is made from:

A. Three 8-bit values (red, green, blue) ✓

B. One 24-bit grey value

True colour uses 8 bits each for red, green and blue. 3 x 8 = 24 bits per pixel.

5. Which change increases the colour depth of an image?

A. Adding more pixels to the image

B. Using more bits for each pixel ✓

Colour depth is about bits per pixel. Adding pixels changes the resolution, not the colour depth.

Match the Pairs

colour depth = 1 bit → Each pixel is stored with 1 bit, so it can only be one of 2 colours, such as black or white

colour depth = 24 bits → Each pixel uses 24 bits, made of three 8-bit values for red, green and blue, giving true colour

Colour depth counts the bits used for each pixel, so a depth of 8 bits gives 2^8 different colours → 8 bits per pixel = 2^8 = 256 colours

Colour depth is bits per pixel, not the number of pixels, so a 4-bit pixel can show 16 colours → 4 bits per pixel = 2^4 = 16 colours

colour depth = 800 x 600 pixels → Colour depth is not the number of pixels. 800 x 600 is the image resolution. Colour depth is the bits used per pixel.

1 bit per pixel = 1 colour → 1 bit has two values (0 and 1), so it gives 2 colours, not 1. The number of colours is 2^bits.

Key Term

colour depth

The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

Fill in the Blanks

1. depth = 1 # black and white

Hint: Type the number of bits per pixel that gives only two colours.

2. colours = 2 ** 8 # 256 colours

Hint: Type the colour depth, in bits per pixel, that gives 256 colours.

3. trueColour = 3 * 8

Hint: Type the number of bits used for each of the three colours (red, green and blue) in 24-bit true colour.

4. colours = 2 ** depth

Hint: Type the base number, because each bit has two possible values.

5. bits = pixels * depth

Hint: Type the operator that multiplies the number of pixels by the bits stored for each pixel.

Spot the Error

Question 1

A colourDepth ← 8

B colours ← 2 * colourDepth

C OUTPUT colours

Line B — The number of colours is 2 raised to the power of the colour depth, so it should be 2 ^ colourDepth. Multiplying gives 16 instead of 256.

Question 2

A red ← 8

B green ← 8

C blue ← 8

D colourDepth ← red * green * blue

E OUTPUT colourDepth

Line D — 24-bit true colour is three 8-bit colours added together (8 + 8 + 8 = 24 bits per pixel). Multiplying gives 512, which is wrong.

Question 3

A bitsPerPixel ← 1

B colours ← 2 ^ bitsPerPixel

C IF colours = 1 THEN

D OUTPUT "Black and white"

E ENDIF

Line C — 1 bit per pixel gives 2 colours (black and white), so the test should be colours = 2. A single colour would need 0 bits.

Question 4

A pixels ← 100

B bitsPerPixel ← 256

C totalBits ← pixels * bitsPerPixel

Line B — Colour depth is the number of bits per pixel, not the number of colours. 256 colours need 8 bits, so it should be bitsPerPixel ← 8.

Question 5

A colourDepth ← 24

B channels ← 3

C bitsEach ← colourDepth * channels

D OUTPUT bitsEach

Line C — 24 bits are shared between the red, green and blue channels, so divide by 3 to get 8 bits each. Multiplying gives 72, which is wrong.

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