1.3.2 | STORAGE AND COMPRESSION

Topics from the Cambridge IGCSE (9-1) Computer Science 0984 syllabus 2023 - 2025.

OBJECTIVES
​1.3.2 Calculate the file size of an image file and a sound file, using information given

ALSO IN THIS TOPIC
1.1.1 NUMBER SYSTEMS
1.1.2 NUMBER SYSTEMS
1.1.3 NUMBER SYSTEMS 
1.1.4 NUMBER SYSTEMS
 ​1.1.5 NUMBER SYSTEMS
1.1.6 NUMBER SYSTEMS
1.2.1 TEXT, SOUND AND IMAGES
​​1.2.2 TEXT, SOUND AND IMAGES
​1.2.3 TEXT, SOUND AND IMAGES
​1.3.1 STORAGE AND COMPRESSION
YOU ARE HERE | ​​1.3.2 STORAGE AND COMPRESSION
​1.3.3 STORAGE AND COMPRESSION
​1.3.4 STORAGE AND COMPRESSION
TOPIC 1 KEY TERMINOLOGY
TOPIC 1 ANSWERS
TOPIC 1 TEACHER RESOURCES (CIE)

---

CALCULATING IMAGE FILE SIZE

IMAGE 1 : 8 x 8 pixels

IMAGE 2 : 16 x 16 pixels

To calculate the file size of the two images above, firstly although only two colours have been used, lets presume a colour depth of 8 bit for both images.
Image 1 calculation:
Firstly; calculate the total amount of pixels used in the image
8 pixels wide by 8 pixels tall:  8 x 8 = 64 (A total of 64 pixels used to represent the entire image)
Secondly; multiply the total pixels used by the colour depth (8 bits have been used to represent the content of each pixel)
64 pixels in total multiplied by 8(colour depth): 64 x 8 = 512
Answer: Image 1 has a file size of 512 bits

Image 2 calculation: 
16 pixel by 16 pixels: 16 x 16 = 256
256 pixels x 8 bit colour depth: 256 x 8 = 2048
Answer: Image 2 has a file size of 2048 bits

In practise images are much larges and the pixel density is much greater than the two example above.

IMAGE 3: Dimensions 1600 pixels x 1200 pixels : Bit Depth 24

Follow the same steps to calculate the file size of image 3, this time convert the file size from bits too Megabytes.
Step 1: Multiply the number of horizontal pixels by the number of vertical pixels: 1600 x 1200 = 1920000 pixels
Step 2: Multiply the total pixels used by the colour/bit depth: 1920000 x 24 = 46080000 bits
Step 3: Divide the answer from step 2 by 8 to give the number of bytes used: 46080000 / 8 = 5760000 bytes
Step 4: Divide the number of bytes used by 1024 to give the value in Kilobytes: 5760000 / 1024 = 5625 Kilobytes
Step 5:  Divide the number of kilobytes used by 1024 to give the value in Megabytes: 5625 / 1024 = 5.4 Megabytes
​
The original image 3 has a file size of 5.2Mb, this value excludes any metadata storage used and was the file size before being compressed for use over the internet

IMAGE SIZE

1: An image has the following properties:

• Dimensions 1024 pixels x 575 pixels
• Bit Depth 24

Calculate the image size, show your workings at each stage and display the result in Mb.

2: An image has the following properties:

• Dimensions 12 inch x 11 inch
• Bit Depth 16
• 72 pixels per inch

Calculate the image size, show your workings at each stage and display the result in Mb.

3: Besides the size and bit depth of an image, what else could have an impact on the total file size?

4:Give two situation where reducing the file size of an image is needed, explain the impact of doing this.

NAVIGATION

1.1.1 NUMBER SYSTEMS | Why Computers use Binary
1.1.2 NUMBER SYSTEMS | Binary, Hex and converting between number systems 
1.1.3 NUMBER SYSTEMS | Benefits of Hex
1.1.4 NUMBER SYSTEMS | Binary Addition and Overflow
1.1.5 NUMBER SYSTEMS | Binary Shifts
1.1.6 NUMBER SYSTEMS | Negative Number Representation, Two's Complement
1.2.1 TEXT, SOUND AND IMAGES | How Computers Represent Text
1.2.2 TEXT, SOUND AND IMAGES | How Computers Represent Sound
1.2.3 TEXT, SOUND AND IMAGES | How Computers Represent Images
1.3.1 STORAGE AND COMPRESSION | How Data Storage is Measured
​1.3.2 STORAGE AND COMPRESSION | Calculating Image and Sound File Size
​1.3.3 STORAGE AND COMPRESSION | Purpose of Data Compression
​1.3.4 STORAGE AND COMPRESSION | Lossy and Lossless, how files are compressed
TOPIC 1 KEY TERMINOLOGY
TOPIC 1 ANSWERS
TOPIC 1 TEACHER RESOURCES (CIE)