Compression (OCR GCSE Computer Science): Revision Notes

Compression

The Need for Compression

• Compression is used to reduce the size of files, making them easier to store and transfer.
• Large files, such as images, videos, and software, can take up significant storage and require a long time to upload/download.
• Compression is necessary to improve transfer speed, save storage space, and make file sharing more efficient.

Types of Compression

Lossy Compression

• Lossy compression permanently removes some data from the file to reduce its size.
• It is commonly used for media files like images, audio, and video (e.g., JPEG, MP3, MP4).

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Advantages:

• Significantly reduces file size, making it ideal for online streaming and storage. Disadvantages:

• Some data and quality are lost, meaning the original cannot be perfectly restored.

• Quality reduction may be noticeable, such as in blurry images or reduced audio clarity.

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Effect on file: The file becomes much smaller, but the quality decreases depending on the compression level.

Lossless Compression

• Lossless compression reduces file size without losing any data.
• Used for files where it is essential to maintain original quality, such as text files, ZIP files, and some image formats (e.g., PNG).

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Advantages:

• No loss of data, so the file can be restored to its original state.

• Ideal for compressing important documents and high-quality images. Disadvantages:

• Less reduction in file size compared to lossy compression.

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Effect on file: The file size is reduced, but quality is preserved.

Common Scenarios for Compression

• Media files: Images, videos, and music are compressed to reduce file size and speed up sharing over the internet.
• Documents: Text and data files (e.g., ZIP archives) are compressed using lossless compression to save space without losing information.
• Websites: Images and files on websites are often compressed to help them load faster, improving the user's experience.

Bitmap Images and File Size

• Bitmap images are made up of tiny squares called pixels. Each pixel stores a specific colour using a binary code.
• The colour depth of an image refers to how many bits are used to store each pixel's colour. More bits = more possible colours:
  • 1 bit can store 2 colours.
  • 2 bits can store 4 colours.
  • 3 bits can store 8 colours.
  • 4 bits can store 16 colours.
• Resolution refers to the number of pixels in an image. Higher resolution = more detail but a larger file size.

Image Terminology

• Pixel: The smallest unit of an image, representing one colour. Each pixel is stored as a binary value.
• Colour Depth: The number of bits used to represent the colour of a pixel. More bits allow more colours but increase file size (e.g., 4 bits = 16 colours, 8 bits = 256 colours).
• Resolution: The density of pixels in an image, usually measured in dots per inch (DPI). Higher DPI = more pixels, higher quality, and larger file size.
• Metadata: Extra information stored with an image, such as its resolution, colour depth, file format, and camera settings (e.g., exposure, ISO, aperture).

Effect of Colour Depth and Resolution on File Size

• Larger colour depth and higher resolution result in better image quality, but also larger file sizes.
• Smaller colour depth and lower resolution create smaller file sizes, but the image quality decreases.

Calculating Image File Size

• File size formula: Multiply the image's resolution by its colour depth (in bits).
• To convert bits to bytes, divide by 8. For example, a 100x100 pixel image with 8-bit colour depth would have a size of:
  • Size = 100 x 100 x 8 bits = 80,000 bits
  • Divide by 8 to get 10,000 bytes (or 10 KB).