Higher Computing information slides relating to compression techniques used when storing bit-mapped file formats

1. Bit-mapped Compression
Techniques
Run Length Encoding (RLE)
Lempel Ziv Welch (LZW)
Colour Look-up Table (CLUT)

2. Colour Lookup Table
• CLUT
True Colour = 24 bits p/pixel
True Colour = 16, 777, 216 colours
This is known as the FULL PALETTE

3. CLUT
• Used in image editing programmes
• Allows a user to specify the colours used in an
image (setting up their own)
• These ‘mini’ palettes use colours taken from
the 24-bit system (True Colour)

4. What does a palette look like?
• This is an example of a colour palette

5. What is RGB?
• The RGB color model is an additive color model
in which red, green, and blue light is added
together in various ways to reproduce a broad
array of colors.
• The name of the model comes from the initials of
the three additive primary colors, red, green, and
blue.
• Each colour created using this technique has a
code:255-250-250 / 139-137-137

6. QUESTION 1
• Explain the purpose of a CLUT in a bit mapped
file format

7. ANSWER 1
• Explain the purpose of a CLUT in a bit mapped file format
The RGB colour code for each colour used in the image is
stored in the colour lookup table
A local palette storing the selection of colours used in the
image
1 mark for any valid point

8. QUESTION 2
• Describe a benefit of using a CLUT when
applying effects to a bitmap image.

9. ANSWER 2
• Describe a benefit of using a CLUT when applying
effects to a bitmap image.
Changes to the palette affect the whole screen at
once, so faster than editing pixel by pixel
Can be used to produce special effects which would
be much slower to produce by updating pixels

10. Run Length Encoding (RLE)
• Colours are used again and again throughout an
image
• RLE gets colour code, stores it, then repeats it
where required in the image.
• Colour code only stored once
• Saves storage (compresses file)

11. QUESTION 3
• Describe how the RLE compression method
reduces the file size.

12. ANSWER 3
• Describe how the RLE compression method reduces
the file size.
• Areas of a particular colour) compressed by storing the
colour to repeat (1 mark)
• And the number of pixels to repeat for (1 mark)
2 mark question

13. QUESTION 4
• Explain why this compression method may not
be effective on this image file.

14. ANSWER 4
• Explain why this compression method may not be effective on a
complex photographic (bit-mapped) image.
The image does not contain that many large blocks of the same colour
(1 mark)
Therefore the amount of compression will be limited (1 mark)
2 mark question

15. Lempel Ziv Welch (LZW)
• Looks for commonly repeated bit patterns
• These patterns are stored in a look-up table
• Patterns allocated an index number
• Index number stored rather than complete
patterns

16. Disclaimer
• Questions taken from 2008 past paper in
accordance with SQA rules:
The information in this publication may be reproduced to support SQA qualifications only on a non-commercial basis.