Colour map
A bitmap image has a colour depth. This determines the number of colours the image can represent.
Every pixel’s colour is represented by a binary number. To know which number represents which colour, we need to have a colour map.
What is a colour map?
A colour map just stores a map (table) of the number and the colour it represents, so we can associate a binary pixel number with a colour.
Pre-set colour map
Some formats, such as Windows’ bmp format, have a hard-coded set of colours that all bitmaps will use.
For example, if we store an image in 4 bits, there are only 16 possible colours, and these colours are the same for all bitmap images of that format.
This isn’t very good if we want to store colours other than the pre-set map of the format’s colours.
Stored colour map
To fix this, we can use what I like to call a ‘stored colour map’. Instead of using the format’s default colour map, we can store a map (like a table) of the colour code, alongside the actual colour it makes up.
That way, if we have a specific colour used in our image, we can replicate exactly that colour, while also using fewer bits per pixel.
This map is stored in the metadata. It might look something like this if we visualise it:
| Code | Colour |
|---|---|
| 0 | black |
| 1 | red |
| 2 | green |
| 3 | blue |
| 4 | yellow |
| … | … |
| 15 | white |
flashcards
| Question | Answer |
|---|---|
| What is a colour map in a bitmap image? | A table that stores a mapping of binary numbers (pixel codes) to their corresponding colours, allowing association of pixel values with colours. |
| How does colour depth relate to the colour map? | Colour depth determines the number of colours an image can represent, and the colour map specifies which binary number corresponds to which colour for each pixel. |
| What is a pre-set colour map (e.g., in Windows BMP format)? | A hard-coded set of colours that all bitmaps of that format use by default, meaning the same colours are used for all images regardless of content. |
| What limitation does a pre-set colour map have? | It is limited to the format’s default colours and cannot represent colours outside that pre-set map. |
| What is a stored colour map and how does it differ from a pre-set colour map? | A stored colour map is a custom table stored in the image metadata that defines colour codes and their exact colours, allowing the image to replicate specific colours while using fewer bits per pixel. |
| Where is a stored colour map located in an image file? | It is stored in the metadata of the image file. |
| How would a stored colour map be visualised? | As a table mapping codes to colours, e.g., |
| What is the advantage of using a stored colour map? | It enables accurate replication of specific colours used in the image while keeping the number of bits per pixel low. |