| Applies To: | Bitmap | Clipboard | Cursor | Icon |
Description
This property defines the table of colours (the colour map) used by a Bitmap or Icon object or by a bitmap stored in the Windows clipboard. Its value is a 3-column integer matrix of numbers in the range 0-255. Each row represents a separate colour which is indexed (0-origin) by values in the Bits property. The 3 columns refer to the intensities of the red, green and blue components of colour respectively.
Please note that Bits and CMap may only be used to represent an image with a colour palette of 256 colours or less. If the colour palette is larger, the values of Bits and CMap reported by ⎕WG will be (0 0). For a high-colour image, use CBits instead.
When you create a Bitmap or Icon by specifying Bits and CMap, the actual colours you obtain are not necessarily those that you specified. This is partly due to hardware restrictions and partly due to the way in which Windows manages colours. Firstly, your display adapter and driver limit the number of pure colours that can be displayed at any one time and therefore define a maximum size for the colour map. For example, on a standard VGA you are limited to 16 different pure colours (additional ones are provided by dithering).
Secondly, Windows reserves a certain number of colours in the colour map for its own use. When an application requests a new colour (i.e. one that is not already installed in the colour map), MS-Windows either assigns it to a spare entry, or allocates the closest match if the colour map is full. The value of Bits and CMap after ⎕WC reflect the actual colours allocated and may bear little resemblance to the values you assigned to these properties initially.
Note that if you are running 16 colours, MS-Windows reserves all 16 entries in the colour map for its own use. This means that on a 16-colour system, you cannot use any colours other than the default ones reserved by MS-Windows. In practice, the "standard" 16-colour CMap is shown in the following table.
| Bits[] | CMap | Colour | ||
|---|---|---|---|---|
| 1 | 0 | 0 | 0 | Black |
| 2 | 128 | 0 | 0 | Dark red |
| 3 | 0 | 128 | 0 | Dark Green |
| 4 | 128 | 128 | 0 | Olive Green |
| 5 | 0 | 0 | 128 | Dark Blue |
| 6 | 128 | 0 | 128 | Dark Magenta |
| 7 | 0 | 128 | 128 | Dark Cyan |
| 8 | 128 | 128 | 128 | Dark Grey |
| 9 | 192 | 192 | 192 | Light Grey |
| 10 | 255 | 0 | 0 | Red |
| 11 | 0 | 255 | 0 | Green |
| 12 | 255 | 255 | 0 | Yellow |
| 13 | 0 | 0 | 255 | Blue |
| 14 | 255 | 0 | 255 | Magenta |
| 15 | 0 | 255 | 255 | Cyan |
| 16 | 255 | 255 | 255 | White |
Table 52: The default 16-colour CMap
If you are using a 256-colour set-up, the first 9 and the last 7 entries of the 256-colour CMap are the same as the first 9 and last 7 entries of the 16-colour CMap shown above. The intervening entries represent additional colours or are initially unused (0 0 0). New colours that you specify will be allocated to unused entries until the table is full.