ColourTrans Colour Calibration (Rev #3)
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» Colour Calibration
Colour Mapping
Colour calibration is achieved by the ColourTrans module using a table that maps device colours (physical colours) to standard colours.
When requesting a colour, ColourTrans takes the palette colour and uses a calibration table to convert the device colours to standard colours, giving a transient palette that maps logical colours to standard colours. The closest match to the standard colour is then selected.
See ColourTrans Transfer Function and Colour Mapping Descriptor for more details.
Calibration Tables
A calibration table is used to map device colours to standard colours. Three separate mapping exist for each component of the device space. e.g red, green and blue on a monitor. Each mapping consists of a series of device component/standard colour pairs.
The format of the pairs is &BBGGRRDD
Where:
BBGGRR |
Standard colour (Blue, Green and Red) |
DD |
Amount of device component |
The calibration table format is as follows:
| Word | Meaning |
|---|---|
| 0 | Number of pairs of component 1 (n1) |
| 1 | Number of pairs of component 2 (n2) |
| 2 | Number of pairs of component 3 (n3) |
| 3 | n1 words giving pairs for component 1 |
| 3 + n1 | n2 words giving pairs for component 2 |
| 3 + n1 + n2 | n3 words giving pairs for component 3 |
The size of the table is therefore 3 + n1 + n2 + n3 words.
The three separate mappings for each device component must be sorted in ascending order. To fill the device colour space there must be entries for device components of 0 and 255. This results in at least two pairs for each component.
An example of a calibration table is shown below.
| Word | Meaning |
|---|---|
| &00000002 | 2 pairs of red component |
| &00000002 | 2 pairs of green component |
| &00000002 | 2 pairs of blue component |
| &02010300 | Device colour 000000 corresponds to standard colour &020103 |
| &0203FDFF | Device colour 0000FF corresponds to standard colour &0203FD |
| &02010300 | Device colour 000000 corresponds to standard colour &020103 |
| &03FC02FF | Device colour 00FF00 corresponds to standard colour &030FC02 |
| &02010300 | Device colour 000000 corresponds to standard colour &020103 |
| &FF0302FF | Device colour FF0000 corresponds to standard colour &FF0302 |
The default mapping is for device colours and standard colours to be the same.
ColourTrans converts a device colour to a standard colour by the following process:
- Splits the device colour into its three component parts
- Looks up each component in the calibration table
- Entries not found in the table is calculated by performing a linear interpolation between the two device components ‘surrounding’ the required device component
- The standard colour obtained for each component is then added together to give the final calibrated standard colour
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