User manual

the blue. If you move the mouse pointer on the histogram, the pixel values is displayed
in abscissa as well as the corresponding red, green and blue values in ordinates on the
right hand side of the histogram. Simultaneously, the final colour is shown in the above
strip.
Transfer functions
By default, Aladin uses a linear function to relate the pixel value => entry for the colour
table. The three cursors below the histogram let you change the slope of this function, or
even to use two slopes, one corresponding to the 128 lowest values in the colour table,
the other corresponding to the 128 highest values.
Basically, moving the right and left cursors
changes the lower and upper thresholds, while
the central cursor when moved to the left will
enhance the contrast, and reduce the contrast
when moved to the right.
If the Ctrl key (resp. “Apple” for Mac) is hold, the
central cursor becomes a diamond and is also
sensitive to the vertical moves with the mouse.
This will lead to reduce or enlarge the range
between the right and left cursors simultaneously.
It is a quick method to readjust the maximum dynamic on the pixel range to display.
The control band also lets you choose other non-linear functions: log, sqrt, pow2. As
indicated below, the log function will lead to an image with a high contrast, while the
sqrt will lead to an image with some contrast and the pow2 will lead to an image with low
contrast.
Colour tables
Aladin has some classical colour tables for astronomy. Not only can they be adjusted via
the transfer function control as described above, but they can also be reverted.
Here is the list of colour tables with a simple linear transfer functions and how they look
in normal and revert mode:
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