Image Display Palettes

Clicking on the palettes icon on the tool-bar or, "Settings->Palette for current image...", or "Settings->Default palette..." calls up the paletter editor form as shown on the right.  Palettes are constructed by additively combining "intensity curves" for each of the red, green and blue channels.  The curves used are Bezier curves as these provide a simple means to obtain smooth, arbitrarily shaped analytic curves.  Since the essential information required to reconstruct the palette is contained in a relatively small amount of data (the Bezier curve vertices and control points), the palettes occupy relatively small storage space in the image file.  Note however that only the Pyxis PXI format allows the palette data to be saved with the image.

The palette is edited by adding vertices to the Bezier curve and/or moving vertices and their associated control points.  The vertices are the points shown with white squares.  The control points are the small dots terminating each line extending out from the vertices.  Up to 21 vertices can be used to form a curve; this doesn't sound like much, until you see what can be done with only 2 or 3 vertices!  Vertices are added by double clicking on the graph somewhere close to where the point is needed.  The new point is automatically inserted at the mid-point of the two nearest points.  The control points for the vertex lie on the line between these two nearby points, on opposite sides of the vertex.

To move any of the points, click and hold the left mouse button while dragging the mouse.  Changes to palette are immediately displayed in the palette colour bar below the Bezier graph.  Clicking on the "Red Curve", "Green Curve" or "Blue Curve" buttons selects the Bezier curve for the respective colour channel to edit. Only the vertices and control points for the colour channel being edited are displayed.

By default, the two control points for each new vertex are set to always be opposite each other about the vertex.  Right clicking near a vertex brings up a popup window with an option to allow the control points for the vertex to be set so that they are independent of one another ("assymmetric" option) or are tied together according to the default ("symmetric" option).  This same popup window also provides the option to remove the vertex entirely.  Note that the popup window will not appear when clicking on a graph that contains only two vertices; the actions in the popup do not apply to this case.


Use of the palettes for image balancing

A common problem with astronomical images is the wide dynamic range of objects that can be present in a single image.  For example, an emission nebula may consist of a bright central core surrounded by dim wispy regions, all of this sprinkled with stars of greatly varying brightness.  A linear scale is poor at representing all of the features in such an image so that a logarithmic scale is often used.  Pyxis does not provide an option for logarithmic scaling because adjustment of the palette provides better control over the displayed image.  Specifically, a palette can be constructed that has a logarithmic-like response over one intensity range, while transitioning smoothly to a linear response for another intensity range.  Of course, Bezier curves can be used to approximate a pure logarithmic response with arbitrary accuracy, although this is rarely what is actually wanted from an aesthetic viewpoint.


Default palette and "Quick-palettes"
 
When an image is acquired from the CCD it is automatically assigned the default palette for the given context; the possible contexts are the main CCD acquisition windows, the guiding CCD form and the composite settings form.  All of the default palettes can be changed by calling the palette settings editor from the three different contexts.   The file names associated with these palettes are saved in the default program settings file at shutdown and the palettes are loaded from these palette files at startup.  Palette files have the extension ".pal" and are saved by default in the "Palettes" path in the main program installation directory.  In the case of the four "Quick-palettes" displayed on the "Image display" tab-sheet of the main Pyxis form, the same palette files are always used; "QuickPalette1.pal", "QuickPalette2.pal", "QuickPalette3.pal", "QuickPalette4.pal".  These palette files may be loaded into the palette editor and modified to accomodate your preferences.


Grey-scale palettes

The three colour curves can be forced to be identical by checking the "grey-scale palette" check-box.  In this case a grey-scale palette is obtained.