Advanced MFC Programming

Chapter 8. DC, Pen, Brush and Palette
colors). Other colors will change from time to time as we open and close graphic applications. We can use
samples 8.8-1\GDI or 8.8-2\GDI to test this.
Please note that if the sample is executed on non-palette device, the logical palette will not represent
the system palette. This is because on the hardware level, palette does not exist at all.
8.10 Palette Animation
Flag PC_RESERVED
Another interesting flag we can use when creating a logical palette is PC_RESERVED, which can be used
to implement palette animation. If we have a logical palette with this flag set for some entries, the colors in
these entries will only be mapped to the unused entries of the system palette. If the mapping is successful,
when we change the colors in the logical palette, the entries in the system palette will also change. If any
portion of window is painted with such entries, this change will affect that portion. This is the reason why a
logical entry with PC_RESERVED flag can not be mapped to an occupied entry.
The following table lists the difference between a normal logical palette entry and an entry with
PC_RESERVED flag:
Normal Entry
Color First unused entry in the system palette is
Mapping looked for, if there is no such an entry, the
Algorithm color is mapped to the nearest color in the
system palette.
Palette If the logical palette is realized, there is no
Realization guarantee that the color will be realized
successfully. The color may be mapped to
a nearest color.
Other Entries can always be created
successfully.
Entry With PC_RESERVED Flag
The entry can only be mapped to an
unused entry of the system palette.
The color contained in this type of
entries will always be realized
successfully.
Entries may not be created successfully.
While we can change the color of any area in a window by painting it again (using a different brush or
pen), the above mentioned method has two advantages:
1) If we have several areas painted with the same color, the new method will cause all of them to change
at the same time once the old color is replaced with a new one in the logical palette. For the traditional
method, we have to draw each area one by one to make this change, it takes longer time.
1) If we draw each area one by one, the change takes place in software level. For the new method, the
color is filled to the system palette directly (This change happens at the hardware level), which is
extremely fast.
Animation
With this method, it is very easy to implement an animation effect. For example, considering an array
of four rectangles that are filled with the following four different colors respectively: red, green, blue and
black. If we paint the four rectangles with green, blue, black, red next time, and blue, black, red, green next
next time, and so on…, this will give us an impression that the rectangles are doing rotating shift. One way
to implement this effect is to redraw four rectangles again and again using different colors (which means
using different entries to draw the same rectangle again and again). Another way is to switch the colors in
the palette directly.
Sample 8-10\GDI demonstrates how to implement palette animation. It is a standard SDI application
generated from Application Wizard. In the sample, the client area is divided into 236 columns, each row is
painted with a different color. The colors in the logical palette will be shifting all the time, and we will see
that the colors in the client window will also shift accordingly.
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