Getting what you ask for

In the article Objectwindows programming - Coloring controls in OWL applications we mention that when you request a color, you don't always get the exact color you ask for. If you specify an RGB value when you create a TColor object, Windows will match the color you request to the nearest color in the extended system palette. The color might be what you expector it might not even be close. There is a way, however, to get exactly the color you need: You can create a logical palette that's always available for your use. Let's review the steps required to do this.

You first fill in a PALETTEENTRY structure by specifying the red, green and blue values for each color you require. You'll usually do this by creating an array of PALETTEENTRY structures. Then, you create a TPalette object using the PALETTEENTRY array.

Once you create a logical palette, you need to implement it. To do so, select the palette into your device context with TDC::SelectObject(). Next, use the TDC function RealizePalette() to map the logical palette to the extended system palette.

Finally, you need to select a color from the palette. You can do this in one of two ways. Your first option is to create a TColor object by specifying the color index from the palette. For instance, if you know that you want color number 8 in your logical palette, you can create a TColor object as follows:

TColor myColor(8);

Now myColor contains the eighth color value in your logical palette.

You can also specify an RGB component, but doing so requires an extra step. If you use

TColor(128, 80, 190);

Windows will find the closest match in the system palettenot in your logical palette. To specify the closest match in your logical palette, you must use TColor::PalRelative(). For example, you might use

TColor myColor(128, 80, 190).PalRelative();

Now Windows will search the extended system palette (to which you've mapped your logical palette) for the closest match to the RGB color you requested, rather than searching the system palette.

The code snippet shown in Figure A creates a logical palette of 16 shades of blue and fills the screen with those colors in a gradient effect. By creating a palette and mapping it to the extended system palette, you can be sure of the colors your applications display.

Figure A - This code creates a logical palette filled with 16 shades of blue.

PALETTEENTRY pe[16];
memset(&pe, 0, sizeof(pe));
int i;
for (i=0;i<16;i++) pe[i].peBlue = i * 16;
pal = new TPalette(pe, 16);
TClientDC dc(*this);
dc.SelectObject(*pal);
dc.RealizePalette();
int step = GetWindowRect().Height()/16;
for (i=0;i<16;i++) {
  TColor color(i);
  dc.SelectObject(TPen(color));
  TRect rect(0, i*step, GetWindowRect().Width(), 
             (i*step) + step);
  dc.FillRect(rect, TBrush(color));
}

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