SAMPLE: SetPal.exe Uses mciSendString() to Change .AVI Palette

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Release Date: Apr-29-1998

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The current video mode of the display must be a palettized video mode, such as 256 colors, for the technique to succeed because the "setvideo palette handle to" string is ignored in non-palettized video modes. Also, the video compressor/decompressor (codec) that decompresses the .avi file handles the palette. Therefore, the exact behavior may depend on the codec in use. For example, the MS Video 1 codec is an 8-bit (256 color) palettized codec while the Indeo and Cinepak Codecs are 24-bit non-palettized Codecs, so their palette behavior might differ.

The following steps show how to supply your own palette:

1. Fill in a LOGPALETTE structure containing the color information for your palette.
2. Create a logical palette using the CreatePalette() function.
3. Call mciSendString() (or mciSendCommand()) to supply that palette to the AVI file using a string such as:
   
   setvideo <alias> palette handle to <palette handle>

The remainder of this article supplies more information and code excerpts from the SETPAL sample that perform these steps.

You can compile the SetPal.exe sample as a 32-bit sample, called SetPal32, or as a 16-bit sample, called SetPal16. As a result, it is compatible with Windows 95, Windows NT, and Windows 3.1. Setpal32.mak and Setpal16.mak make files are provided. SetPal16 requires the Video for Windows 1.1 Development Kit to build successfully.

SetPal.exe is a sample application that allows you to open an .avi file through the open common dialog box. Menu choices allow the .avi file to be opened, played, stopped, or closed. MCI strings perform the underlying work. The following MCI string is used to supply a palette for the .avi file:

Excerpts from Setpal.c

   // Include files.
   #include <windows.h>  // Required for all Windows applications
   #include "windowsx.h" // for GlobalAllocPtr/GlobalFreePtr in
                         // CreateSamplePalette.
   #include "mmsystem.h" // For the MCI calls.

   // Global variables.
   static HPALETTE g_hPal = NULL;    // Palette handle.
   .
   .
   .

   // CreateSamplePalette() demonstrates how to fill in a LOGPALETTE
   // structure and create a logical palette.
   VOID CreateSamplePalette(void)

   {
       LPLOGPALETTE lpLogPal;
       int          i;
       int          nPalEntries = 236;   // Number of entries in our
                                         // palette.
                                         // 256 are possible, but the
                                         // system reserves 20 of them.

       lpLogPal = (LPLOGPALETTE) GlobalAllocPtr (GHND,
                   sizeof (LOGPALETTE) + nPalEntries * sizeof
                   (PALETTEENTRY));

          lpLogPal->palVersion    = 0x300;
       lpLogPal->palNumEntries = nPalEntries;

       for (i = nPalEntries; i > 0; i--)
       {
           // Fill in the red, green, and blue values for our palette.
           // This particular palette is a wash from green to black.
           lpLogPal->palPalEntry[i].peRed   = 0;
           lpLogPal->palPalEntry[i].peGreen = i;
           lpLogPal->palPalEntry[i].peBlue  = 0;

           // Create unique palette entries. This flag may change depending
           // on your purposes. See the Windows API documentation
           // about the PALETTEENTRY structure for more information.
           lpLogPal->palPalEntry[i].peFlags = PC_NOCOLLAPSE;
       }

       // Create the logical palette.
       g_hPal = CreatePalette (lpLogPal);

       // Clean up.
       GlobalFreePtr (lpLogPal);
   }

   // The ProcessAVICommands() function in SETPAL.C handles the open, set
   // palette, play, and close for the AVI file. Once the AVI file has been
   // opened, issue an mciSendString() such as the following to set the
   // palette:

   static char szAlias[10] = "paltest";  // The movie alias to use in
   mciSendString                         // buffer to hold the MCI
   char szBuffer[128];                   // string we built.
   .
   .
   .
      wsprintf(szBuffer, "setvideo %s palette handle to %d",
              (LPSTR)szAlias, g_hPal);
      mciSendString(szBuffer, NULL, 0, NULL);