PRB: Large DIBs May Not Display Under Win32s

• Microsoft Win32s versions 1.10, 1.15, or 1.20

SYMPTOMS

DIB functions fail when using large DIBs under Win32s.

CAUSE

There is a two-megabyte limit on the size of the area of a DIB that can be blitted using blting functions under Win32s. In versions of Win32s up to 1.2, Microsoft set this size to accommodate DIB blts of 1024*768*24 bits- per-pixel. In version 1.25, the maximum size of the blitted area will be enlarged to accommodate 1280*1024*24 bits-per-pixel.

The following functions are affected:

  SetDIBits
  SetDIBitsToDevice
  CreateDIBitmap
  StretchDIBits

WORKAROUND

To work around the problem, break down large blts into bands that are smaller than two megabytes. Please keep in mind that the biSizeImage field of the BITMAPINFOHEADER used with the blting functions will need to be set to a value smaller than the DIB size limit.

The following code demonstrates a simple implementation of StretchDIBits() that can be used with large DIBs under Win32s.

/* Macro to determine the bytes in a DWORD aligned DIB scanline */
#define BYTESPERLINE(Width, BPP) ((WORD)((((DWORD)(Width) *

int NewStretchDIBits(
    HDC  hdc,     // handle of device context
    int  XDest,     // x-coordinate of upper-left corner of dest. rect.
    int  YDest,     // y-coordinate of upper-left corner of dest. rect.
    int  nDestWidth,  // width of destination rectangle
    int  nDestHeight, // height of destination rectangle
    int  XSrc,     // x-coordinate of upper-left corner of source rect.
    int  YSrc,     // y-coordinate of upper-left corner of source rect.
    int  nSrcWidth,   // width of source rectangle
    int  nSrcHeight,  // height of source rectangle
    VOID  *lpBits,    // address of bitmap bits
    BITMAPINFO *lpBitsInfo,   // address of bitmap data
    UINT  iUsage,  // usage
    DWORD  dwRop   // raster operation code
   )
{
    BITMAPINFOHEADER  bmiTemp;
    float fDestYDelta;
    LPBYTE lpNewBits;
    int i;

    // Check for NULL pointers and return error
    if (lpBits == NULL) return 0;
    if (lpBitsInfo == NULL) return 0;

    // Get increment value for Y axis of destination
    fDestYDelta = (float)nDestHeight / (float)nSrcHeight;

    // Make backup copy of BITMAPINFOHEADER
    bmiTemp = lpBitsInfo->bmiHeader;

   // Adjust image sizes for one scan line
    lpBitsInfo->bmiHeader.biSizeImage =
                       BYTESPERLINE(lpBitsInfo->bmiHeader.biWidth,
                                    lpBitsInfo->bmiHeader.biBitCount);
    lpBitsInfo->bmiHeader.biHeight = 1;

    // Initialize pointer to the image data
    lpNewBits = (LPBYTE)lpBits;

    // Do the stretching
    for (i = 0; i < nSrcHeight; i++)
   if (!StretchDIBits(hdc,
          XDest, YDest + (int)floor(fDestYDelta * (nSrcHeight - (i+1))),
          nDestWidth, (int)ceil(fDestYDelta),
          XSrc, 0,
          nSrcWidth, 1,
          lpNewBits, lpBitsInfo,
          iUsage, SRCCOPY))
        break; // Error!
   else
     // Increment image pointer by one scan line
     lpNewBits += lpBitsInfo->bmiHeader.biSizeImage;

    // Restore BITMAPINFOHEADER
    lpBitsInfo->bmiHeader = bmiTemp;

    return(i);

STATUS

This behavior is by design.