PRB: Bitmap Displays Upside Down Using StretchDIBits (151920)

CAUSE

Two situations could cause a bitmap to display upside down in Win32:

1. The signs of the cxSrc and cxDest parameters differ. The documentation for StretchDIBits states that the function creates a mirror image of a bitmap along the x-axis if the signs of the cxSrc and cxDest parameters differ. Whether intentional or unintentional, this could cause a bitmap to display upside down.
2. The sign of the biHeight value in the BITMAPINFOHEADER does not correctly reflect how the bitmap bits are stored in the DIB file. In Win16, all DIBs are stored bottom-up, with the bottom-most scan line stored first in the DIB file. In Win32, DIBs may also be stored top-down, with the top-most scan line stored first. Top-down DIBs are denoted by a negative biHeight value in the BITMAPINFOHEADER structure; bottom-up DIBS are denoted by a positive biHeight value:

      DIB stored as      Should have a biHeight value of
      -------------      -------------------------------
      top-down           negative
      bottom-up          positive
   						

Make sure the biHeight value is appropriately set as positive or negative, depending on how the bitmap bits were stored in the DIB file. A mismatch in the way this value is set could result to an upside down bitmap because the DIB engine interprets the bits incorrectly. For example, a positive biHeight value for a DIB stored as top-down should cause a bitmap to display upside down.