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The information in this article applies to:
SYMPTOMS
Calling StretchDIBits on a DIB causes it to display upside down.
CAUSE
Two situations could cause a bitmap to display upside down in Win32:
- 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.
- 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 will interpret the bits incorrectly. For example, a positive
biHeight value for a DIB stored as top-down should cause a bitmap to
display upside down.
STATUS
This behavior is by design.
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