wiasDownSampleBuffer function

The wiasDownSampleBuffer function takes in a buffer of DWORD-aligned pixel data and downsamples it (produces image data of lower resolution) to the specified size and resolution.


HRESULT wiasDownSampleBuffer(
  LONG                  lFlags,



Specifies a set of flags that determine the behavior of this function. Currently, only the following flag is defined.

Flag Meaning
WIAS_GET_DOWNSAMPLED_SIZE_ONLY Do not copy the downsampled data to the destination buffer. Instead, set the following members of the WIAS_DOWN_SAMPLE_INFO structure: ulDownSampledHeight, ulDownSampleWidth, ulAlignedHeight, ulAlignedWidth.


Pointer to the WIAS_DOWN_SAMPLE_INFO structure that contains all of the information needed for the downsampling operation.

Return Value

On success, the function returns S_OK. If the function fails, it returns a standard COM error or one of the WIA_ERROR_XXX errors (described in the Microsoft Windows SDK documentation).


The wiasDownSampleBuffer function can be used in either of the following two ways:

  1. The caller specifies the downsampled (that is, output) width and height by setting the ulDownSampledWidth and ulDownSampledHeight members of the WIA_DOWN_SAMPLE_INFO structure.

  2. The caller sets the ulDownSampledWidth and ulDownSampledHeight members of the WIA_DOWN_SAMPLE_INFO structure to zero, indicating that the function should choose the output width and height.

    To see what output width and height values the function chooses, call this function with the lFlags parameter set to WIAS_GET_DOWNSAMPLED_SIZE_ONLY. On return, the ulDownSampledWidth and ulDownSampledHeight members are set to their new values. No downsampling is performed in this case.

The caller of this function is required to fill in the following members of the WIA_DOWN_SAMPLE_INFO structure:

  • ulOriginalWidth

  • ulOriginal Height

  • ulBitsPerPixel

  • ulXRes

  • ulYRes

  • pSrcBuffer

Note This function expects ulBitsPerPixel to be 1, 8, or 24, corresponding to 1-, 8-, and 24-bit-per-pixel data.
The caller can also specify the size of the downsampled data by filling in the following WIA_DOWN_SAMPLE_INFO structure members:

  • ulDownSampledWidth

  • ulDownSampledHeight

If the buffer that receives the downsampled data has already been allocated, the caller should fill in these WIA_DOWN_SAMPLE_INFO structure members:

  • ulDestBufSize

  • ulSrcBufSize

  • pDestBuffer

If the caller sets pDestBuffer to NULL, the destination buffer is allocated by the WIA service. On return from this function, pDestBuffer points to the destination buffer. The caller is responsible for freeing this memory when the operation is finished, and does this by calling CoTaskMemFree (described in the Microsoft Windows SDK documentation) on the buffer.

Because this function is not able to produce partial output lines, the number of scan lines in the input buffer must be an integer multiple of the scaling factor. For example, suppose the input buffer contains an image sampled at 600 dpi, which you intend to downsample to an equivalent 50 dpi image. In this case, you are scaling down the original image by a factor of 12 (because 600 / 50 = 12). This means that the function must receive 12 input lines for each output line that it produces.

More generally, if the original image has a resolution of Rin dpi, and is to be scaled down to an image with a resolution of Rout dpi, the scale-down factor is Rin / Rout, and the number of lines in the input buffer should be a multiple of Rin / Rout. If the scan head reaches the last band of the original image, and there are too few scan lines in the input buffer to produce an output line, pad the input buffer so that it contains the required number of data lines. Failure to do so causes unpredictable results, and can even result in a driver crash.


Windows version Available in Microsoft Windows Me and in Windows XP and later versions of the Windows operating systems.
Target Platform Desktop
Header wiamdef.h (include Wiamdef.h)
Library Wiaservc.lib
DLL Wiaservc.dll

See Also