Function

vImageContrastStretch_ARGBFFFF(_:_:_:_:_:_:_:)

Stretches the contrast of an ARGBFFFF source image.

Declaration

func vImageContrastStretch_ARGBFFFF(_ src: UnsafePointer<vImage_Buffer>, _ dest: UnsafePointer<vImage_Buffer>, _ tempBuffer: UnsafeMutableRawPointer!, _ histogram_entries: UInt32, _ minVal: Pixel_F, _ maxVal: Pixel_F, _ flags: vImage_Flags) -> vImage_Error

Parameters

src

A pointer to a vImage buffer structure that contains the source image.

dest

A pointer to a vImage buffer data structure. You are responsible for filling out the height, width, and rowBytes fields of this structure, and for allocating a data buffer of the appropriate size. On return, the data buffer pointed to by this structure contains the destination image data. When you no longer need the data buffer, you must deallocate the memory.

tempBuffer

A pointer to a temporary buffer. If you pass NULL, the function allocates the buffer, then deallocates it before returning. Alternatively, you can allocate the buffer yourself, in which case you are responsible for deallocating it when you is no longer need it.

If you want to allocate the buffer yourself, see the Discussion for information on how to determine the minimum size that you must allocate.

histogram_entries

The number of histogram entries, or bins, to use in histograms for this operation.

minVal

A minimum pixel value, the low end of the histogram. Any pixel value less than this will be clipped to this value (for the purposes of histogram calculation), and assigned to the first histogram entry. This minimum value is applied to each of the four channels separately.

maxVal

A maximum pixel value, the high end of the histogram. Any pixel value greater than this will be clipped to this value (for the purposes of histogram calculation), and assigned to the last histogram entry. This maximum value is applied to each of the four channels separately.

flags

The options to use. Set the kvImageDoNotTile flag if you plan to perform your own tiling or use multithreading.

Set the kvImageLeaveAlphaUnchanged flag to copy the alpha channel to the destination image unchanged.

Return Value

kvImageNoError; otherwise, one of the error codes described in Data Types and Constants.

Discussion

The contrast stretch operation alters the image histogram so that pixel values can be found at both the lowest and highest end of the histogram, with values in between “stretched” in a linear fashion. The contrast stretch operation is done separately for each of the four channels—alpha, red, green, and blue. However, the size and range values are the same for each of the four histograms.

The source and destination buffers must have the same height and the same width.

If you want to allocate the memory for the tempBuffer parameter yourself, call this function twice, as follows:

  1. To determine the minimum size for the temporary buffer, the first time you call this function pass the kvImageGetTempBufferSize flag. Pass the same values for all other parameters that you intend to use in for the second call. The function returns the required minimum size, which should be a positive value. (A negative returned value indicates an error.) The kvImageGetTempBufferSize flag prevents the function from performing any processing other than to determine the minimum buffer size.

  2. After you allocate enough space for a buffer of the returned size, call the function a second time, passing a valid pointer in the tempBuffer parameter. This time, do not pass the kvImageGetTempBufferSize flag.

See Also

Stretching the Contrast