vImageEndsInContrastStretch_PlanarF(_:_:_:_:_:_:_:_:_:)
Performs ends-in contrast stretching on a 32-bit planar buffer.
Declaration
func vImageEndsInContrastStretch_PlanarF(_ src: UnsafePointer<vImage_Buffer>, _ dest: UnsafePointer<vImage_Buffer>, _ tempBuffer: UnsafeMutableRawPointer!, _ percent_low: UInt32, _ percent_high: UInt32, _ histogram_entries: UInt32, _ minVal: Pixel_F, _ maxVal: Pixel_F, _ flags: vImage_Flags) -> vImage_ErrorParameters
- src:
The source vImage buffer.
- dest:
A pointer to the destination vImage buffer structure. You’re 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 this structure points to contains the destination image data. When you no longer need the data buffer, deallocate the memory to prevent memory leaks.
- tempBuffer:
A pointer to workspace memory the function uses as it operates on an image. Pass
nilto instruct the function to allocate, use, and then free its own temporary buffer. - percent_low:
The percentage of pixels that the operation maps to the lowest end of the transformed image’s histogram.
- percent_high:
The percentage of pixels that the operation maps to the highest end of the transformed image’s histogram.
- histogram_entries:
The number of histogram entries.
- minVal:
The minimum pixel value. The operation assigns pixel values less than
minValto the first histogram entry. - maxVal:
The maximum pixel value. The operation assigns pixel values greater than
maxValto the last histogram entry. - flags:
The options to use when performing the operation. If your code implements its own tiling or its own multithreading, pass Kvimagedonottile; otherwise, pass Kvimagenoflags.
To instruct the function to return the minimum size of the workspace memory, set the Kvimagegettempbuffersize flag.
Return Value
kvImageNoError; otherwise, a negative value indicates one of the error codes that Data Types and Constants describes, and a positive value indicates the required size for the temporary buffer.
Discussion
Contrast stretching evenly distributes a histogram’s pixel values across the full range of available pixel values. This technique is ideal for enhancing the contrast of an image with pixel values concentrated in one area of the intensity spectrum.
Optimize performance with temporary buffers
This function uses a multiple-pass algorithm that saves intermediate pixel values between passes. In some cases, the destination buffer may not be large enough to store that intermediate data, so the operation requires additional storage.
Pass nil to the tempBuffer parameter to have vImage create and manage this temporary storage for you.
In cases where your code calls the function frequently (for example, when processing video), create and manage this temporary buffer yourself and reuse it across function calls. Reusing a buffer avoids vImage allocating the temporary storage with each call.
To use your own temporary buffer, first call the function with the same values for all other parameters that you intend to use for subsequent calls. In addition, pass the kvImageGetTempBufferSize flag. The kvImageGetTempBufferSize instructs the function not to perform any processing, and to return a positive value that represents the minimum size, in bytes, of the temporary buffer. A negative return value represents an error.
After you allocate the memory for the temporary buffer, pass that to the tempBuffer parameter for subsequent calls to the function, and don’t pass the kvImageGetTempBufferSize flag.
You can use the same workspace memory for a group of images that are different sizes. To do this, create a vImage buffer structure with a size that shares the maximum width and maximum height of the images that you’re working with. For example, to create a workspace memory that’s suitable for 4x4, 5x4, and 4x5 images, pass a buffer with the size 5x5.