Contents

applyLookup(alphaTable:redTable:greenTable:blueTable:destination:)

Applies a set of four lookup tables to transform an interleaved, four-channel 8-bit image.

Declaration

func applyLookup(alphaTable: [Pixel_8]?, redTable: [Pixel_8]?, greenTable: [Pixel_8]?, blueTable: [Pixel_8]?, destination: vImage.PixelBuffer<vImage.Interleaved8x4>)

Parameters

  • alphaTable:

    A lookup table for the alpha channel that contains 256 Pixel_8 values. Pass nil to specify that the function copies the alpha channel unchanged to the destination buffer.

  • redTable:

    A lookup table for the red channel that contains 256 Pixel_8 values. Pass nil to specify that the function copies the red channel unchanged to the destination buffer.

  • greenTable:

    A lookup table for the green channel that contains 256 Pixel_8 values. Pass nil to specify that the function copies the green channel unchanged to the destination buffer.

  • blueTable:

    A lookup table for the blue channel that contains 256 Pixel_8 values. Pass nil to specify that the function copies the blue channel unchanged to the destination buffer.

  • destination:

    The destination pixel buffer.

Discussion

Use this function to apply individual lookup tables to each channel in an interleaved, four-channel image. Adjust the order of the tables for images that don’t use ARGB channel ordering. For example, use the blueTable parameter for the alpha lookup table to transform an RGBA image.

The following code creates a simple lookup table that transforms a vImage.Interleaved8x4 pixel buffer into its negative. For example, when an input pixel has the value 255, 255, 255, 255, the output pixel has the value 0, 0, 0, 0. Conversely, when an input pixel has the value 0, 0, 0, 0, the output pixel has the value 255, 255, 255, 255.


let lookup: [Pixel_8] = (0 ..< 256).map { i in
    Pixel_8(255 - i)
}

let destinationBuffer = vImage.PixelBuffer(
    size: sourceBuffer.size,
    pixelFormat: vImage.Interleaved8x4.self)

sourceBuffer.applyLookup(alphaTable: nil,
                         redTable: lookup,
                         greenTable: lookup,
                         blueTable: lookup,
                         destination: destinationBuffer)

The images below show an example source image on the left and the negative result on the right.

[Image]

See Also

Transforming with a lookup table