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Use vImage to accelerate alpha premultiplication. #51

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Use vImage to accelerate alpha premultiplication. #51

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32 changes: 12 additions & 20 deletions swift/ThumbHash.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -557,6 +557,7 @@ func thumbHashToImage(hash: Data) -> NSImage {

#if os(iOS)
import UIKit
import Accelerate

func imageToThumbHash(image: UIImage) -> Data {
let size = image.size
Expand Down Expand Up @@ -610,26 +611,17 @@ func imageToThumbHash(image: UIImage) -> Data {
func thumbHashToImage(hash: Data) -> UIImage {
var (w, h, rgba) = thumbHashToRGBA(hash: hash)
rgba.withUnsafeMutableBytes { rgba in
// Convert from unpremultiplied alpha to premultiplied alpha
var rgba = rgba.baseAddress!.bindMemory(to: UInt8.self, capacity: rgba.count)
let n = w * h
var i = 0
while i < n {
let a = UInt16(rgba[3])
if a < 255 {
var r = UInt16(rgba[0])
var g = UInt16(rgba[1])
var b = UInt16(rgba[2])
r = min(255, r * a / 255)
g = min(255, g * a / 255)
b = min(255, b * a / 255)
rgba[0] = UInt8(r)
rgba[1] = UInt8(g)
rgba[2] = UInt8(b)
}
rgba = rgba.advanced(by: 4)
i += 1
}
guard let ptr = rgba.baseAddress else { return }

var buffer = vImage_Buffer(
data: ptr,
height: vImagePixelCount(h),
width: vImagePixelCount(w),
rowBytes: w * 4
)

// Convert from unpremultiplied alpha to premultiplied alpha using vImage
vImagePremultiplyData_RGBA8888(&buffer, &buffer, vImage_Flags(kvImageNoFlags))
}
let image = CGImage(
width: w,
Expand Down