mastodon-ios/MastodonSDK/Sources/MastodonExtension/CGImage.swift

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//
// CGImage.swift
// Mastodon
//
// Created by MainasuK Cirno on 2021-3-31.
//
import CoreImage
extension CGImage {
// Reference
// https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.709-6-201506-I!!PDF-E.pdf
// Luma Y = 0.2126R + 0.7152G + 0.0722B
public var brightness: CGFloat? {
let context = CIContext() // default with metal accelerate
let ciImage = CIImage(cgImage: self)
let rec709Image = context.createCGImage(
ciImage,
from: ciImage.extent,
format: .RGBA8,
colorSpace: CGColorSpace(name: CGColorSpace.itur_709) // BT.709 a.k.a Rec.709
)
guard let image = rec709Image,
image.bitsPerPixel == 32,
let data = rec709Image?.dataProvider?.data,
let pointer = CFDataGetBytePtr(data) else { return nil }
let length = CFDataGetLength(data)
guard length > 0 else { return nil }
var luma: CGFloat = 0.0
for i in stride(from: 0, to: length, by: 4) {
let r = pointer[i]
let g = pointer[i + 1]
let b = pointer[i + 2]
let Y = 0.2126 * CGFloat(r) + 0.7152 * CGFloat(g) + 0.0722 * CGFloat(b)
luma += Y
}
luma /= CGFloat(width * height)
return luma
}
}