How to get correct RGB color values from PNG with PLTE palette using NSBitmapImageRep in Swift? - swift

Question

Given a PNG image, I would like to get the RGB color at a given pixel using Swift on macOS.
This is simply done by:
let d = Data(base64Encoded: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAADElEQVQImWP4z8AAAAMBAQCc479ZAAAAAElFTkSuQmCC")!
let nsImage = NSImage(data: d)!
let bitmapImageRep = NSBitmapImageRep(cgImage: nsImage.cgImage(forProposedRect: nil, context: nil, hints: [:])!)
let color = bitmapImageRep.colorAt(x: 0, y: 0)
print(color!.redComponent, color!.greenComponent, color!.blueComponent, separator: " ")
// 1.0 0.0 0.0
This minimal example contains a one pixel PNG in RGB encoding with a red pixel in base64 encoding to make this easily reproducible.
printf iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAADElEQVQImWP4z8AAAAMBAQCc479ZAAAAAElFTkSuQmCC | base64 --decode | pngcheck -v
File: stdin (140703128616967 bytes)
chunk IHDR at offset 0xfffffffffffffffb, length 13
1 x 1 image, 24-bit RGB, non-interlaced
chunk IDAT at offset 0xfffffffffffffffb, length 12
zlib: deflated, 256-byte window, default compression
chunk IEND at offset 0xfffffffffffffffb, length 0
No errors detected in stdin (3 chunks, -133.3% compression).
When changing this PNG to an indexed PNG with a PLTE Palette
printf iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAMAAAAoyzS7AAAAA1BMVEX/AAAZ4gk3AAAACklEQVQImWNgAAAAAgAB9HFkpgAAAABJRU5ErkJggg== | base64 --decode | pngcheck -v
File: stdin (140703128616967 bytes)
chunk IHDR at offset 0xfffffffffffffffb, length 13
1 x 1 image, 8-bit palette, non-interlaced
chunk PLTE at offset 0xfffffffffffffffb, length 3: 1 palette entry
chunk IDAT at offset 0xfffffffffffffffb, length 10
zlib: deflated, 256-byte window, default compression
chunk IEND at offset 0xfffffffffffffffb, length 0
No errors detected in stdin (4 chunks, -600.0% compression).
the snippet above changes to
let d = Data(base64Encoded: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAMAAAAoyzS7AAAAA1BMVEX/AAAZ4gk3AAAACklEQVQImWNgAAAAAgAB9HFkpgAAAABJRU5ErkJggg==")!
let nsImage = NSImage(data: d)!
let bitmapImageRep = NSBitmapImageRep(cgImage: nsImage.cgImage(forProposedRect: nil, context: nil, hints: [:])!)
let color = bitmapImageRep.colorAt(x: 0, y: 0)
print(color!.redComponent, color!.greenComponent, color!.blueComponent, separator: " ")
// 0.984313725490196 0.0 0.027450980392156862
It produces the unexpected output with a red component close to one and some non-zero blue component. Where does this change in color come from?