Is it possible to load a 24-bits BMP with a native iOS API?

Question

I'm programming an app for a device whose API give us a const char* to an array of bytes that correspond to the pixel data of a BMP with a colour depth of 24-bit, which Core Graphics doesn't support, so I'm looping over the bytes adding a fourth one for the alpha channel, and latter skipping it with CGImageAlphaInfo.noneSkipLast:

let imageProperties = /* call to the device API that returns the image metadata */
let imageData = UnsafeMutablePointer<CChar>(mutating: /* call to the device API that returns the pointer */)

let imageWidth = Int(imageProperties.width)
let imageHeight = Int(imageProperties.height)
let bytesPerPixelWithoutAlpha = 3
let imageSize = imageWidth * imageHeight * bytesPerPixelWithoutAlpha
var imageDataWithAlpha = [CChar]()
for i in stride(from: 0, to: imageSize, by: bytesPerPixelWithoutAlpha) {
    imageDataWithAlpha.append(imageData![i])
    imageDataWithAlpha.append(imageData![i + 1])
    imageDataWithAlpha.append(imageData![i + 2])
    imageDataWithAlpha.append(127)
}
let bytesPerPixelWithAlpha = 4
let bytesPerRow = imageWidth * bytesPerPixelWithAlpha
let imageDataWithAlphaPointer = UnsafeMutablePointer<CChar>.allocate(capacity: imageDataWithAlpha.count)
imageDataWithAlphaPointer.initialize(from: &imageDataWithAlpha, count: imageDataWithAlpha.count)

let bmp = CGContext(
    data: imageDataWithAlphaPointer,
    width: imageWidth,
    height: imageHeight,
    bitsPerComponent: 8,
    bytesPerRow: bytesPerRow,
    space: CGColorSpaceCreateDeviceRGB(),
    bitmapInfo: CGImageAlphaInfo.noneSkipLast.rawValue
)!.makeImage()!

bmp is passed to CIContext().writeJPEGRepresentation to write the JPEG to disk, and it works, but my code smells, and I could bet there's some other API I could use instead of my loop

Answer 1

I think you can wrap a CVPixelBuffer with an appropriate format around your data and pass that to Core Image. Something like that:

var pixelBuffer: CVPixelBuffer?
let result = CVPixelBufferCreateWithBytes(kCFAllocatorDefault,
                                          imageWidth,
                                          imageHeight,
                                          kCVPixelFormatType_24RGB,
                                          imageData,
                                          3 * imageWidth,
                                          nil,
                                          nil,
                                          nil,
                                          &pixelBuffer)
guard result == kCVReturnSuccess, let pixelBuffer = pixelBuffer else {
    fatalError()
}
let image = CIImage(cvPixelBuffer: pixelBuffer)
// ...

Core Image should automatically fill the alpha channel if needed.