Re: Get pixel data as array from UIImage/CGImage i

Question

This code works for me (Swift 2 playground) in Gray scale, loading a colour .BMP image...

Get pixel data as array from UIImage/CGImage in swift

... but if I change bytesPerPixel to 3 (or 4) and use

let colorSpace = CGColorSpaceCreateDeviceRGB()

...the code still runs with no errors, but pixel values are all zeros. Any suggestions as to how to fix that?

Answer 1

You probably forgot the CGImageAlphaInfo parameter. For color images, if you assume bytesPerPixel to be 4, you need to set either RGBA (or ARGB) when creating the context. Following is an example for RGBA without the alpha channel.

// RGBA format
let ctx = CGBitmapContextCreate(&data, pixelsWide, pixelsHigh, 8,   
    bitmapBytesPerRow, colorSpace, CGImageAlphaInfo.NoneSkipLast.rawValue)

According to the documentation, you have these options:

enum CGImageAlphaInfo : UInt32 {
    case None
    case PremultipliedLast
    case PremultipliedFirst
    case Last
    case First
    case NoneSkipLast
    case NoneSkipFirst
    case Only
}