Re: Get pixel data as array from UIImage/CGImage in swift

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?

Never mind what the other post says. Please post a link to the image you're starting with and show in full the actual code you are using. — matt
– matt, Commented Mar 11, 2016 at 16:19

megakilo · Accepted Answer · 2016-03-11 20:16:56Z

0

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
}

answered Mar 11, 2016 at 20:16

megakilo

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1 Comment

Simon Youens Over a year ago

Thanks megakilo. I knew alpha was the problem and I had found the parameters, but wasn't aware of the crucial .rawvalue! :-)

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