Convert UIImage to UInt8 Array in Swift

Question

all!

I've been doing a lot of research into this and I've integrated several different solutions into my project, but none of them seem to work. My current solution has been borrowed from this thread.

When I run my code, however, two things happen:

The pixel array remains initialized but unpopulated (Full of 0s)
I get two errors:

CGBitmapContextCreate: unsupported parameter combination: set CGBITMAP_CONTEXT_LOG_ERRORS environmental variable to see the details

and

CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set

Any ideas? Here is my current built function that I'm calling for my Image class:

init?(fromImage image: UIImage!) {
    let imageRef = image!.CGImage
    self.width = CGImageGetWidth(imageRef)
    self.height = CGImageGetHeight(imageRef)
    let colorspace = CGColorSpaceCreateDeviceRGB()
    let bytesPerRow = (4 * width);
    let bitsPerComponent :UInt = 8
    let pixels = UnsafeMutablePointer<UInt8>(malloc(width*height*4))

    var context = CGBitmapContextCreate(pixels, width, height, Int(bitsPerComponent), bytesPerRow, colorspace, 0);

    CGContextDrawImage(context, CGRectMake(0, 0, CGFloat(width), CGFloat(height)), imageRef)

Any pointers would help a lot, as I'm new to understanding how all of this CGBitmap stuff works.

Thanks a ton!

First step would be to remove the implicitly unwrapped and force unwrapped Optional. They are just asking for runtime crashes and errors. It's much better to check an Optional and properly handle each case. — user887210
– user887210, Commented Apr 5, 2016 at 4:28

世人都晓神仙好 · Accepted Answer · 2016-04-05 04:45:44Z

4

You should not pass an 0 as the bitmapInfo param for CGBitmapContextCreate. For RGBA you shall pass CGImageAlphaInfo.PremultipliedLast.rawValue.

Supported combinations of bitsPerComponent, bytesPerRow, colorspace and bitmapInfo can be found here: https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_context/dq_context.html#//apple_ref/doc/uid/TP30001066-CH203-BCIBHHBB

Note that 32 bits per pixel (bpp) is 4 bytes per pixel and you use it to calculate bytesPerRow

answered Apr 5, 2016 at 4:45

世人都晓神仙好

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1 Comment

Ted Lasso Over a year ago

This definitely helped a ton for getting the array populated, although I'm not sure it's being populated correctly. When I convert it back into an image straight away, the channels seems very messed up. Should I continue this here or start another question?

Christian Abella · Accepted Answer · 2016-04-05 04:37:59Z

-1

You need to convert the image to NSData and then convert NSData to UInt8 array.

let data: NSData = UIImagePNGRepresentation(image)

// or let data: NSData = UIImageJPGRepresentation(image)

let count = data.length / sizeof(UInt8)

// create an array of Uint8
var array = [UInt8](count: count, repeatedValue: 0)

// copy bytes into array
data.getBytes(&array, length:count * sizeof(UInt8))

answered Apr 5, 2016 at 4:37

Christian Abella

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1 Comment

Martin R Over a year ago

That would give you the PNG representation, not the raw pixel data.

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