Core Graphics messing changes UIimage format - ios

Question

I wrote a simple algorithm that detects the edges in the UIImages. It works perfectly fine with the images taken from bundle (look at first image).
After I am doing some image manipulations (apply filters, masks, crop and etc) and I pass the image to the same function it comes up messed up (image 2). I assume that that the CoreGrahics is changing something internally in the image. The question is what?
That's how I start processing the image:
public struct PixelData {
var a:UInt8 = 255
var r:UInt8
var g:UInt8
var b:UInt8
}
func findEdges(cgImage:CGImageRef)->UIImage{
var pixelData = CGDataProviderCopyData(CGImageGetDataProvider(cgImage))
//var data = CFDataGetMutableBytePtr
var mdata: UnsafePointer<UInt8> = CFDataGetBytePtr(pixelData)
var data = UnsafeMutablePointer<UInt8>(mdata)
let height = CGImageGetHeight(cgImage)
let width = CGImageGetWidth(cgImage)
var start = CACurrentMediaTime()
//create an empty buffer
let emptyPixel = PixelData(a: 0, r: 0, g: 0, b: 0)
let blackPixel = PixelData(a: 255, r: 255, g: 255, b: 255)
var buffer = [PixelData](count: Int(width * height), repeatedValue: emptyPixel)
var booleanArray = [Bool](count: Int(width * height), repeatedValue: false)
for var y = 0; y < height-1; y++ {
for var x = 0; x < width; x++ {
//Current one
var currentPixelInfo: Int = ((Int(width) * Int(y)) + Int(x)) * 4
var currentAlpha = CGFloat(data[currentPixelInfo+3]) / CGFloat(255.0)
var downPixelInfo: Int = ((Int(width) * Int(y+1)) + Int(x)) * 4
var downAlpha = CGFloat(data[downPixelInfo+3]) / CGFloat(255.0)
if y == 0 && currentAlpha != 0{ // Top Edge
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
if y > 0 && y < height - 2{
//one up
var topPixelInfo: Int = ((Int(width) * Int(y - 1)) + Int(x )) * 4
var topAlpha = CGFloat(data[topPixelInfo+3]) / CGFloat(255.0)
if downAlpha == 0 && currentAlpha != 0 {//edge
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
if topAlpha == 0 && currentAlpha != 0 {//edge
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
}
if y == height - 2 && downAlpha != 0 {
booleanArray[downPixelInfo/4] = true
buffer[downPixelInfo/4] = blackPixel
}
}
}
for var y = 0; y < height-1; y++ {
for var x = 0; x < width-1; x++ {
//Current one
var currentPixelInfo: Int = ((Int(width) * Int(y)) + Int(x)) * 4
var currentAlpha = CGFloat(data[currentPixelInfo+3]) / CGFloat(255.0)
//Next
var nextPixelInfo: Int = ((Int(width) * Int(y)) + Int(x + 1)) * 4
var nextAlpha = CGFloat(data[nextPixelInfo+3]) / CGFloat(255.0)
//check horizontally
if x == 0 && currentAlpha != 0{ // Edge case
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
if x > 0 && x < width - 2{
//One before
var previousPixelInfo: Int = ((Int(width) * Int(y)) + Int(x - 1)) * 4
var previousAlpha = CGFloat(data[previousPixelInfo+3]) / CGFloat(255.0)
if nextAlpha == 0 && currentAlpha != 0 {//Living on the edge
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
if previousAlpha == 0 && currentAlpha != 0 {//Living on the edge
booleanArray[currentPixelInfo/4] = true
buffer[currentPixelInfo/4] = blackPixel
}
}
if x == width - 2 && nextAlpha != 0 {
booleanArray[nextPixelInfo/4] = true
buffer[nextPixelInfo/4] = blackPixel
}
}
}
var stop = CACurrentMediaTime()
let image = imageFromARGB32Bitmap(buffer, width: width, height: height)
println(stop - start)
return image!;
//self.imageView.image = image
}
func imageFromARGB32Bitmap(pixels:[PixelData], width:Int, height:Int)->UIImage? {
let bitsPerComponent:Int = 8
let bitsPerPixel:Int = 32
assert(pixels.count == Int(width * height))
var data = pixels // Copy to mutable []
let providerRef = CGDataProviderCreateWithCFData(
NSData(bytes: &data, length: data.count * sizeof(PixelData))
)
// let redPixel = PixelData(a: 255, r: 192, g: 0, b: 0)
let cgim = CGImageCreate(
width,
height,
bitsPerComponent,
bitsPerPixel,
width * Int(sizeof(PixelData)),
rgbColorSpace,
bitmapInfo,
providerRef,
nil,
true,
kCGRenderingIntentDefault
)
return UIImage(CGImage: cgim)
}
[][3]

Answer 1

Although I didn't figure out what exactly UIImage is doing to raw pixel data, the function that I wrote fixes the problem. Key point here is using CGImageAlphaInfo.PremultipliedFirst value of bitmap info since my data structure is expecting ARGB format.
func imageFromBitmapContext(image:CGImageRef, width:Int, height:Int)->UIImage?
{
let colorSpace:CGColorSpace = CGColorSpaceCreateDeviceRGB()
let bitmapInfo = CGBitmapInfo(CGImageAlphaInfo.PremultipliedFirst.rawValue)
let bytesPerRow = 4 * width
let context = CGBitmapContextCreate(nil, Int(width), Int(height), 8, Int(bytesPerRow), colorSpace, bitmapInfo)
CGContextDrawImage(context, CGRectMake(0, 0, CGFloat(width), CGFloat(height)), image)
let image = CGBitmapContextCreateImage(context)
return UIImage(CGImage: image)
}