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extract pixel from a CGImage
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// extract pixel from a CGImage | |
/* use case: | |
let extractor = PixelExtractor(img: UIImage(named: "gauge_vertical")!.CGImage!) | |
let color = extractor.color_at(x: 10, y: 20) | |
*/ | |
class PixelExtractor { | |
// taken from http://stackoverflow.com/questions/24049313/ | |
// and adapted to swift 1.2 | |
let image: CGImage | |
let context: CGContextRef | |
var width: Int { | |
get { | |
return CGImageGetWidth(image) | |
} | |
} | |
var height: Int { | |
get { | |
return CGImageGetHeight(image) | |
} | |
} | |
init(img: CGImage) { | |
image = img | |
context = PixelExtractor.create_bitmap_context(img) | |
} | |
private class func create_bitmap_context(img: CGImage)->CGContextRef { | |
// Get image width, height | |
let pixelsWide = CGImageGetWidth(img) | |
let pixelsHigh = CGImageGetHeight(img) | |
// Declare the number of bytes per row. Each pixel in the bitmap in this | |
// example is represented by 4 bytes; 8 bits each of red, green, blue, and | |
// alpha. | |
let bitmapBytesPerRow = pixelsWide * 4 | |
let bitmapByteCount = bitmapBytesPerRow * Int(pixelsHigh) | |
// Use the generic RGB color space. | |
let colorSpace = CGColorSpaceCreateDeviceRGB() | |
// Allocate memory for image data. This is the destination in memory | |
// where any drawing to the bitmap context will be rendered. | |
let bitmapData = malloc(bitmapByteCount) | |
let bitmapInfo = CGBitmapInfo(CGImageAlphaInfo.PremultipliedFirst.rawValue) | |
// Create the bitmap context. We want pre-multiplied ARGB, 8-bits | |
// per component. Regardless of what the source image format is | |
// (CMYK, Grayscale, and so on) it will be converted over to the format | |
// specified here by CGBitmapContextCreate. | |
let context = CGBitmapContextCreate(bitmapData, pixelsWide, pixelsHigh, 8, | |
bitmapBytesPerRow, colorSpace, bitmapInfo) | |
// draw the image onto the context | |
let rect = CGRect(x: 0, y: 0, width: pixelsWide, height: pixelsHigh) | |
CGContextDrawImage(context, rect, img) | |
return context | |
} | |
func color_at(#x: Int, y: Int)->UIColor { | |
assert(0<=x && x<width) | |
assert(0<=y && y<height) | |
let uncasted_data = CGBitmapContextGetData(context) | |
let data = UnsafePointer<UInt8>(uncasted_data) | |
let offset = 4 * (y * width + x) | |
let alpha = data[offset] | |
let red = data[offset+1] | |
let green = data[offset+2] | |
let blue = data[offset+3] | |
let color = UIColor(red: CGFloat(red)/255.0, green: CGFloat(green)/255.0, blue: CGFloat(blue)/255.0, alpha: CGFloat(alpha)/255.0) | |
return color | |
} | |
} |
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Great thanks for sharing!
Regarding the memory leakage, you need to dstroy and dealloc the bitmapData after using; otherwise, memory is being allocated over and over without being released.