我有一个UIImage (Cocoa Touch)。从那里，我很高兴得到一个CGImage或任何你想要的可用的东西。我想写这个函数:
- (int)getRGBAFromImage:(UIImage *)image atX:(int)xx andY:(int)yy {
// [...]
// What do I want to read about to help
// me fill in this bit, here?
// [...]
int result = (red << 24) | (green << 16) | (blue << 8) | alpha;
return result;
}
当前回答
真不敢相信这里竟然没有一个正确答案。不需要分配指针，未乘的值仍然需要规范化。开门见山地说，下面是Swift 4的正确版本。对于UIImage，只需使用。cgimage。
extension CGImage {
func colors(at: [CGPoint]) -> [UIColor]? {
let colorSpace = CGColorSpaceCreateDeviceRGB()
let bytesPerPixel = 4
let bytesPerRow = bytesPerPixel * width
let bitsPerComponent = 8
let bitmapInfo: UInt32 = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Big.rawValue
guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo),
let ptr = context.data?.assumingMemoryBound(to: UInt8.self) else {
return nil
}
context.draw(self, in: CGRect(x: 0, y: 0, width: width, height: height))
return at.map { p in
let i = bytesPerRow * Int(p.y) + bytesPerPixel * Int(p.x)
let a = CGFloat(ptr[i + 3]) / 255.0
let r = (CGFloat(ptr[i]) / a) / 255.0
let g = (CGFloat(ptr[i + 1]) / a) / 255.0
let b = (CGFloat(ptr[i + 2]) / a) / 255.0
return UIColor(red: r, green: g, blue: b, alpha: a)
}
}
}
必须首先将图像绘制/转换为缓冲区的原因是，图像可以有几种不同的格式。需要此步骤将其转换为您可以读取的一致格式。
其他回答
Swift 5版本
这里给出的答案要么是过时的，要么是不正确的，因为他们没有考虑到以下因素:
The pixel size of the image can differ from its point size that is returned by image.size.width/image.size.height. There can be various layouts used by pixel components in the image, such as BGRA, ABGR, ARGB etc. or may not have an alpha component at all, such as BGR and RGB. For example, UIView.drawHierarchy(in:afterScreenUpdates:) method can produce BGRA images. Color components can be premultiplied by the alpha for all pixels in the image and need to be divided by alpha in order to restore the original color. For memory optimization used by CGImage, the size of a pixel row in bytes can be greater than the mere multiplication of the pixel width by 4.
下面的代码提供了一个通用的Swift 5解决方案，以获得所有此类特殊情况下像素的UIColor。代码优化的目标是可用性和清晰度，而不是性能。
public extension UIImage {
var pixelWidth: Int {
return cgImage?.width ?? 0
}
var pixelHeight: Int {
return cgImage?.height ?? 0
}
func pixelColor(x: Int, y: Int) -> UIColor {
assert(
0..<pixelWidth ~= x && 0..<pixelHeight ~= y,
"Pixel coordinates are out of bounds")
guard
let cgImage = cgImage,
let data = cgImage.dataProvider?.data,
let dataPtr = CFDataGetBytePtr(data),
let colorSpaceModel = cgImage.colorSpace?.model,
let componentLayout = cgImage.bitmapInfo.componentLayout
else {
assertionFailure("Could not get a pixel of an image")
return .clear
}
assert(
colorSpaceModel == .rgb,
"The only supported color space model is RGB")
assert(
cgImage.bitsPerPixel == 32 || cgImage.bitsPerPixel == 24,
"A pixel is expected to be either 4 or 3 bytes in size")
let bytesPerRow = cgImage.bytesPerRow
let bytesPerPixel = cgImage.bitsPerPixel/8
let pixelOffset = y*bytesPerRow + x*bytesPerPixel
if componentLayout.count == 4 {
let components = (
dataPtr[pixelOffset + 0],
dataPtr[pixelOffset + 1],
dataPtr[pixelOffset + 2],
dataPtr[pixelOffset + 3]
)
var alpha: UInt8 = 0
var red: UInt8 = 0
var green: UInt8 = 0
var blue: UInt8 = 0
switch componentLayout {
case .bgra:
alpha = components.3
red = components.2
green = components.1
blue = components.0
case .abgr:
alpha = components.0
red = components.3
green = components.2
blue = components.1
case .argb:
alpha = components.0
red = components.1
green = components.2
blue = components.3
case .rgba:
alpha = components.3
red = components.0
green = components.1
blue = components.2
default:
return .clear
}
// If chroma components are premultiplied by alpha and the alpha is `0`,
// keep the chroma components to their current values.
if cgImage.bitmapInfo.chromaIsPremultipliedByAlpha && alpha != 0 {
let invUnitAlpha = 255/CGFloat(alpha)
red = UInt8((CGFloat(red)*invUnitAlpha).rounded())
green = UInt8((CGFloat(green)*invUnitAlpha).rounded())
blue = UInt8((CGFloat(blue)*invUnitAlpha).rounded())
}
return .init(red: red, green: green, blue: blue, alpha: alpha)
} else if componentLayout.count == 3 {
let components = (
dataPtr[pixelOffset + 0],
dataPtr[pixelOffset + 1],
dataPtr[pixelOffset + 2]
)
var red: UInt8 = 0
var green: UInt8 = 0
var blue: UInt8 = 0
switch componentLayout {
case .bgr:
red = components.2
green = components.1
blue = components.0
case .rgb:
red = components.0
green = components.1
blue = components.2
default:
return .clear
}
return .init(red: red, green: green, blue: blue, alpha: UInt8(255))
} else {
assertionFailure("Unsupported number of pixel components")
return .clear
}
}
}
public extension UIColor {
convenience init(red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8) {
self.init(
red: CGFloat(red)/255,
green: CGFloat(green)/255,
blue: CGFloat(blue)/255,
alpha: CGFloat(alpha)/255)
}
}
public extension CGBitmapInfo {
enum ComponentLayout {
case bgra
case abgr
case argb
case rgba
case bgr
case rgb
var count: Int {
switch self {
case .bgr, .rgb: return 3
default: return 4
}
}
}
var componentLayout: ComponentLayout? {
guard let alphaInfo = CGImageAlphaInfo(rawValue: rawValue & Self.alphaInfoMask.rawValue) else { return nil }
let isLittleEndian = contains(.byteOrder32Little)
if alphaInfo == .none {
return isLittleEndian ? .bgr : .rgb
}
let alphaIsFirst = alphaInfo == .premultipliedFirst || alphaInfo == .first || alphaInfo == .noneSkipFirst
if isLittleEndian {
return alphaIsFirst ? .bgra : .abgr
} else {
return alphaIsFirst ? .argb : .rgba
}
}
var chromaIsPremultipliedByAlpha: Bool {
let alphaInfo = CGImageAlphaInfo(rawValue: rawValue & Self.alphaInfoMask.rawValue)
return alphaInfo == .premultipliedFirst || alphaInfo == .premultipliedLast
}
}
一种方法是将图像绘制到一个位图上下文中，这个位图上下文由给定的缓冲区支持，用于给定的颜色空间(在这种情况下是RGB):(注意，这将把图像数据复制到该缓冲区中，所以你想要缓存它，而不是每次你需要获取像素值时都做这个操作)
请看下面的例子:
// First get the image into your data buffer
CGImageRef image = [myUIImage CGImage];
NSUInteger width = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = malloc(height * width * 4);
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextDrawImage(context, CGRectMake(0, 0, width, height));
CGContextRelease(context);
// Now your rawData contains the image data in the RGBA8888 pixel format.
int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
red = rawData[byteIndex];
green = rawData[byteIndex + 1];
blue = rawData[byteIndex + 2];
alpha = rawData[byteIndex + 3];
Apple的技术问答QA1509展示了以下简单的方法:
CFDataRef CopyImagePixels(CGImageRef inImage)
{
return CGDataProviderCopyData(CGImageGetDataProvider(inImage));
}
使用CFDataGetBytePtr来获得实际的字节(以及各种CGImageGet*方法来理解如何解释它们)。
根据不同的答案，但主要是在这一点上，这是我需要的:
UIImage *image1 = ...; // The image from where you want a pixel data
int pixelX = ...; // The X coordinate of the pixel you want to retrieve
int pixelY = ...; // The Y coordinate of the pixel you want to retrieve
uint32_t pixel1; // Where the pixel data is to be stored
CGContextRef context1 = CGBitmapContextCreate(&pixel1, 1, 1, 8, 4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipFirst);
CGContextDrawImage(context1, CGRectMake(-pixelX, -pixelY, CGImageGetWidth(image1.CGImage), CGImageGetHeight(image1.CGImage)), image1.CGImage);
CGContextRelease(context1);
由于这些行，您将拥有一个AARRGGBB格式的像素，alpha总是设置为FF，在4字节无符号整数pixel1中。
这是一个SO线程，其中@Matt通过替换图像只渲染所需的像素到1x1上下文，以便所需的像素与上下文中的一个像素对齐。
