相关疑难解决方法(0)

我目前正在尝试获取UIImageView中像素的alpha值.我从[UIImageView image]中获取了CGImage,并从中创建了一个RGBA字节数组.Alpha是预乘的.

CGImageRef image = uiImage.CGImage;
NSUInteger width = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
rawData = malloc(height * width * 4);
bytesPerPixel = 4;
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), image);
CGContextRelease(context);

然后,我使用UIImageView中的坐标计算给定alpha通道的数组索引.

int byteIndex = (bytesPerRow * uiViewPoint.y) + uiViewPoint.x * bytesPerPixel;
unsigned char alpha = rawData[byteIndex + 3];

但是我没有得到我期望的价值.对于图像的完全黑色透明区域,我得到alpha通道的非零值.我是否需要翻译UIKit和Core Graphics之间的坐标 - …