我有一个掩蔽CGContext
有两种类型的像素:颜色和 Alpha(不透明和透明像素)。如何计算上下文中 alpha 像素的百分比?
我没有测试它,但这应该可以解决问题 - 只需将 ReportAlphaPercent 传递给 CGImageRef:
UIImage *foo; //The image you want to analyze
float alphaPercent = ReportAlphaPercent(foo.CGImage);
float ReportAlphaPercent(CGImageRef imgRef)
{
size_t w = CGImageGetWidth(imgRef);
size_t h = CGImageGetHeight(imgRef);
unsigned char *inImage = malloc(w * h * 4);
memset(inImage, 0, (h * w * 4));
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(inImage, w, h, 8, w * 4, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
CGContextSetShouldAntialias(context, NO);
CGContextDrawImage(context, CGRectMake(0, 0, w, h), imgRef);
int byteIndex = 0;
int alphaCount = 0;
for(int i = 0; i < (w * h); i++) {
if (inImage[byteIndex + 3]) { // if the alpha value is not 0, count it
alphaCount++;
}
byteIndex += 4;
}
free(inImage);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
return (float) alphaCount / (w * h);
}
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