在项目开发过程中，遇到一种很奇怪的现象：同一张图片，在iOS、Mac端能很正常的显示出来，但在H5、Android端却显示不出来。经过对比正常的图片与这张异常图片，发现区别在于异常图片Alpha通道一项的值为“是”，而正常能够显示出来的图片该值为“否”。查看图片简介如下：
image
那么怎么去掉一张截图的Alpha通道呢？
1. 方法一
-(void)converter:(NSString*)name{
NSLog(@"NAME: %@",name);
NSURL *url = [NSURL fileURLWithPath:name];
CGImageSourceRef source;
NSImage *srcImage =[[NSImage alloc] initWithContentsOfURL:url];;
NSLog(@"URL: %@",url);
source = CGImageSourceCreateWithData((__bridge CFDataRef)[srcImage TIFFRepresentation], NULL);
CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, 0, NULL);
CGRect rect = CGRectMake(0.f, 0.f, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
rect.size.width,
rect.size.height,
CGImageGetBitsPerComponent(imageRef),
CGImageGetBytesPerRow(imageRef),
CGImageGetColorSpace(imageRef),
kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Little
);
CGContextDrawImage(bitmapContext, rect, imageRef);
CGImageRef decompressedImageRef = CGBitmapContextCreateImage(bitmapContext);
NSImage *finalImage = [[NSImage alloc] initWithCGImage:decompressedImageRef size:NSZeroSize];
NSData *imageData = [finalImage TIFFRepresentation];
NSBitmapImageRep *imageRep = [NSBitmapImageRep imageRepWithData:imageData];
NSDictionary *imageProps = [NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:0.9] forKey:NSImageCompressionFactor];
imageData = [imageRep representationUsingType:NSPNGFileType properties:imageProps];
[imageData writeToFile:name atomically:NO];
CGImageRelease(decompressedImageRef);
CGContextRelease(bitmapContext);
}
2. 方法二
CGImageRef imageRef = [image CGImageForProposedRect:&((CGRect){0,0,image.size.width,image.size.height}) context:nil hints:nil];
NSInteger bytesPerRow = CGImageGetBytesPerRow(imageRef);
NSInteger bitsPerPixel = CGImageGetBitsPerPixel(imageRef);
NSInteger bitsPerSample = 16;
bool hasAlpha = NO;
NSInteger samplesPerPixel = hasAlpha ? 4 : 3;
///画布
NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
initWithBitmapDataPlanes:NULL
pixelsWide:image.size.width
pixelsHigh:image.size.height
bitsPerSample:bitsPerSample
samplesPerPixel:samplesPerPixel
hasAlpha:NO
isPlanar:NO
colorSpaceName:NSCalibratedRGBColorSpace
bytesPerRow:bytesPerRow*(bitsPerSample/8)
bitsPerPixel:bitsPerPixel*(bitsPerSample/8)];
NSGraphicsContext *context = [NSGraphicsContext graphicsContextWithBitmapImageRep:rep];
[NSGraphicsContext setCurrentContext:context];
[image drawInRect:NSMakeRect(0, 0, image.size.width, image.size.height) fromRect:NSZeroRect operation:NSCompositingOperationCopy fraction:1.0];
NSData *data = [rep representationUsingType:NSBitmapImageFileTypePNG properties:@{}];
NSImage *resultImage = [[NSImage alloc]initWithData:data];
方法不一样，思路一致，可以根据自己的需求调整参数。输出结果后，再次打开图片简介查看Alpha通道值。
