CGAffineTransform旋转90度

Question

我需要将图像旋转90度，即交换宽度和高度（翻转图像）。在将图像提供给编码器以创建视频之前，需要完成此操作。我用下面的代码（参考：How do I export UIImage array as a movie?）

- (CVPixelBufferRef) newPixelBufferFromCGImage: (CGImageRef) image 
{ 
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys: 
     [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey, 
     [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey, 
     nil]; 
    CVPixelBufferRef pxbuffer = NULL; 
    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, CGImageGetWidth(image), 
     CGImageGetHeight(image), kCVPixelFormatType_32ARGB, (CFDictionaryRef) options, 
     &pxbuffer); 
    NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL); 

    CVPixelBufferLockBaseAddress(pxbuffer, 0); 
    void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer); 
    NSParameterAssert(pxdata != NULL); 

    CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB(); 
    CGContextRef context = CGBitmapContextCreate(pxdata, CGImageGetWidth(image), 
     CGImageGetHeight(image), 8, 4*CGImageGetWidth(image), rgbColorSpace, 
     kCGImageAlphaNoneSkipFirst); 
    NSParameterAssert(context); 
    CGContextConcatCTM(context, CGAffineTransformMakeRotation(1.571)); // ROTATION 
    CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image), 
     CGImageGetHeight(image)), image); 
    CGColorSpaceRelease(rgbColorSpace); 
    CGContextRelease(context); 

    CVPixelBufferUnlockBaseAddress(pxbuffer, 0); 

    return pxbuffer; 
} 

然而，通过90度的旋转不工作（通过评论表示）。你能找出问题吗？

Answer 1

您应用的转换是旋转上下文，问题在于原点位于角落而不是上下文的中心。通过旋转90度的图像，你完全移动到负边界，因此看不到任何东西。 假设在您的代码中frameSize是旋转图像的大小（具有倒置的宽度/高度），您需要将该原点移动到中心并在其中应用旋转。请尝试：

CGContextTranslateCTM(context, frameSize.width/2, frameSize.height/2); 
CGContextRotateCTM(context, M_PI_2); 
CGContextTranslateCTM(context, -frameSize.height/2, -frameSize.width/2); 
CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image), CGImageGetHeight(image)), image); 


- (CVPixelBufferRef) newPixelBufferFromCGImage: (CGImageRef) image 
{ 
    CGSize frameSize = CGSizeMake(CGImageGetHeight(image), CGImageGetWidth(image)); 
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys: 
     [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey, 
     [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey, 
     nil]; 
    CVPixelBufferRef pxbuffer = NULL; 
    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, frameSize.width, 
     frameSize.height, kCVPixelFormatType_32ARGB, (CFDictionaryRef) options, 
     &pxbuffer); 
    NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL); 

    CVPixelBufferLockBaseAddress(pxbuffer, 0); 
    void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer); 
    NSParameterAssert(pxdata != NULL); 

    CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB(); 
    CGContextRef context = CGBitmapContextCreate(pxdata, frameSize.width, 
     frameSize.height, 8, 4 * frameSize.width, rgbColorSpace, 
     kCGImageAlphaNoneSkipFirst); 
    NSParameterAssert(context); 
    CGContextTranslateCTM(context, frameSize.width/2, frameSize.height/2); 
    CGContextRotateCTM(context, M_PI_2); 
    CGContextTranslateCTM(context, -frameSize.height/2, -frameSize.width/2); 
    CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image), CGImageGetHeight(image)), image); 

    CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image), 
     CGImageGetHeight(image)), image); 
    CGColorSpaceRelease(rgbColorSpace); 
    CGContextRelease(context); 

    CVPixelBufferUnlockBaseAddress(pxbuffer, 0); 

    return pxbuffer; 
}