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）。这需要在我将图像提供给编码器以从中创建视频之前完成。我使用以下代码（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;
}

但是，90 度的旋转不起作用（由注释指示）。你能找出问题吗？

Answer 1

您正在应用的变换是旋转上下文，问题是原点位于角落而不是上下文的中心。通过将图像旋转 90 度，您可以将其完全移动到负边界，因此什么也看不到。 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），您需要将此原点移动到中心并在那里应用旋转。试试：

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;
}