借鉴 CoreMLHelpers 的灵感，我们可以创建一个C函数来完成你的需求。根据你的像素格式要求，我认为这个解决方案将是最高效的选项。 我在测试中使用了AVCaptureVideoDataOutput。
希望这能有所帮助！
AVCaptureVideoDataOutputSampleBufferDelegate 实现 。这里的大部分工作是创建一个居中裁剪矩形。利用AVMakeRectWithAspectRatioInsideRect 是关键(它正好做你想做的事情)。
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection; {
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
if (pixelBuffer == NULL) { return; }
size_t height = CVPixelBufferGetHeight(pixelBuffer);
size_t width = CVPixelBufferGetWidth(pixelBuffer);
CGRect videoRect = CGRectMake(0, 0, width, height);
CGSize scaledSize = CGSizeMake(299, 299);
CGRect centerCroppingRect = AVMakeRectWithAspectRatioInsideRect(scaledSize, videoRect);
CVPixelBufferRef croppedAndScaled = createCroppedPixelBuffer(pixelBuffer, centerCroppingRect, scaledSize);
CIImage *image = [CIImage imageWithCVImageBuffer:croppedAndScaled];
CVPixelBufferRelease(croppedAndScaled);
}
方法1：数据操作和加速
此函数的基本原理是先将图像裁剪到指定的矩形，然后缩放到最终需要的大小。裁剪是通过简单地忽略矩形外的数据来实现的。缩放是通过加速的vImageScale_ARGB8888函数实现的。再次感谢CoreMLHelpers提供的见解。
void assertCropAndScaleValid(CVPixelBufferRef pixelBuffer, CGRect cropRect, CGSize scaleSize) {
CGFloat originalWidth = (CGFloat)CVPixelBufferGetWidth(pixelBuffer);
CGFloat originalHeight = (CGFloat)CVPixelBufferGetHeight(pixelBuffer);
assert(CGRectContainsRect(CGRectMake(0, 0, originalWidth, originalHeight), cropRect));
assert(scaleSize.width > 0 && scaleSize.height > 0);
}
void pixelBufferReleaseCallBack(void *releaseRefCon, const void *baseAddress) {
if (baseAddress != NULL) {
free((void *)baseAddress);
}
}
CVPixelBufferRef createCroppedPixelBuffer(CVPixelBufferRef sourcePixelBuffer, CGRect croppingRect, CGSize scaledSize) {
OSType inputPixelFormat = CVPixelBufferGetPixelFormatType(sourcePixelBuffer);
assert(inputPixelFormat == kCVPixelFormatType_32BGRA
|| inputPixelFormat == kCVPixelFormatType_32ABGR
|| inputPixelFormat == kCVPixelFormatType_32ARGB
|| inputPixelFormat == kCVPixelFormatType_32RGBA);
assertCropAndScaleValid(sourcePixelBuffer, croppingRect, scaledSize);
if (CVPixelBufferLockBaseAddress(sourcePixelBuffer, kCVPixelBufferLock_ReadOnly) != kCVReturnSuccess) {
NSLog(@"Could not lock base address");
return nil;
}
void *sourceData = CVPixelBufferGetBaseAddress(sourcePixelBuffer);
if (sourceData == NULL) {
NSLog(@"Error: could not get pixel buffer base address");
CVPixelBufferUnlockBaseAddress(sourcePixelBuffer, kCVPixelBufferLock_ReadOnly);
return nil;
}
size_t sourceBytesPerRow = CVPixelBufferGetBytesPerRow(sourcePixelBuffer);
size_t offset = CGRectGetMinY(croppingRect) * sourceBytesPerRow + CGRectGetMinX(croppingRect) * 4;
vImage_Buffer croppedvImageBuffer = {
.data = ((char *)sourceData) + offset,
.height = (vImagePixelCount)CGRectGetHeight(croppingRect),
.width = (vImagePixelCount)CGRectGetWidth(croppingRect),
.rowBytes = sourceBytesPerRow
};
size_t scaledBytesPerRow = scaledSize.width * 4;
void *scaledData = malloc(scaledSize.height * scaledBytesPerRow);
if (scaledData == NULL) {
NSLog(@"Error: out of memory");
CVPixelBufferUnlockBaseAddress(sourcePixelBuffer, kCVPixelBufferLock_ReadOnly);
return nil;
}
vImage_Buffer scaledvImageBuffer = {
.data = scaledData,
.height = (vImagePixelCount)scaledSize.height,
.width = (vImagePixelCount)scaledSize.width,
.rowBytes = scaledBytesPerRow
};
vImage_Error error = vImageScale_ARGB8888(&croppedvImageBuffer, &scaledvImageBuffer, nil, 0);
CVPixelBufferUnlockBaseAddress(sourcePixelBuffer, kCVPixelBufferLock_ReadOnly);
if (error != kvImageNoError) {
NSLog(@"Error: %ld", error);
free(scaledData);
return nil;
}
OSType pixelFormat = CVPixelBufferGetPixelFormatType(sourcePixelBuffer);
CVPixelBufferRef outputPixelBuffer = NULL;
CVReturn status = CVPixelBufferCreateWithBytes(nil, scaledSize.width, scaledSize.height, pixelFormat, scaledData, scaledBytesPerRow, pixelBufferReleaseCallBack, nil, nil, &outputPixelBuffer);
if (status != kCVReturnSuccess) {
NSLog(@"Error: could not create new pixel buffer");
free(scaledData);
return nil;
}
return outputPixelBuffer;
}
方法二：CoreImage
这种方法更容易理解，并且具有传递的像素缓冲区格式相当不可知性的优点，对于某些使用情况来说是一个加分项。但是需要注意，您只能使用CoreImage支持的格式。
CVPixelBufferRef createCroppedPixelBufferCoreImage(CVPixelBufferRef pixelBuffer,
CGRect cropRect,
CGSize scaleSize,
CIContext *context) {
assertCropAndScaleValid(pixelBuffer, cropRect, scaleSize);
CIImage *image = [CIImage imageWithCVImageBuffer:pixelBuffer];
image = [image imageByCroppingToRect:cropRect];
CGFloat scaleX = scaleSize.width / CGRectGetWidth(image.extent);
CGFloat scaleY = scaleSize.height / CGRectGetHeight(image.extent);
image = [image imageByApplyingTransform:CGAffineTransformMakeScale(scaleX, scaleY)];
image = [image imageByApplyingTransform:CGAffineTransformMakeTranslation(-image.extent.origin.x, -image.extent.origin.y)];
CVPixelBufferRef output = NULL;
CVPixelBufferCreate(nil,
CGRectGetWidth(image.extent),
CGRectGetHeight(image.extent),
CVPixelBufferGetPixelFormatType(pixelBuffer),
nil,
&output);
if (output != NULL) {
[context render:image toCVPixelBuffer:output];
}
return output;
}
在调用处可以创建CIContext，也可以创建并存储在属性中。有关选项的信息，请参见文档。
if (self.context == nil) {
self.context = [CIContext context];
}
