在裁剪之前调整 UIImage 或 CIImage 的大小会拉伸图像吗？

Question

好的，我已经尝试了每一个我能找到的例子来做到这一点，遗憾的是它们中的大多数都是从 2008 年到 2009 年，而 iOS5 似乎非常不同。我只是想调整大小和图像，以便短边是我指定的大小，并且保持成比例。

我正在使用 AVFoundation 从相机中获取图像，我通过 UIImage 将其转换为 CIImage，这样我就可以应用过滤器并对其进行处理，然后再转换回 UIImage 进行保存。

- (void) startCamera {

session = [[AVCaptureSession alloc] init];
session.sessionPreset = AVCaptureSessionPresetPhoto;

AVCaptureVideoPreviewLayer *captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];
captureVideoPreviewLayer.frame = _cameraView.bounds;
[_cameraView.layer addSublayer:captureVideoPreviewLayer];
captureVideoPreviewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;

AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
NSError *error = nil;

AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
if (!input) {
    //no cam, handle error - need to put up a nice happy note here
    NSLog(@"ERROR: %@", error);
}

[session addInput:input];

stillImage = [[AVCaptureStillImageOutput alloc] init];
NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys:AVVideoCodecJPEG , AVVideoCodecKey, nil];
[stillImage setOutputSettings:outputSettings];

[session addOutput:stillImage];
[session startRunning];
}

- (IBAction) _cameraButtonPress:(id)sender {

AVCaptureConnection *videoConnection = nil;
for (AVCaptureConnection *connection in stillImage.connections)
{
    for (AVCaptureInputPort *port in [connection inputPorts])
    {
        if ([[port mediaType] isEqual:AVMediaTypeVideo] )
        {
            videoConnection = connection;
            break;
        }
    }
    if (videoConnection) {
        break;
    }
}

[stillImage captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler:^(CMSampleBufferRef imageSampleBuffer, NSError *error) {

    NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageSampleBuffer];
    UIImage *startImage = [[UIImage alloc] initWithData:imageData];

    //resizing of image to take place before anything else
    UIImage *image = [startImage imageScaledToFitSize:_exportSSize];  //resizes to the size given in the prefs

    //change the context to render using cpu, so on app exit renders get completed
    context = [CIContext contextWithOptions:
               [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
                                           forKey:kCIContextUseSoftwareRenderer]];

    //set up the saving library
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

    //create a new ciimage
    CIImage *greyImage = [[CIImage alloc] initWithCGImage:[image CGImage]];

    //create a CIMonochrome filter 
    desaturate = [CIFilter filterWithName:@"CIColorMonochrome" keysAndValues:kCIInputImageKey, greyImage, @"inputIntensity", [NSNumber numberWithFloat:0.00], nil];

    //[crop setValue:ciImage forKey:@"inputImage"];
    //[crop setValue:[CIVector vectorWithX:0.0f Y:0.0f Z:_exportSize W:_exportSize] forKey:@"inputRectangle"];

    CIImage *croppedColourImage = [desaturate valueForKey:@"outputImage"];
    //convert it to a cgimage for saving
    CGImageRef cropColourImage = [context createCGImage:croppedColourImage fromRect:[croppedColourImage extent]];

     //saving of the images         
     [library writeImageToSavedPhotosAlbum:cropColourImage metadata:[croppedColourImage properties] completionBlock:^(NSURL *assetURL, NSError *error){
     if (error) {
         NSLog(@"ERROR in image save: %@", error);
     } else
     {
         NSLog(@"SUCCESS in image save");
         //in here we'll put a nice animation or something
         CGImageRelease(cropColourImage);
     }

     }];
}];
}

此代码是测试代码，因此会有各种尝试，对此表示歉意。

我最接近的是使用 Matt Gemmell 的代码 here

但是，无论我尝试什么，总是会拉伸图像。我想调整图像大小，然后将其裁剪为 512 像素正方形。如果我只是做一个 CICrop 过滤器，它会占用左上角的 512 像素，所以我会丢失很多图像（抓取高分辨率照片，因为稍后我还想导出 1800x1800 图像）。我的想法是先调整图像大小，然后裁剪。但无论我做什么，图像都会被拉伸。

所以我的问题是，在 iOS5+ 中是否有适当的建议可识别方法来调整大小和图像？

谢谢。

Answer 1

我找到了一种方法：

UIGraphicsBeginImageContext( newSize );

[image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];

UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();