问题
I would like to take photos of A4 pieces of paper with writing on. Importantly, I want the text to be readable, but I do not want images with resolutions like 2592x1936 pixel or 3264x2448 pixel as that would be too big. Also, I assume that rescaling the photo after capturing takes extra time, so I would like to avoid this too.
We can choose between the following qualities:
UIImagePickerControllerQualityTypeHigh = 0
UIImagePickerControllerQualityTypeMedium = 1 default value
UIImagePickerControllerQualityTypeLow = 2
UIImagePickerControllerQualityType640x480 = 3,
UIImagePickerControllerQualityTypeIFrame1280x720 = 4,
UIImagePickerControllerQualityTypeIFrame960x540 = 5
If we were using the AVFoundation, we could choose resolutions from this nice table (under headline "Capturing Still Images").
But is there a similar table for UIImagePickerController, which for example says that UIImagePickerControllerQualityTypeHigh equals 1920x1080 on iPhone 3gs?
回答1:
UIImagepickerController quality is used for video recording (used in UIImagepickerController property "videoQuality").
I guess that if you want to specify what the exact photo resolution should be, you should use the AV Foundation framework instead of the UIImagepickerController. Or as you said, convert the picture afterwards.
To resize it afterwards (found here):
// ==============================================================
// resizedImage
// ==============================================================
// Return a scaled down copy of the image.
UIImage* resizedImage(UIImage *inImage, CGRect thumbRect)
{
CGImageRef imageRef = [inImage CGImage];
CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(imageRef);
// There's a wierdness with kCGImageAlphaNone and CGBitmapContextCreate
// see Supported Pixel Formats in the Quartz 2D Programming Guide
// Creating a Bitmap Graphics Context section
// only RGB 8 bit images with alpha of kCGImageAlphaNoneSkipFirst, kCGImageAlphaNoneSkipLast, kCGImageAlphaPremultipliedFirst,
// and kCGImageAlphaPremultipliedLast, with a few other oddball image kinds are supported
// The images on input here are likely to be png or jpeg files
if (alphaInfo == kCGImageAlphaNone)
alphaInfo = kCGImageAlphaNoneSkipLast;
// Build a bitmap context that's the size of the thumbRect
CGContextRef bitmap = CGBitmapContextCreate(
NULL,
thumbRect.size.width, // width
thumbRect.size.height, // height
CGImageGetBitsPerComponent(imageRef), // really needs to always be 8
4 * thumbRect.size.width, // rowbytes
CGImageGetColorSpace(imageRef),
alphaInfo
);
// Draw into the context, this scales the image
CGContextDrawImage(bitmap, thumbRect, imageRef);
// Get an image from the context and a UIImage
CGImageRef ref = CGBitmapContextCreateImage(bitmap);
UIImage* result = [UIImage imageWithCGImage:ref];
CGContextRelease(bitmap); // ok if NULL
CGImageRelease(ref);
return result;
}
Hope this helps!
回答2:
In a word: no.
There is no "similar table" because you're misunderstanding how the likes of UIImagePickerControllerQualityTypeHigh are used by UIImagePickerController.
If you're capturing still images with UIImagePickerController you will get them at the default (i.e. maximum) quality of the device in question—anywhere from 8 MP with iPhone 4s to < 1 MP with iPod touch or iPad 2).
With AVFoundation, however, you have choices, thanks to the session presets to which you refer.
But unlike these AVFoundation session presets, the UIImagePickerController UIImagePickerControllerQualityType options only apply to motion video, not still image capture.
So you have the choice of using AVFoundation to control capture size, or re-sizing the full-size images before saving them; but UIImagePickerController can't do what you want, I'm afraid.
回答3:
The problem with Thermometer's answer is that it screws up with the image orientation.
I've found this solution that solves the orientation problem and has a faster performance:
- (UIImage *)scaleAndRotateImage:(UIImage *)image {
int kMaxResolution = 320; // Or whatever
CGImageRef imgRef = image.CGImage;
CGFloat width = CGImageGetWidth(imgRef);
CGFloat height = CGImageGetHeight(imgRef);
CGAffineTransform transform = CGAffineTransformIdentity;
CGRect bounds = CGRectMake(0, 0, width, height);
if (width > kMaxResolution || height > kMaxResolution) {
CGFloat ratio = width/height;
if (ratio > 1) {
bounds.size.width = kMaxResolution;
bounds.size.height = bounds.size.width / ratio;
}
else {
bounds.size.height = kMaxResolution;
bounds.size.width = bounds.size.height * ratio;
}
}
CGFloat scaleRatio = bounds.size.width / width;
CGSize imageSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef));
CGFloat boundHeight;
UIImageOrientation orient = image.imageOrientation;
switch(orient) {
case UIImageOrientationUp: //EXIF = 1
transform = CGAffineTransformIdentity;
break;
case UIImageOrientationUpMirrored: //EXIF = 2
transform = CGAffineTransformMakeTranslation(imageSize.width, 0.0);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
break;
case UIImageOrientationDown: //EXIF = 3
transform = CGAffineTransformMakeTranslation(imageSize.width, imageSize.height);
transform = CGAffineTransformRotate(transform, M_PI);
break;
case UIImageOrientationDownMirrored: //EXIF = 4
transform = CGAffineTransformMakeTranslation(0.0, imageSize.height);
transform = CGAffineTransformScale(transform, 1.0, -1.0);
break;
case UIImageOrientationLeftMirrored: //EXIF = 5
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(imageSize.height, imageSize.width);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
transform = CGAffineTransformRotate(transform, 3.0 * M_PI / 2.0);
break;
case UIImageOrientationLeft: //EXIF = 6
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(0.0, imageSize.width);
transform = CGAffineTransformRotate(transform, 3.0 * M_PI / 2.0);
break;
case UIImageOrientationRightMirrored: //EXIF = 7
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeScale(-1.0, 1.0);
transform = CGAffineTransformRotate(transform, M_PI / 2.0);
break;
case UIImageOrientationRight: //EXIF = 8
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(imageSize.height, 0.0);
transform = CGAffineTransformRotate(transform, M_PI / 2.0);
break;
default:
[NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"];
}
UIGraphicsBeginImageContext(bounds.size);
CGContextRef context = UIGraphicsGetCurrentContext();
if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) {
CGContextScaleCTM(context, -scaleRatio, scaleRatio);
CGContextTranslateCTM(context, -height, 0);
}
else {
CGContextScaleCTM(context, scaleRatio, -scaleRatio);
CGContextTranslateCTM(context, 0, -height);
}
CGContextConcatCTM(context, transform);
CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, width, height), imgRef);
UIImage *imageCopy = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return imageCopy;
}
回答4:
For your Swift 2.1-Project, this is the translation of @marcelosalloum's code:
func scaleAndRotateImage(image: UIImage, kMaxResolution: CGFloat) -> UIImage {
var imageCopy: UIImage = image
if let imgRef: CGImageRef = image.CGImage {
let width = CGFloat(CGImageGetWidth(imgRef))
let height = CGFloat(CGImageGetHeight(imgRef))
var transform = CGAffineTransformIdentity
var bounds = CGRectMake(0, 0, width, height)
if width > kMaxResolution || height > kMaxResolution {
let ratio = width/height
if ratio > 1 {
bounds.size.width = kMaxResolution
bounds.size.height = bounds.size.width / ratio
} else {
bounds.size.height = kMaxResolution
bounds.size.width = bounds.size.height * ratio
}
}
let scaleRatio = bounds.size.width / width
let imageSize = CGSizeMake(width, height)
let boundHeight: CGFloat
let orient: UIImageOrientation = image.imageOrientation
switch orient {
case .Up:
transform = CGAffineTransformIdentity
case .UpMirrored:
transform = CGAffineTransformMakeTranslation(imageSize.width, 0.0)
transform = CGAffineTransformScale(transform, -1.0, 1.0)
case .Down:
transform = CGAffineTransformMakeTranslation(imageSize.width, imageSize.height)
transform = CGAffineTransformRotate(transform, CGFloat(M_PI))
case .DownMirrored: //EXIF = 4
transform = CGAffineTransformMakeTranslation(0.0, imageSize.height);
transform = CGAffineTransformScale(transform, 1.0, -1.0);
case .LeftMirrored: //EXIF = 5
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(imageSize.height, imageSize.width);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
transform = CGAffineTransformRotate(transform, 3.0 * CGFloat(M_PI) / 2.0);
case .Left: //EXIF = 6
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(0.0, imageSize.width);
transform = CGAffineTransformRotate(transform, 3.0 * CGFloat(M_PI) / 2.0);
case .RightMirrored: //EXIF = 7
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeScale(-1.0, 1.0);
transform = CGAffineTransformRotate(transform, CGFloat(M_PI) / 2.0);
case .Right: //EXIF = 8
boundHeight = bounds.size.height;
bounds.size.height = bounds.size.width;
bounds.size.width = boundHeight;
transform = CGAffineTransformMakeTranslation(imageSize.height, 0.0);
transform = CGAffineTransformRotate(transform, CGFloat(M_PI) / 2.0);
}
UIGraphicsBeginImageContext(bounds.size)
if let context: CGContextRef = UIGraphicsGetCurrentContext() {
if orient == .Right || orient == .Left {
CGContextScaleCTM(context, -scaleRatio, scaleRatio)
CGContextTranslateCTM(context, -height, 0)
} else {
CGContextScaleCTM(context, scaleRatio, -scaleRatio)
CGContextTranslateCTM(context, 0, -height)
}
CGContextConcatCTM(context, transform)
CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0,0,width,height), imgRef)
imageCopy = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
}
}
return imageCopy
}
回答5:
A swift 3 translation of marcelosalloum's answer:
private func scale(image originalImage: UIImage, toLessThan maxResolution: CGFloat) -> UIImage? {
guard let imageReference = originalImage.cgImage else { return nil }
let rotate90 = CGFloat.pi/2.0 // Radians
let rotate180 = CGFloat.pi // Radians
let rotate270 = 3.0*CGFloat.pi/2.0 // Radians
let originalWidth = CGFloat(imageReference.width)
let originalHeight = CGFloat(imageReference.height)
let originalOrientation = originalImage.imageOrientation
var newWidth = originalWidth
var newHeight = originalHeight
if originalWidth > maxResolution || originalHeight > maxResolution {
let aspectRatio: CGFloat = originalWidth / originalHeight
newWidth = aspectRatio > 1 ? maxResolution : maxResolution * aspectRatio
newHeight = aspectRatio > 1 ? maxResolution / aspectRatio : maxResolution
}
let scaleRatio: CGFloat = newWidth / originalWidth
var scale: CGAffineTransform = .init(scaleX: scaleRatio, y: -scaleRatio)
scale = scale.translatedBy(x: 0.0, y: -originalHeight)
var rotateAndMirror: CGAffineTransform
switch originalOrientation {
case .up:
rotateAndMirror = .identity
case .upMirrored:
rotateAndMirror = .init(translationX: originalWidth, y: 0.0)
rotateAndMirror = rotateAndMirror.scaledBy(x: -1.0, y: 1.0)
case .down:
rotateAndMirror = .init(translationX: originalWidth, y: originalHeight)
rotateAndMirror = rotateAndMirror.rotated(by: rotate180 )
case .downMirrored:
rotateAndMirror = .init(translationX: 0.0, y: originalHeight)
rotateAndMirror = rotateAndMirror.scaledBy(x: 1.0, y: -1.0)
case .left:
(newWidth, newHeight) = (newHeight, newWidth)
rotateAndMirror = .init(translationX: 0.0, y: originalWidth)
rotateAndMirror = rotateAndMirror.rotated(by: rotate270)
scale = .init(scaleX: -scaleRatio, y: scaleRatio)
scale = scale.translatedBy(x: -originalHeight, y: 0.0)
case .leftMirrored:
(newWidth, newHeight) = (newHeight, newWidth)
rotateAndMirror = .init(translationX: originalHeight, y: originalWidth)
rotateAndMirror = rotateAndMirror.scaledBy(x: -1.0, y: 1.0)
rotateAndMirror = rotateAndMirror.rotated(by: rotate270)
case .right:
(newWidth, newHeight) = (newHeight, newWidth)
rotateAndMirror = .init(translationX: originalHeight, y: 0.0)
rotateAndMirror = rotateAndMirror.rotated(by: rotate90)
scale = .init(scaleX: -scaleRatio, y: scaleRatio)
scale = scale.translatedBy(x: -originalHeight, y: 0.0)
case .rightMirrored:
(newWidth, newHeight) = (newHeight, newWidth)
rotateAndMirror = .init(scaleX: -1.0, y: 1.0)
rotateAndMirror = rotateAndMirror.rotated(by: CGFloat.pi/2.0)
}
UIGraphicsBeginImageContext(CGSize(width: newWidth, height: newHeight))
guard let context = UIGraphicsGetCurrentContext() else { return nil }
context.concatenate(scale)
context.concatenate(rotateAndMirror)
context.draw(imageReference, in: CGRect(x: 0, y: 0, width: originalWidth, height: originalHeight))
let copy = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return copy
}
来源：https://stackoverflow.com/questions/12258280/capturing-photos-with-specific-resolution-using-the-uiimagepickercontroller