Capturing photos with specific resolution using the UIImagePickerController

Question

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?

Answer 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!

Answer 2

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
}