CoreGraphics 图像调整大小

Question

此代码来自 Apple 的 WWDC 2011 Session 318 - iOS Performance in Depth，并使用 CoreGraphics 从服务器托管的图像创建缩略图。

CGImageSourceRef src = CGImageSourceCreateWithURL(url);
NSDictionary *options = (CFDictionaryRef)[NSDictionary 
dictionaryWithObject:[NSNumber numberWithInt:1024
forKey:(id)kCGImageSourceThumbnailMaxPixelSize];

CGImageRef thumbnail = CGImageSourceCreateThumbnailAtIndex(src,0,options);
UIImage *image = [UIImage imageWithCGImage:thumbnail];
CGImageRelease(thumbnail);
CGImageSourceRelease(src); 

但它不起作用，文档也没有真正的帮助。在 iOS 文档中 CGImageSource CGImageSourceRef CGImageSourceCreateThumbnailAtIndex 可用

在 Mac OS X v10.4 或更高版本中

我怎样才能让它工作？

编辑

这些是我得到的编译器错误：

• 使用未声明的标识符“CGImageSourceRef”
• 使用未声明的标识符“kCGImageSourceThumbnailMaxPixelSize”
• 使用未声明的标识符“src”
• 函数“CGImageSourceCreateThumbnailAtIndex”的隐式声明在 C99 中无效
• 函数“CGImageSourceRelease”的隐式声明在 C99 中无效
• 函数“CGImageSourceCreateWithURL”的隐式声明在 C99 中无效

Answer 1

小学生的错误。

没有添加#import <ImageIO/ImageIO.h>

Answer 2

尝试调整图片大小：

-(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;
}

我在我的代码中使用了一段时间，但我不记得它的来源

也试试这个 resizing a UIImage without loading it entirely into memory?