ALAssetLibrary aspectRatioThumbnail iOS4

Question

我正在从事一个项目，需要浏览和显示目前在iPhone画廊的图片。

我使用ALAssetLibrary来做到这一点。为了生成缩略图，我使用了"aspectRatioThumbnail“。在我的3GS iOS 5上，一切都运行得很好、很流畅。但iOS4中不存在这些方法。

我尝试用此函数手动生成缩略图比例图像，但由于内存警告而崩溃。日志显示，生成的图像大小不符合给定的最大大小120的约束。

有什么想法吗？

    NSDate* firstDate = [NSDate date];
    uint8_t* buffer = (Byte*)malloc(_asset.defaultRepresentation.size);

    NSUInteger length = [_asset.defaultRepresentation getBytes:buffer fromOffset:0.0 length:_asset.defaultRepresentation.size error:nil];

NSData* data = [[NSData alloc] initWithBytesNoCopy:buffer length:_asset.defaultRepresentation.size freeWhenDone:YES];
                                  nil];

        NSDictionary* options = [NSDictionary dictionaryWithObjectsAndKeys:
                           (id)kCFBooleanTrue, kCGImageSourceShouldAllowFloat,
                           (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailWithTransform,
                           (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailFromImageAlways,
                           [NSNumber numberWithInt:120], kCGImageSourceThumbnailMaxPixelSize,
                           nil];

        CGImageSourceRef sourceRef = CGImageSourceCreateWithData((CFDataRef)data, (CFDictionaryRef) options);
        CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(sourceRef, 0, NULL);

        CGImageRef imageRef = CGImageSourceCreateImageAtIndex(sourceRef, 0, imageProperties);

        image = [UIImage imageWithCGImage:imageRef];

        NSTimeInterval tic = [[NSDate date] timeIntervalSinceDate:firstDate];
        NSLog(@"Thumbnail generation from ios4 method %f [size %f * %f", tic, image.size.width, image.size.height);

        [data release];
        CFRelease(sourceRef);
        CFRelease(imageRef);
        CFRelease(imageProperties);

Answer 1

上面的代码有很多不必要的拷贝。相反，只需以屏幕分辨率(小于完整图像大小)加载图像，并从那里缩小到缩略图：

CGImageRef img = [[asset defaultRepresentation] fullScreenImage];
int w = CGImageGetWidth(img);
int h = CGImageGetHeight(img);
float scale = 120 / (float)(MAX(w, h));
CGSize newsize = CGSizeMake(w * scale, h * scale);

UIGraphicsBeginImageContext(newsize);
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSetInterpolationQuality(ctx, kCGInterpolationHigh);
CGContextTranslateCTM(ctx, 0, newsize.height);
CGContextScaleCTM(ctx, 1.0, -1.0);
CGContextDrawImage(ctx, CGRectMake(0, 0, newsize.width, newsize.height), iref);
UIImage* thumbnail = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

运行此代码后，缩略图将具有与aspectRatioThumbnail等效的UIImage。