ALAssetsLibrary aspect Ratio Thumbnail iOS 4

Posted 2023-03-07

【中文标题】ALAssetsLibrary aspect Ratio Thumbnail iOS 4

【英文标题】：ALAssetLibrary aspectRatioThumbnail iOS4

【发布时间】：2012-09-22 15:41:59

【问题描述】：

我正在处理一个需要浏览和显示 iPhone 画廊中的图片的项目。

我使用 ALAssetLibrary 来做到这一点。要生成缩略图图像，我使用“aspectRatioThumbnail”。在我的 3GS ios 5 上一切正常。但这些方法在 iOS4 中不存在。

我尝试使用此功能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尺寸不遵守最大尺寸 120 的给定约束。

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);

【参考方案1】：

上面的代码有很多不必要的复制。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为缩略图：

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，thumbnail 将有一个与 aspectRatioThumbnail 等效的 UIImage。