为什么这个代码比天真的方法更好地解压缩UIImage？ [英] Why does this code decompress a UIImage so much better than the naive approach?

问题描述

在我的应用程序中，我需要加载大型JPEG图像并在滚动视图中显示它们。为了保持UI响应，我决定在后台加载图像，然后在主线程上显示它们。为了在后台完全加载它们，我强制每个图像被解压缩。我 使用此代码解压缩图像（注意我的应用只是iOS 7，所以我知道在后台线程上使用这些方法是可以的）：

In my app I need to load large JPEG images and display them in a scroll view. In order to keep the UI responsive, I decided to load the images in the background, then display them on the main thread. In order to fully load them in the background, I force each image to be decompressed. I was using this code to decompress an image (note that my app is iOS 7 only, so I understand that using these methods on a background thread is OK):

+ (UIImage *)decompressedImageFromImage:(UIImage *)image {
    UIGraphicsBeginImageContextWithOptions(image.size, YES, 0);
    [image drawAtPoint:CGPointZero];
    UIImage *decompressedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return decompressedImage;
}

然而，我仍然有很长的加载时间，UI口吃，以及很多记忆压力。我刚刚找到另一种解决方案：

However, I still had long load times, UI stutter, and a lot of memory pressure. I just found another solution:

+ (UIImage *)decodedImageWithImage:(UIImage *)image {
    CGImageRef imageRef = image.CGImage;
    // System only supports RGB, set explicitly and prevent context error
    // if the downloaded image is not the supported format
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 CGImageGetWidth(imageRef),
                                                 CGImageGetHeight(imageRef),
                                                 8,
                                                 // width * 4 will be enough because are in ARGB format, don't read from the image
                                                 CGImageGetWidth(imageRef) * 4,
                                                 colorSpace,
                                                 // kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little
                                                 // makes system don't need to do extra conversion when displayed.
                                                 kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
    CGColorSpaceRelease(colorSpace);

    if ( ! context) {
        return nil;
    }
    CGRect rect = (CGRect){CGPointZero, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)};
    CGContextDrawImage(context, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef];
    CGImageRelease(decompressedImageRef);
    return decompressedImage;
}

此代码的数量级更好。图像几乎立即加载，没有UI口吃，内存使用量也有所下降。

This code is orders of magnitude better. The image loads almost immediately, there is no UI stutter, and the memory usage has gone way down.

所以我的问题是双重的：

So my question is two-fold:

1. 为什么第二种方法比第一种方法好得多？


2. 如果第二种方法由于唯一参数更好该设备有没有办法确保它对所有iOS设备，现在和将来都能同样有效？我不想假设我的原生位图格式发生了变化，重新引入了这个问题。

推荐答案

我假设你在Retina设备上运行它。在 UIGraphicsBeginImageContextWithOptions 中，您询问了默认比例，即主屏幕的比例，即2.这意味着它生成的位图大小为4倍。在第二个函数中，你以1x的比例绘制。

I assume that you're running this on a Retina device. In UIGraphicsBeginImageContextWithOptions, you asked for the default scale, which is the scale of the main screen, which is 2. This means that it's generating a bitmap 4x as large. In the second function, you're drawing at 1x scale.

尝试将1的标度传递给 UIGraphicsBeginImageContextWithOptions 并且看看你的表现是否相似。

Try passing a scale of 1 to UIGraphicsBeginImageContextWithOptions and see if your performance is similar.

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