解码后台线​​程中的图像？ [英] Decode images in background thread?

问题描述

我有一个后台线程加载图像，并在主线程中显示它们。我注意到后台线程几乎没有什么可做，因为实际的图像解码似乎是在主线程中完成：

I have a background thread that loads images and displays them in the main thread. I noticed that the background thread has almost nothing to do, because the actual image decoding seems to be done in the main thread:

到目前为止，我试过调用 [UIImage imageNamed ：] ， [UIImage imageWithData：] 和 CGImageCreateWithJPEGDataProvider 。有没有办法强制解码在后台线程上完成？

So far I’ve tried calling [UIImage imageNamed:], [UIImage imageWithData:] and CGImageCreateWithJPEGDataProvider in the background thread with no difference. Is there a way to force the decoding to be done on the background thread?

已经有一个类似的问题在这里，但它没有帮助。正如我在那里写的那样，我尝试了以下技巧：

There’s already a similar question here, but it does not help. As I wrote there, I tried the following trick:

@implementation UIImage (Loading)

- (void) forceLoad
{
    const CGImageRef cgImage = [self CGImage];  

    const int width = CGImageGetWidth(cgImage);
    const int height = CGImageGetHeight(cgImage);

    const CGColorSpaceRef colorspace = CGImageGetColorSpace(cgImage);
    const CGContextRef context = CGBitmapContextCreate(
        NULL, /* Where to store the data. NULL = don’t care */
        width, height, /* width & height */
        8, width * 4, /* bits per component, bytes per row */
        colorspace, kCGImageAlphaNoneSkipFirst);

    NSParameterAssert(context);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), cgImage);
    CGContextRelease(context);
}

@end

解码方法），但是它也会触发对 ImageIO_BGR_A_TO_RGB_A_8Bit 的显然昂贵的调用。

That works (forces the image to decode), but it also triggers an apparently expensive call to ImageIO_BGR_A_TO_RGB_A_8Bit.

推荐答案

我遇到了类似的问题，在新的视网膜iPad上的高分辨率图像。大于屏幕尺寸的图像（大致）会导致UI响应性的主要问题。这些是JPG，所以让他们在背景上解码似乎是正确的事情。我仍然在努力收紧所有这一切，但Tommy的解决方案对我很好。我只是想贡献一些代码来帮助下一个人，当他们试图找出为什么他们的UI是大的图像口吃。这是我最后做的（这个代码运行在后台队列的NSOperation）。示例是我的代码和上面的代码的混合：

I ran into similar issues with hi-res images on the new retina iPad. Images larger than the screen size (roughly) would cause major problems with UI responsiveness. These were JPGs, so getting them to decode on the background seemed to be the right thing to do. I'm still working on tightening all of this up, but Tommy's solution worked great for me. I just wanted to contribute some code to help the next person along when they're trying to identify why their UI is stuttering with large images. Here's what I ended up doing (this code runs in an NSOperation on a background queue). The example is a blend of my code and the code above:

  CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((CFDataRef)self.data);
  CGImageRef newImage = CGImageCreateWithJPEGDataProvider(dataProvider,
                                    NULL, NO, 
                                    kCGRenderingIntentDefault);


  //////////
  // force DECODE

  const int width = CGImageGetWidth(newImage);
  const int height = CGImageGetHeight(newImage);

  const CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
  const CGContextRef context = CGBitmapContextCreate(
                                                     NULL, /* Where to store the data. NULL = don’t care */
                                                     width, height, /* width & height */
                                                     8, width * 4, /* bits per component, bytes per row */
                                                     colorspace, kCGImageAlphaNoneSkipFirst);

  NSParameterAssert(context);
  CGContextDrawImage(context, CGRectMake(0, 0, width, height), newImage);
  CGImageRef drawnImage = CGBitmapContextCreateImage(context);
  CGContextRelease(context);
  CGColorSpaceRelease(colorspace);

  //////////

  self.downloadedImage = [UIImage imageWithCGImage:drawnImage];

  CGDataProviderRelease(dataProvider);
  CGImageRelease(newImage);
  CGImageRelease(drawnImage);

我仍然在进行优化。但到目前为止，它似乎做得很好。

I'm still optimizing this. But it seems to do pretty well so far.

这篇关于解码后台线​​程中的图像？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！