有什么方法可以比UIImagePNGRepresentation更快地编码PNG？ [英] Any way to encode a PNG faster than UIImagePNGRepresentation?

问题描述

我正在为 CATiledLayer 生成一堆磁贴。在iPhone 4S上以4个级别的细节生成120个256 x 256的图块需要大约11秒钟。图像本身适合2048 x 2048.

我的瓶颈是 UIImagePNGRepresentation 。生成每个256 x 256图像需要大约0.10-0.15秒。

我尝试在不同的背景队列上生成多个图块，但这只会将其减少到约9-10秒。

我也尝试使用ImageIO框架，代码如下：

   - （void）writeCGImage：（CGImageRef）image toURL：（NSURL *）url andOptions：（CFDictionaryRef）options 
 {
 CGImageDestinationRef myImageDest = CGImageDestinationCreateWithURL（（__ bridge CFURLRef）url ，（__ bridge CFStringRef）@public.png，1，nil）; 
 CGImageDestinationAddImage（myImageDest，image，options）; 
 CGImageDestinationFinalize（myImageDest）; 
 CFRelease（myImageDest）; 
} 
  

虽然这会产生较小的PNG文件（赢！），但大约需要13秒比以前多2秒。

有没有办法更快地从 CGImage 编码PNG图像？也许是一个使用 NEON ARM扩展（iPhone 3GS +）的库，如。本文提供了另一种加载图像的方法：

  NSDictionary * dict = [NSDictionary dictionaryWithObject：[NSNumber numberWithBool：YES] 
 forKey：（id）kCGImageSourceShouldCache]; 
 CGImageSourceRef source = CGImageSourceCreateWithURL（（__ bridge CFURLRef）[[NSURL alloc] initFileURLWithPath：path]，NULL）; 
 CGImageRef cgImage = CGImageSourceCreateImageAtIndex（source，0，（__ bridge CFDictionaryRef）dict）; 
 UIImage * image = [UIImage imageWithCGImage：cgImage]; 
 CGImageRelease（cgImage）; 
 CFRelease（来源）; 
  

现在同样的过程大约需要3秒钟！使用GCD并行生成切片可以更加显着地减少时间。

上面的 writeCGImage 函数大约需要5秒。由于文件大小较小，我怀疑zlib压缩处于较高级别。

I'm generating a bunch of tiles for CATiledLayer. It takes about 11 seconds to generate 120 tiles at 256 x 256 with 4 levels of detail on an iPhone 4S. The image itself fits within 2048 x 2048.

My bottleneck is UIImagePNGRepresentation. It takes about 0.10-0.15 seconds to generate every 256 x 256 image.

I've tried generating multiple tiles on different background queue's, but this only cuts it down to about 9-10 seconds.

I've also tried using the ImageIO framework with code like this:

- (void)writeCGImage:(CGImageRef)image toURL:(NSURL*)url andOptions:(CFDictionaryRef) options
{
    CGImageDestinationRef myImageDest = CGImageDestinationCreateWithURL((__bridge CFURLRef)url, (__bridge CFStringRef)@"public.png", 1, nil);
    CGImageDestinationAddImage(myImageDest, image, options);
    CGImageDestinationFinalize(myImageDest);
    CFRelease(myImageDest);
}

While this produces smaller PNG files (win!), it takes about 13 seconds, 2 seconds more than before.

Is there any way to encode a PNG image from CGImage faster? Perhaps a library that makes use of NEON ARM extension (iPhone 3GS+) like libjpeg-turbo does?

Is there perhaps a better format than PNG for saving tiles that doesn't take up a lot of space?

The only viable option I've been able to come up with is to increase the tile size to 512 x 512. This cuts the encoding time by half. Not sure what that will do to my scroll view though. The app is for iPad 2+, and only supports iOS 6 (using iPhone 4S as a baseline).

解决方案

It turns out the reason why UIImageRepresentation was performing so poorly was because it was decompressing the original image every time even though I thought I was creating a new image with CGImageCreateWithImageInRect.

You can see the results from Instruments here:

Notice _cg_jpeg_read_scanlines and decompress_onepass.

I was force-decompressing the image with this:

UIImage *image = [UIImage imageWithContentsOfFile:path];
UIGraphicsBeginImageContext(CGSizeMake(1, 1));
[image drawAtPoint:CGPointZero];
UIGraphicsEndImageContext();

The timing of this was about 0.10 seconds, almost equivalent to the time taken by each UIImageRepresentation call.

There are numerous articles over the internet that recommend drawing as a way of force decompressing an image.

There's an article on Cocoanetics Avoiding Image Decompression Sickness. The article provides an alternate way of loading the image:

NSDictionary *dict = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
                                                 forKey:(id)kCGImageSourceShouldCache];
CGImageSourceRef source = CGImageSourceCreateWithURL((__bridge CFURLRef)[[NSURL alloc] initFileURLWithPath:path], NULL);
CGImageRef cgImage = CGImageSourceCreateImageAtIndex(source, 0, (__bridge CFDictionaryRef)dict);
UIImage *image = [UIImage imageWithCGImage:cgImage];
CGImageRelease(cgImage);
CFRelease(source);

And now the same process takes about 3 seconds! Using GCD to generate tiles in parallel reduces the time more significantly.

The writeCGImage function above takes about 5 seconds. Since the file sizes are smaller, I suspect the zlib compression is at a higher level.

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