在 Cocoa 中创建动画 GIF - 定义帧类型

我已经能够改编在 SO 上找到的一些代码，从我的视图的“屏幕截图”生成动画 GIF，但结果是不可预测的。 GIF 帧有时是完整图像、完整帧（“替换”模式，正如 GIMP 所标记的那样），其他时候只是与上一层的“差异”（“组合”模式）。

据我所知，当涉及的帧较少和/或较小时，CG 以“组合”模式写入 GIF，但无法获得正确的颜色。事实上，移动部件的颜色是正确的，背景是错误的。 当CG将GIF保存为全帧时，颜色还可以。文件大小较大，但是，嘿，显然你无法两全其美。 :)

有没有办法：

    a) force CG to create "full frames" when saving the GIF
    b) fix the colors (color table?)

我所做的是（ARC模式）：

捕获视图的可见部分

[[scrollView contentView] dataWithPDFInsideRect:[[scrollView contentView] visibleRect]];

将其转换并调整大小为 PNG 类型的 NSImageBitmapRep

-(NSMutableDictionary*) pngImageProps:(int)quality {
  NSMutableDictionary *pngImageProps;  
  pngImageProps = [[NSMutableDictionary alloc] init];
  [pngImageProps setValue:[NSNumber numberWithBool:NO] forKey:NSImageInterlaced];
  double compressionF = 1;

  [pngImageProps setValue:[NSNumber numberWithFloat:compressionF] forKey:NSImageCompressionFactor];
  return pngImageProps;
}


-(NSData*) resizeImageToData:(NSData*)data toDimX:(int)xdim andDimY:(int)ydim withQuality:(int)quality{
  NSImage *image = [[NSImage alloc] initWithData:data];
  NSRect inRect = NSZeroRect;
  inRect.size = [image size];

  NSRect outRect = NSMakeRect(0, 0, xdim, ydim);
  NSImage *outImage = [[NSImage alloc] initWithSize:outRect.size];

  [outImage lockFocus];
    [image drawInRect:outRect fromRect:inRect operation:NSCompositeCopy fraction:1];
    NSBitmapImageRep* bitmapRep = [[NSBitmapImageRep alloc] initWithFocusedViewRect:outRect];
  [outImage unlockFocus];

  NSMutableDictionary *imageProps = [self pngImageProps:quality];
  NSData* imageData = [bitmapRep representationUsingType:NSPNGFileType properties:imageProps];
  return [imageData copy];
}

获取 BitmapReps 数组并创建 GIF

-(CGImageRef) pngRepDataToCgImageRef:(NSData*)data {
  CFDataRef imgData = (__bridge CFDataRef)data;
  CGDataProviderRef imgDataProvider = CGDataProviderCreateWithCFData (imgData);
  CGImageRef image = CGImageCreateWithPNGDataProvider(imgDataProvider, NULL, true, kCGRenderingIntentDefault);
  return image;
}

////////// create GIF from 

NSArray *images;  // holds all BitmapReps

CGImageDestinationRef destination = CGImageDestinationCreateWithURL((__bridge CFURLRef)[NSURL fileURLWithPath:pot],
                                                                    kUTTypeGIF,
                                                                    allImages,
                                                                    NULL);
// set frame delay
NSDictionary *frameProperties = [NSDictionary
                                 dictionaryWithObject:[NSDictionary
                                                       dictionaryWithObject:[NSNumber numberWithFloat:0.2f]
                                                       forKey:(NSString *) kCGImagePropertyGIFDelayTime]
                                 forKey:(NSString *) kCGImagePropertyGIFDictionary];

// set gif color properties
NSMutableDictionary *gifPropsDict = [[NSMutableDictionary alloc] init];
[gifPropsDict setObject:(NSString *)kCGImagePropertyColorModelRGB forKey:(NSString *)kCGImagePropertyColorModel];
[gifPropsDict setObject:[NSNumber numberWithBool:YES] forKey:(NSString *)kCGImagePropertyGIFHasGlobalColorMap];

// set gif loop
NSDictionary *gifProperties = [NSDictionary
                               dictionaryWithObject:gifPropsDict
                               forKey:(NSString *) kCGImagePropertyGIFDictionary];

// loop through frames and add them to GIF 
for (int i=0; i < [images count]; i++) {
  NSData *imageData = [images objectAtIndex:i];
  CGImageRef imageRef = [self pngRepDataToCgImageRef:imageData];
    CGImageDestinationAddImage(destination, imageRef, (__bridge CFDictionaryRef) (frameProperties));
}

// save the GIF
CGImageDestinationSetProperties(destination, (__bridge CFDictionaryRef)(gifProperties));
CGImageDestinationFinalize(destination);
CFRelease(destination);

我检查了 ImageBitmapReps，当单独保存为 PNG 时，它们就很好。 据我了解，颜色表应该由 CG 处理，还是我负责生成抖动颜色？怎么做？

即使重复制作相同的动画，生成的 GIF 也可能会有所不同。

这是单个 BitmapRep

_{(source: andraz.eu http://andraz.eu/stuff/gif/frame.png)}

And this is the GIF with the invalid colors ("combine" mode)
_{(source: andraz.eu http://andraz.eu/stuff/gif/anim2.gif)}

我读了你的代码。创建 CGImageDestinationRef 时，请仔细检查“allImages”和“[images count]”。

以下测试代码工作正常：

NSDictionary *prep = [NSDictionary dictionaryWithObject:[NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:0.2f] forKey:(NSString *) kCGImagePropertyGIFDelayTime] forKey:(NSString *) kCGImagePropertyGIFDictionary];

CGImageDestinationRef dst = CGImageDestinationCreateWithURL((__bridge CFURLRef)(fileURL), kUTTypeGIF, [filesArray count], nil);

for (int i=0;i<[filesArray count];i++)
{
    //load anImage from array
    ...

    CGImageRef imageRef=[anImage CGImageForProposedRect:nil context:nil hints:nil];
    CGImageDestinationAddImage(dst, imageRef,(__bridge CFDictionaryRef)(prep));

}

bool fileSave = CGImageDestinationFinalize(dst);
CFRelease(dst);