使用CoreImage框架，实现绿幕抠图功能

绿幕抠图代码实现和使用P图软件完成绿幕抠图的步骤是一致的，总共包括四步:

1、初始化前景图，需要P图的部分填充为纯色背景（绿幕/蓝幕）

2、把纯色部分的bitmap设置为透明

3、准备背景图（尺寸和前景图一致），前景图叠加到背景图生成目标图片

4、导出目标图片，完成抠图功能

纯色部分的bitmap设置为透明

- (CIFilter*) chromaKeyFilterHuesFrom:(CGFloat)minHue to:(CGFloat)maxHue｛
     // 1
     const unsigned int size = 64;
     const size_t cubeDataSize = size * size * size * 4;
     NSMutableData* cubeData = [[NSMutableData alloc] initWithCapacity:
                                (cubeDataSize * sizeof(float))];
     // 2
     for (int z = 0; z < size; z++) {
         CGFloat blue = ((double)z)/(size-1);
         for (int y = 0; y < size; y++) {
             CGFloat green = ((double)y)/(size-1);
             for (int x = 0; x < size; x++) {
                 CGFloat red = ((double)x)/(size-1);
               
                 // 3
                 CGFloat hue = [self hueFromRed:red green:green blue:blue];
                 float alpha = (hue >= minHue && hue <= maxHue) ? 0 : 1;
               
                 // 4
                 float premultipliedRed = red * alpha;
                 float premultipliedGreen = green * alpha;
                 float premultipliedBlue = blue * alpha;
                 [cubeData AppendBytes:&premultipliedRed length:sizeof(float)];
                 [cubeData appendBytes:&premultipliedGreen length:sizeof(float)];
                 [cubeData appendBytes:&premultipliedBlue length:sizeof(float)];
                 [cubeData appendBytes:&alpha length:sizeof(float)];
             }
         }
     }

    // 5
     CIFilter* chromaKeyFilter = [CIFilter filterWithName:@"CIColorCube"];
     [chromaKeyFilter setValue:@(size) forKey:@"inputCubeDimension"];
     [chromaKeyFilter setValue:cubeData forKey:@"inputCubeData"];
     return chromaKeyFilter;
}

获取每个bitmap的颜色值

- (CGFloat) hueFromRed:(CGFloat)red green:(CGFloat)green blue:(CGFloat)blue {
    UIColor* color = [UIColor colorWithRed:red green:green blue:blue alpha:1];
    CGFloat hue, saturation, brightness;
     [color getHue:&hue saturation:&saturation brightness:&brightness alpha:nil];
    return hue; 
}

前景图叠加到背景图，生成目标图片

- (CIImage *)outputImage{
     CIFilter* chromaKeyFilter = [self chromaKeyFilterHuesFrom:0.3 to:0.4];
     CIImage *myImage = [[CIImage alloc] initWithImage:self.inputFilterImage];
     [chromaKeyFilter setValue:myImage forKey:kCIInputImageKey];

     CIFilter* compositor = [CIFilter filterWithName:@"CISourceOverCompositing"];

     CIImage* souceCIImage = chromaKeyFilter.outputImage;
     [compositor setValue:souceCIImage forKey:kCIInputImageKey];

     CIImage *backgroundCIImage = [[CIImage alloc] initWithImage:self.backgroundImage];
     [compositor setValue:backgroundCIImage forKey:kCIInputBackgroundImageKey];

     return compositor.outputImage; 
}

最后，把生成的图片保存到系统相册，完成抠图功能。