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綠幕摳圖代碼實現和使用P圖軟件完成綠幕摳圖的步驟是一致的，總共包括四步:

1、初始化前景圖，需要P圖的部分填充為純色背景（綠幕/藍幕）

2、把純色部分的bitmap設置為透明

3、準備背景圖（尺寸和前景圖一致），前景圖疊加到背景圖生成目標圖片

4、導出目標圖片，完成摳圖功能

純色部分的bitmap設置為透明

- (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;
}

獲取每個bitmap的顏色值

- (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; 
}

前景圖疊加到背景圖，生成目標圖片

- (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，把生成的圖片保存到系統相冊，完成摳圖功能。