GPUImage混合滤镜可以用GPUImageFilterGroup和GPUImageFilterPipeline

GPUImageFilterGroup用法

+ (UIImage *)applyGroupFilter:(UIImage *)image              //实际是把filter一个个链接起来的
{
    GPUImagePicture *pic=[[GPUImagePicture alloc]initWithImage:image];
    GPUImageFilterGroup *filterGroup = [[GPUImageFilterGroup alloc] init];
    
    // 添加 filter
    /**
     原理：
     1. filterGroup(addFilter) 滤镜组添加每个滤镜
     2. 按添加顺序（可自行调整）前一个filter(addTarget) 添加后一个filter
     3. filterGroup.initialFilters = @[第一个filter]];
     4. filterGroup.terminalFilter = 最后一个filter;
     */
    GPUImageRGBFilter *filter1         = [[GPUImageRGBFilter alloc] init];
    GPUImageToonFilter *filter2        = [[GPUImageToonFilter alloc] init];
    GPUImageColorInvertFilter *filter3 = [[GPUImageColorInvertFilter alloc] init];
    GPUImageSepiaFilter       *filter4 = [[GPUImageSepiaFilter alloc] init];
    [filterGroup addFilter:filter1];
    [filterGroup addFilter:filter2];
    [filterGroup addFilter:filter3];
    [filterGroup addFilter:filter4];
    [filter1 addTarget:filter2];
    [filter2 addTarget:filter3];
    [filter3 addTarget:filter4];
    filterGroup.initialFilters=@[filter1];
    filterGroup.terminalFilter=filter4;
    [filterGroup forceProcessingAtSize:image.size];
    [pic removeAllTargets];
    [pic addTarget:filterGroup];
    [pic processImage];
    [filterGroup useNextFrameForImageCapture];
    return [filterGroup imageFromCurrentFramebuffer];
}

filterGroup的很多操作都是利用最后一个Filter实现的，例如：

//GPUImageFilterGroup.m文件
- (void)useNextFrameForImageCapture;
{
    [self.terminalFilter useNextFrameForImageCapture];
}

- (CGImageRef)newCGImageFromCurrentlyProcessedOutput;
{
    return [self.terminalFilter newCGImageFromCurrentlyProcessedOutput];
}

GPUImageFilterPipeline用法

+ (UIImage *)applyPipelineFilter:(UIImage *)image
{
    GPUImagePicture *pic=[[GPUImagePicture alloc]initWithImage:image];
    //RGB滤镜
    GPUImageRGBFilter * RGBFilter = [[GPUImageRGBFilter alloc] init];
    //卡通滤镜
    GPUImageToonFilter * toonFilter = [[GPUImageToonFilter alloc] init];
    NSMutableArray *arrayTemp=[NSMutableArray array];
    [arrayTemp addObject:RGBFilter];
    [arrayTemp addObject:toonFilter];
    GPUImageFilterPipeline * filterPipeline = [[GPUImageFilterPipeline alloc] initWithOrderedFilters:arrayTemp input:pic output:[[GPUImageView alloc]init]];
    [pic processImage];
    [toonFilter useNextFrameForImageCapture];//这个filter是toonFilter
    return [filterPipeline currentFilteredFrame];
}

Pieline内部也把Filter串连了起来：

//GPUImageFilterPipeline.m文件
- (void)_refreshFilters {
    
    id prevFilter = self.input;
    GPUImageOutput<GPUImageInput> *theFilter = nil;
    
    for (int i = 0; i < [self.filters count]; i++) {
        theFilter = [self.filters objectAtIndex:i];
        [prevFilter removeAllTargets];
        [prevFilter addTarget:theFilter];
        prevFilter = theFilter;
    }
    
    [prevFilter removeAllTargets];
    //output为显示视图，可使用GPUImageView，也可为nil
    if (self.output != nil) {
        [prevFilter addTarget:self.output];
    }
}

混合滤镜原理

模仿group与pipeline，快速实现混合滤镜：

+ (UIImage *)applyChainFilter:(UIImage *)image
{
    GPUImagePicture *pic=[[GPUImagePicture alloc]initWithImage:image];
    GPUImageRGBFilter *filter1         = [[GPUImageRGBFilter alloc] init];
    GPUImageToonFilter *filter2        = [[GPUImageToonFilter alloc] init];
    GPUImageColorInvertFilter *filter3 = [[GPUImageColorInvertFilter alloc] init];
    //串连起来
    [filter1 addTarget:filter2];
    [filter2 addTarget:filter3];
    [pic removeAllTargets];
    [pic addTarget:filter1];
    
    [filter1 forceProcessingAtSize:image.size];
    [filter2 forceProcessingAtSize:image.size];
    [filter3 forceProcessingAtSize:image.size];
    //处理得到图片
    [pic processImage];
    [filter3 useNextFrameForImageCapture];
    return [filter3 imageFromCurrentFramebuffer];
}