如何通过GPUImage调整图像的亮度和对比度？

Question

我编写了一种使用亮度因子和对比度因子过滤图像的方法，如下所示：

- (UIImage*)image:(UIImage*)image withBrightness:(float)brightness contrast:(float)contrast{
    GPUImagePicture *imagePicture = [[GPUImagePicture alloc] initWithImage:image];
    GPUImageBrightnessFilter *brightnessFilter = [[GPUImageBrightnessFilter alloc] init];
    GPUImageContrastFilter *contrastFilter = [[GPUImageContrastFilter alloc] init];

    [imagePicture addTarget:brightnessFilter];
    [brightnessFilter addTarget:contrastFilter];

    brightnessFilter.brightness = brightness; // set brightness
    contrastFilter.contrast = contrast;       // set contrast

    [imagePicture processImage];
    UIImage *filteredImage = [contrastFilter imageFromCurrentlyProcessedOutput];
    return filteredImage;
}

调用此实现将引发错误： <Error>: CGContextDrawImage: invalid context 0x0 。

如何使用GPUImage正确实现亮度和对比度调整？

Answer 1

您可能应该创建一个组过滤器，请检查此示例代码

    GPUImageFilterGroup *groupFilter = [[GPUImageFilterGroup alloc]init];
    [groupFilter addTarget:edit.currentFilter];

    //create a brightness filter
    GPUImageBrightnessFilter *brightnessFilter = [[GPUImageBrightnessFilter alloc]init];
    [brightnessFilter setBrightness:brightnessValue];

    //create a contrast filter
    GPUImageContrastFilter *contrastFilter = [[GPUImageContrastFilter alloc]init];
    [contrastFilter setContrast:contrastValue];

    //create a exposure filter
    GPUImageExposureFilter *exposureFilter =[[GPUImageExposureFilter alloc]init];
    [exposureFilter setExposure:exposureValue];

    //add brightness,contrast,exposure filters to group filter
    [groupFilter addFilter:brightnessFilter];
    [groupFilter addFilter:contrastFilter];
    [groupFilter addFilter:exposureFilter];


    [edit.currentFilter addTarget:brightnessFilter];
    [brightnessFilter addTarget: contrastFilter];
    [contrastFilter addTarget: exposureFilter];
    //setting initial and terminal filters
    [(GPUImageFilterGroup *) groupFilter setInitialFilters:[NSArray arrayWithObject:edit.currentFilter]];
    [(GPUImageFilterGroup *) groupFilter setTerminalFilter:exposureFilter];


    //apply it to the GPUImagePicture