GPUImage简单滤镜使用之色阶（三）

　　色阶是表示图像亮度强弱的指数标准，图像的色彩丰满度和精细度是由色阶决定的。在GPUImage中GPUImageLevelsFilter提供了此功能。

　　GPUImageLevelsFilter定义了修改红色，绿色，蓝色，和所有通道的方法，有效值范围时0.0-1.0之间，如果你想想像ps中那样，你必须先转为［0，1］。

/** Set levels for the red channel */

- (void)setRedMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max minOut:(CGFloat)minOut maxOut:(CGFloat)maxOut;

- (void)setRedMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max;

/** Set levels for the green channel */

- (void)setGreenMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max minOut:(CGFloat)minOut maxOut:(CGFloat)maxOut;

- (void)setGreenMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max;

/** Set levels for the blue channel */

- (void)setBlueMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max minOut:(CGFloat)minOut maxOut:(CGFloat)maxOut;

- (void)setBlueMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max;

/** Set levels for all channels at once */

- (void)setMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max minOut:(CGFloat)minOut maxOut:(CGFloat)maxOut;

- (void)setMin:(CGFloat)min gamma:(CGFloat)mid max:(CGFloat)max;

    filter = [[GPUImageLevelsFilter alloc] init];
    [filter setMin:[[(UITextField *)[self viewWithTag:1001] text] floatValue]     gamma:[[(UITextField *)[self viewWithTag:1002] text] floatValue] max:    [[(UITextField *)[self viewWithTag:1003] text] floatValue] minOut:[[(UITextField *)[self viewWithTag:1004] text] floatValue] maxOut:[[(UITextField *)[self viewWithTag:1005] text] floatValue]];

    [filter forceProcessingAtSize:img.size];
    GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:img];
    [pic addTarget:filter];
    [pic processImage];
    [filter useNextFrameForImageCapture];
    self.imgView.image = [filter imageFromCurrentFramebuffer];