Andrey gave you the basics. To be very specific.
Here is your current code:
- (IBAction)sliderBrightness:(id)sender {
UIImage *inputImage = imgView.image; // <--- This line refers to an instance var
sourcePicture = [[GPUImagePicture alloc] initWithImage:inputImage smoothlyScaleOutput:YES];
brightnessFilter = [[GPUImageBrightnessFilter alloc] init];
GPUImageView *imgView = (GPUImageView *)self.view; <--- This line defines a local
[brightnessFilter forceProcessingAtSize:imgView.sizeInPixels];
[sourcePicture addTarget:brightnessFilter];
[brightnessFilter addTarget:imgView];
[sourcePicture processImage];
}
Look at the to comments I added that start with " <--- This line". The first one marks your reference to an instance variable imgView.
The second comment marks a place where you define a different, local variable with the same name, imgView.
If you intend to use a different local variable, change that name to something else, like
GPUImageView *theGPUimgView = (GPUImageView *)self.view;
If you intend to change the instance variable, then make it an assignment, and get rid of the variable declaration:
imgView = (GPUImageView *)self.view;
I'm guessing that you want the second case, where you change the contents of the instance variable, but I'm not sure. It's your code.