GPUImage for large sized image

Question

I am using GPUImage framework for adding filter Effects that change the contrast,brightness of the image.

When i tried to change the contrast,brightness for large sized image it is pixelating the original image. The quality of the image is gone. here some code that i tried to change the contrast of the image.

GPUImageContrastFilter *contrastfilter =[[GPUImageContrastFilter alloc]init];
[contrastfilter setContrast:contrastSlider.value];
UIImage *ima= [contrastfilter imageByFilteringImage:visibleScrollViewImage1];
imageView.image= ima;

it seems like the image is bit blurred. Is there anything i missed? Please suggest some better way to add filter effects for large sized image.