Half the times the captured photo comes up totally dark when using flash - the ios camera app doesn't misfire like this.
the gpuimage code for above is at https://github.com/BradLarson/GPUImage/blob/master/framework/Source/GPUImageStillCamera.m
-(void)capturePhotoProcessedUpToFilter:(GPUImageOutput<GPUImageInput> *)finalFilterInChain withImageOnGPUHandler:(void (^)(NSError *error))block