Change back page alpha value when Page Curl

You may know that page curl effect in iphone, ipad... How can I change back page alpha value. Help

fingerDelta = distan开发者_高级运维ceBetweenPoints(currentPos, lastPos);
filter = [[CAFilter filterWithType:kCAFilterPageCurl] retain];
[filter setDefaults];
CGFloat _angleRad = angleBetweenCGPoints(currentPos, lastPos);
[filter setValue:[NSNumber numberWithFloat:_angleRad] forKey:@"inputAngle"];
[filter setValue:[NSNumber numberWithFloat:fingerDelta] forKey:@"inputTime"];
myView.layer.filters = [NSArray arrayWithObject:filter];

Thanks.