页面卷曲时更改后页面alpha值

Question

你可能知道iphone，ipad中的页面卷曲效果...... 如何更改返回页面的alpha值。帮助

fingerDelta = distanceBetweenPoints(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];

感谢。