Cocos2D 2.0 - bizarre behavior during a screenshot capture - cocos2d-iphone

Question

I have a layer based class that contains two sprites as children. Lets call them: background and tree.
Inside this class I have a method to capture a screenshot of that layer. The method is working fine. Every time I take a screenshot of that layer, I obtain the composition of the tree over the background. Fine. At a certain point, I want to get the screenshot of that layer without the tree. So what I do is, to hide the tree, take the screenshot and show the tree again... like this:
[myLayer hideTree];
UIImage *screenshot = [myLayer screenshot];
[myLayer showTree];
To my surprise, the screenshots produced this way always contain the tree.
this is hideTree and showTree:
- (void) hideTree {
[treeLayer setOpacity:0];
// I have also tried [treelayer setVisible:NO];
}
- (void) showTree {
[treeLayer setOpacity:255];
// I have also tried [treelayer setVisible:YES];
}
I am using this method for screenshots, from cocos2d forums:
-(UIImage*) screenshot
{
CGSize displaySize = [[CCDirector sharedDirector] winSize];
CGSize winSize = [self winSize];
//Create buffer for pixels
GLuint bufferLength = displaySize.width * displaySize.height * 4;
GLubyte* buffer = (GLubyte*)malloc(bufferLength);
//Read Pixels from OpenGL
glReadPixels(0, 0, displaySize.width, displaySize.height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
//Make data provider with data.
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, bufferLength, NULL);
//Configure image
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4 * displaySize.width;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef iref = CGImageCreate(displaySize.width, displaySize.height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
uint32_t* pixels = (uint32_t*)malloc(bufferLength);
CGContextRef context = CGBitmapContextCreate(pixels, winSize.width, winSize.height, 8, winSize.width * 4, CGImageGetColorSpace(iref), kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGContextTranslateCTM(context, 0, displaySize.height);
CGContextScaleCTM(context, 1.0f, -1.0f);
UIDeviceOrientation orientation = [[UIDevice currentDevice] orientation];
switch (orientation)
{
case UIDeviceOrientationPortrait: break;
case UIDeviceOrientationPortraitUpsideDown:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(180));
CGContextTranslateCTM(context, -displaySize.width, -displaySize.height);
break;
case UIDeviceOrientationLandscapeLeft:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(-90));
CGContextTranslateCTM(context, -displaySize.height, 0);
break;
case UIDeviceOrientationLandscapeRight:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(90));
CGContextTranslateCTM(context, displaySize.width * 0.5f, -displaySize.height);
break;
case UIDeviceOrientationUnknown:
break;
case UIDeviceOrientationFaceUp:
break;
case UIDeviceOrientationFaceDown:
break;
}
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, displaySize.width, displaySize.height), iref);
CGImageRef imageRef = CGBitmapContextCreateImage(context);
UIImage *outputImage = [UIImage imageWithCGImage:imageRef];
//Dealloc
CGImageRelease(imageRef);
CGDataProviderRelease(provider);
CGImageRelease(iref);
CGColorSpaceRelease(colorSpaceRef);
CGContextRelease(context);
free(buffer);
free(pixels);
return outputImage;
}
what am I missing? Why is the tree always showing?

Answer 1

Your code is running during the update process of the game loop.
The screenshot is getting the current pixels to use to create the UIImage.
When you tell the tree to not be visible or have zero opacity, that will not take effect until the next draw cycle.
To fix your problem, you need to hide the tree, do a draw cycle, and then in the next update capture the frame with the tree not showing.
Make sense?

Answer 2

When you change the tree opacity, it will only take effect the next time the screen updates.
Once I had this problem and I did some schedules. Instead of
[myLayer hideTree];
UIImage *screenshot = [myLayer screenshot];
[myLayer showTree];
Do
[myLayer hideTree];
[self schedule:#selector(takeScreenshot)interval:0.05];
And add the following method to your scene:
-(void)takeScreenshot {
[self unschedule:#selector(takeScreenshot)];
UIImage *screenshot = [myLayer screenshot];
[myLayer showTree];
}
Basically, we're putting a 0.05 seconds gap to allow the screen to update. That should work.
The above method worked for me, but I think it also works if you call [self visit] after hiding the tree. But I never tested.