Q:

How do you render OpenGL-ES to an external screen using the VGA out adapter? 

I've been developing a 3D program for the iPad and iPhone and would like to be able to render it to an external screen.  According to my understanding, you have to do something similar to the below code to implement it, (found at: Sunsetlakesoftware.com):
if ([[UIScreen screens] count] > 1)
{
    // External screen attached
}
else
{
    // Only local screen present
}

CGRect externalBounds = [externalScreen bounds];
externalWindow = [[UIWindow alloc] initWithFrame:externalBounds];

UIView *backgroundView = [[UIView alloc]  initWithFrame:externalBounds];
backgroundView.backgroundColor = [UIColor whiteColor];

[externalWindow addSubview:backgroundView];

[backgroundView release];

externalWindow.screen = externalScreen;
[externalWindow makeKeyAndVisible];

However, I'm not sure what to change to do this to an OpenGL project.  Does anyone know what you would do to implement this into the defualt openGL project for iPad or iPhone in XCode? 

A:

I've figured out how to get ANY OpenGL-ES content to render onto an external display.  It's actually really straightforward.  You just copy your renderbuffer to a UIImage then display that UIImage on the external screen view.  The code to take a snapshot of your renderbuffer is below:
- (UIImage*)snapshot:(UIView*)eaglview
{
// Get the size of the backing CAEAGLLayer
GLint backingWidth, backingHeight;
glBindRenderbufferOES(GL_RENDERBUFFER_OES, defaultFramebuffer);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

NSInteger x = 0, y = 0, width = backingWidth, height = backingHeight;
NSInteger dataLength = width * height * 4;
GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));

// Read pixel data from the framebuffer
glPixelStorei(GL_PACK_ALIGNMENT, 4);
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);

// Create a CGImage with the pixel data
// If your OpenGL ES content is opaque, use kCGImageAlphaNoneSkipLast to ignore the alpha channel
// otherwise, use kCGImageAlphaPremultipliedLast
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
                                ref, NULL, true, kCGRenderingIntentDefault);

// OpenGL ES measures data in PIXELS
// Create a graphics context with the target size measured in POINTS
NSInteger widthInPoints, heightInPoints;
if (NULL ! = UIGraphicsBeginImageContextWithOptions) {
    // On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
    // Set the scale parameter to your OpenGL ES view's contentScaleFactor
    // so that you get a high-resolution snapshot when its value is greater than 1.0
    CGFloat scale = eaglview.contentScaleFactor;
    widthInPoints = width / scale;
    heightInPoints = height / scale;
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(widthInPoints, heightInPoints), NO, scale);
}
else {
    // On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
    widthInPoints = width;
    heightInPoints = height;
    UIGraphicsBeginImageContext(CGSizeMake(widthInPoints, heightInPoints));
}

CGContextRef cgcontext = UIGraphicsGetCurrentContext();

// UIKit coordinate system is upside down to GL/Quartz coordinate system
// Flip the CGImage by rendering it to the flipped bitmap context
// The size of the destination area is measured in POINTS
CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);

// Retrieve the UIImage from the current context
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

// Clean up
free(data);
CFRelease(ref);
CFRelease(colorspace);
CGImageRelease(iref);

return image;
}

Although, for some reason I've never been able to get glGetRenderbufferParameterivOES to return the proper backingwidth and backingheight, so I've had to use my own function to calculate those.  Just pop this into your rendering implementation and place the result onto the external screen using a timer.  If anyone can make any improvements to this method, please let me know.