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copied!<p>Looks like option (D) is the best, with some changes. Apparently OS X has a pretty good idea about the real-world size of the display and you can <a href="https://stackoverflow.com/questions/12589198/how-to-read-the-physical-screen-size-of-osx">get it via CGDisplayScreenSize</a>. It's then pretty straightforward to pick the largest one and assume that's the presentation screen. </p> <p>Granted this doesn't accurately measure projectors, but my informal testing shows that the function returns pretty good pixel per inch values for each screen:</p> <ul> <li>Macbook Air 13": {290, 180} mm, 126 ppi</li> <li>Apple Cinema Display: {596, 336} mm, 109 ppi</li> <li>An Epson Overhead Projector: {799, 450} mm, 61 ppi</li> </ul> <p>(the above were converted with a constant 25.4 millimeters per inch).</p> <p>Here's the code that I used:</p> <pre><code>#import <Foundation/Foundation.h> #import <AppKit/AppKit.h> #import <ApplicationServices/ApplicationServices.h> int main(int argc, char *argv[]) { @autoreleasepool { NSArray* screens = [NSScreen screens]; CGFloat __block biggestArea = 0; NSScreen* __block presentationScreen; NSUInteger __block presentationScreenIndex; [screens enumerateObjectsUsingBlock:^(NSScreen* screen,NSUInteger idx,BOOL* stop){ NSDictionary *description = [screen deviceDescription]; NSSize displayPixelSize = [[description objectForKey:NSDeviceSize] sizeValue]; CGSize displayPhysicalSize = CGDisplayScreenSize( [[description objectForKey:@"NSScreenNumber"] unsignedIntValue]); NSLog(@"Screen %d Physical Size: %@ ppi is %0.2f",(int) idx, NSStringFromSize(displayPhysicalSize), (displayPixelSize.width / displayPhysicalSize.width) * 25.4f); // there being 25.4 mm in an inch CGFloat screenArea = displayPhysicalSize.width * displayPhysicalSize.height; if(screenArea > biggestArea) { presentationScreen = screen; biggestArea = screenArea; presentationScreenIndex = idx; } }]; NSLog(@"Presentation screen: index: %d %@",(int) presentationScreenIndex,presentationScreen); } } </code></pre>

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