CGDirectDisplayID display = CGMainDisplayID();
size_t width = CGDisplayPixelsWide(display);
size_t height = CGDisplayPixelsHigh(display);
UInt8 *frameBuffer = CGDisplayBaseAddress(display);
NSBitmapImageRep *imageRep = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:&frameBuffer 
                                                       pixelsWide:width pixelsHigh:height bitsPerSample:CGDisplayBitsPerSample(display) 
                                                      samplesPerPixel:CGDisplaySamplesPerPixel(display) 
                                                      hasAlpha:NO isPlanar:NO colorSpaceName:NSDeviceRGBColorSpace 
                                                      bitmapFormat:NSAlphaFirstBitmapFormat 
                                                      bytesPerRow:CGDisplayBytesPerRow(display) bitsPerPixel:CGDisplayBitsPerPixel(display)];