How to record an off-screen NSView at 60fps

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I want to record the video output (encoded or not) from an off-screen NSView running on macOS. I'm pretty sure there is no API to do this, however I believe it is feasible by rendering it frame-by-frame into a framebuffer.

The problem is that I can't find a way to render the view at a fast enough rate. Methods I've tried without success (tested on a MacBook M1 Pro running Monterey) :

  • [view dataWithPDFInsideRect:] and [view dataWithEPSInsideRect:] : takes about 200ms to execute.
  • [view.layer renderInContext:] : takes about 350ms to execute.
  • [view cacheDisplayInRect: toBitmapImageRep:] : takes about 100ms to execute.

I also tried to embed the view in a window and capture the window. Window capturing functions (such as CGWindowListCreateImage) are much faster, but does not work when windows are off-screen.

Considering the view can be rendered at 60fps in a window without issue, why do theese methods take so much time? Is there any method I missed to render an NSView into a framebuffer?

swift macos objective-c cocoa
1 Answers

I finally found a performant way of doing it. By capturing the view this way I am able to reach 60+ fps.

NSView* view = ...;
NSBitmapImageRep* bitmap = [
    [NSBitmapImageRep alloc]
    initWithBitmapDataPlanes:nil
    pixelsWide:view.bounds.size.width
    pixelsHigh:view.bounds.size.height
    bitsPerSample:8
    samplesPerPixel:4
    hasAlpha:YES
    isPlanar:NO
    colorSpaceName:NSCalibratedRGBColorSpace
    bitmapFormat:0
    bytesPerRow:(4 * view.bounds.size.width)
    bitsPerPixel:32
];

NSGraphicsContext* graphicsContext = [NSGraphicsContext graphicsContextWithBitmapImageRep:bitmap];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:graphicsContext];
[view displayRectIgnoringOpacity:view.bounds inContext:graphicsContext];
[NSGraphicsContext restoreGraphicsState];

// pixels are in format [R, G, B, A, R, G, B, A, ...]
unsigned char* pixels = [bitmap bitmapData];
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