MacOS Metal: Failing to capture GPU Frame from command line app

Question

I'm trying to programmatically capture GPU frames using MTLCaptureManager in a command line application.

So far, the capture manager fails to support the MTLCaptureDestinationGPUTraceDocument destination.

I tried to create a very minimal repro case using XCode :

#import <Foundation/Foundation.h>
#import <Metal/Metal.h>

int main(int argc, const char * argv[]) {
  @autoreleasepool {

      id<MTLDevice> device = MTLCreateSystemDefaultDevice();
      
      MTLCaptureManager* captureManager = [MTLCaptureManager sharedCaptureManager];
      if (![captureManager supportsDestination:MTLCaptureDestinationGPUTraceDocument])
      {
        NSLog(@"********** captureManager does not support MTLCaptureDestinationGPUTraceDocument ************");
      }
      else
      {
        NSLog(@"captureManager support is fine");
      }
  }
  return 0;
}

When run with XCode, it seems to be willing to work : the output is :

2020-09-02 16:25:59.712217+0200 testMetalCapture[20095:416447] Metal GPU Frame Capture Enabled
2020-09-02 16:25:59.712503+0200 testMetalCapture[20095:416447] Metal API Validation Enabled
2020-09-02 16:26:00.669092+0200 testMetalCapture[20095:416447] captureManager support is fine
Program ended with exit code: 0

But when I archive the build result, and run from a terminal, it fails :

2020-09-02 16:32:57.607 testMetalCapture[20126:419837] ********** captureManager does not support MTLCaptureDestinationGPUTraceDocument ************

Is there any runtime environment I could reproduce in terminal to get the MTLCaptureManager working ?

(Environment is XCode 11.6 + MacOS 10.15 Catalina)

score 2 · Accepted Answer · answered Oct 02 '20 at 13:55

From what I understood (I could not find any official documentation) :

MTLCaptureManager needs an authorization from the Info.plist: the MetalCaptureEnabled should set to YES.
The proper way is to bundle the command line application with such a plist
- This is quite involved, see for example Building OSX App Bundle

I found by accident that MTLCaptureManager also works if there's an Info.plist in the same directory as the command line application.

This Info.plist can be almost empty, like this :

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>MetalCaptureEnabled</key>
    <true/>
</dict>
</plist>

With this simple setup, I can run my test program (and my real one too)

% ./testMetalCapture
2020-10-02 15:53:08.507 testMetalCapture[28559:686864] Metal GPU Frame Capture Enabled
2020-10-02 15:53:08.523 testMetalCapture[28559:686864] captureManager support is fine

(I'm currently using MacOSX 10.15.6, it may break in the future)

score 1 · Answer 2 · answered Apr 17 '22 at 21:48

The trick from @rotoglup was working for me, until it wasn't anymore: some buffers were full of zeros, and no image appeared in the 'screenshots' inside the metal debugger. I don't know if this was because of an update of the OS or of XCode or something else.

I still needed an Info.plist with this content, as @rotoglup suggested:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>MetalCaptureEnabled</key>
    <true/>
</dict>
</plist>

but I also needed to set the METAL_DEVICE_WRAPPER_TYPE env variable to 1 in the terminal that launches the command line app:

export METAL_DEVICE_WRAPPER_TYPE=1

With this, buffers are filled up correctly, and I get non-empty screenshots.

MacOS Metal: Failing to capture GPU Frame from command line app

2 Answers2