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

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)

Answer 1

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)

Answer 2

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.