Ways to handle custom Windows message

n_korobeynikov · May 9, 2025, 9:33am

Hello everyone,

I’m integrating Nvidia PCL stats into a SDL application.
One of challenges is handling of a custom windows message in message loop. The message is used to measure input latency.

github.com/NVIDIA-RTX/Streamline

docs/ProgrammingGuidePCL.md

main


Streamline - PCL Stats
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>The focus of this guide is on using Streamline to integrate PCL (PC Latency) Stats into an application.

Version 2.7.30
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The application should not explicitly check for GPU HW, vendor, and driver version.

PCL Stats enables real time measurement of per-frame PC latency (PCL) during gameplay. E2E system latency = PCL + peripheral latency + display latency. And PCL = input sampling latency + simulation start to present + present to displayed.

Windows messages with ID == `sl::PclState::statsWindowMessage` are sent to the application periodically. The time this ping message is sent is recorded and is used to measure the latency between the simulated input and the application picking up the input. This is the input sampling latency.

Typically, the application's message pump would process keyboard and mouse input messages into a queue to be read in the upcoming frame. On seeing the ping message, the application must either call `slPclSetMarker` or `sl::PclMarker::ePCLatencyPing` to send the ping marker right away, or put it in the queue, or set a flag for the next simulation to send the ping marker.

> **NOTE:**
> The exact timing of the ping marker does not matter. It is the frame index parameter that identify which frame picks up the simulated input. For example, since the frame index is usually incremented at simulation start, in the case of the ping marker being sent at message pump before simulation start, use current frame index +1.

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So my question is:
Can I handle this windows message trough SDL_Event somehow, or is the only way to override WndProc() function?

flowCRANE · May 9, 2025, 11:15am

You can install message hook callback and receive all Windows messages in it. Just use the SDL_SetWindowsMessageHook() and provide a pointer to the callback function.

I’m using such hook to catch modal loop messages (menu and sizing loops) and it works like a charm.