Backends: SDL2,SDL3: Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. (#6189, #6114, #3644)

backends/imgui_impl_sdl2.cpp

@@ -17,6 +17,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2023-02-23: Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. (#6189, #6114, #3644)
 //  2023-02-07: Implement IME handler (io.SetPlatformImeDataFn will call SDL_SetTextInputRect()/SDL_StartTextInput()).
 //  2023-02-07: *BREAKING CHANGE* Renamed this backend file from imgui_impl_sdl.cpp/.h to imgui_impl_sdl2.cpp/.h in prevision for the future release of SDL3.
 //  2023-02-02: Avoid calling SDL_SetCursor() when cursor has not changed, as the function is surprisingly costly on Mac with latest SDL (may be fixed in next SDL version).
@@ -610,8 +611,11 @@ void ImGui_ImplSDL2_NewFrame()
         io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
 
     // Setup time step (we don't use SDL_GetTicks() because it is using millisecond resolution)
+    // (Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. Happens in VMs and Emscripten, see #6189, #6114, #3644)
     static Uint64 frequency = SDL_GetPerformanceFrequency();
     Uint64 current_time = SDL_GetPerformanceCounter();
+    if (current_time <= bd->Time)
+        current_time = bd->Time + 1;
     io.DeltaTime = bd->Time > 0 ? (float)((double)(current_time - bd->Time) / frequency) : (float)(1.0f / 60.0f);
     bd->Time = current_time;
 

backends/imgui_impl_sdl3.cpp

@@ -18,6 +18,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2023-02-23: Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. (#6189, #6114, #3644)
 //  2023-02-07: Forked "imgui_impl_sdl2" into "imgui_impl_sdl3". Removed version checks for old feature. Refer to imgui_impl_sdl2.cpp for older changelog.
 
 #include "imgui.h"
@@ -543,8 +544,11 @@ void ImGui_ImplSDL3_NewFrame()
         io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
 
     // Setup time step (we don't use SDL_GetTicks() because it is using millisecond resolution)
+    // (Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. Happens in VMs and Emscripten, see #6189, #6114, #3644)
     static Uint64 frequency = SDL_GetPerformanceFrequency();
     Uint64 current_time = SDL_GetPerformanceCounter();
+    if (current_time <= bd->Time)
+        current_time = bd->Time + 1;
     io.DeltaTime = bd->Time > 0 ? (float)((double)(current_time - bd->Time) / frequency) : (float)(1.0f / 60.0f);
     bd->Time = current_time;
 

docs/CHANGELOG.txt

@@ -53,6 +53,8 @@ Other changes:
   avoid inteferences. (#5817, #6183) [@DimaKoltun]
 - Backends: Win32: Use WM_NCMOUSEMOVE / WM_NCMOUSELEAVE to track mouse positions over
   non-client area (e.g. OS decorations) when app is not focused. (#6045, #6162)
+- Backends: SDL2, SDL3:  Accept SDL_GetPerformanceCounter() not returning a monotonically
+  increasing value. (#6189, #6114, #3644) [@adamkewley]
 
 
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