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Backends: Emscripten: Add runtime device pixel ratio setter

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This commit is contained in:
SlowRiot 2026-09-04 13:01:50 +01:00
parent 796b15e894
commit cfa0d9f4e2
2 changed files with 28 additions and 15 deletions
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33
backends/imgui_impl_emscripten.cpp
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@ -4,6 +4,7 @@
//
// CHANGELOG
// (minor and older changes stripped away, please see git history for details)
// 2026-09-04: Emscripten: Replaced TargetDevicePixelRatio global with an Init() parameter and runtime setter.
// 2026-09-03: Updated coding style and simplified key translation table setup.
// 2026-04-02: Inputs: Replaced custom KeyboardEvent.code parser with ImHashStr()/ImGuiStorage lookup to match Dear ImGui backend style.
// 2026-03-31: Emscripten: Added configurable TargetDevicePixelRatio to control how browser device pixels map to Dear ImGui pixels.
@ -25,8 +26,6 @@
extern ImGuiID ImHashStr(char const* data, size_t data_size = 0, ImGuiID seed = 0); // Declared in imgui_internal.h.
float ImGui_ImplEmscripten_TargetDevicePixelRatio = 1.0f;
// W3C UI Events KeyboardEvent.code translation helpers
static ImGuiKey ImGui_ImplEmscripten_TranslateKey(char const* emscripten_key);
@ -94,6 +93,7 @@ static void ImGui_ImplEmscripten_SetBrowserCursor(char const* new_cursor);
struct ImGui_ImplEmscripten_Data
{
float TargetDevicePixelRatio = 1.0f;
float CssToImGuiScale = 1.0f;
ImGui_ImplEmscripten_Cursor CurrentCursor = ImGui_ImplEmscripten_Cursor_Invalid;
char* CursorToRestore = nullptr;
@ -101,15 +101,9 @@ struct ImGui_ImplEmscripten_Data
bool LastNoMouseCursorChange = false;
};
static float ImGui_ImplEmscripten_GetTargetDevicePixelRatio()
{
IM_ASSERT(ImGui_ImplEmscripten_TargetDevicePixelRatio > 0.0f && "ImGui_ImplEmscripten_TargetDevicePixelRatio must be positive.");
return ImGui_ImplEmscripten_TargetDevicePixelRatio > 0.0f ? ImGui_ImplEmscripten_TargetDevicePixelRatio : 1.0f;
}
static void ImGui_ImplEmscripten_UpdateDisplayProperties(ImGuiIO& io, ImGui_ImplEmscripten_Data* bd, float css_width, float css_height)
{
float const target_device_pixel_ratio = ImGui_ImplEmscripten_GetTargetDevicePixelRatio();
float const target_device_pixel_ratio = bd->TargetDevicePixelRatio;
float const css_to_imgui_scale = (float)emscripten_get_device_pixel_ratio() / target_device_pixel_ratio;
bd->CssToImGuiScale = css_to_imgui_scale;
io.DisplaySize.x = css_width * css_to_imgui_scale;
@ -123,13 +117,16 @@ static ImGui_ImplEmscripten_Data* ImGui_ImplEmscripten_GetBackendData()
return ImGui::GetCurrentContext() ? (ImGui_ImplEmscripten_Data*)ImGui::GetIO().BackendPlatformUserData : nullptr;
}
bool ImGui_ImplEmscripten_Init()
bool ImGui_ImplEmscripten_Init(float target_device_pixel_ratio)
{
// Initialise the Emscripten backend, setting input callbacks
ImGuiIO& io = ImGui::GetIO();
IMGUI_CHECKVERSION();
IM_ASSERT(io.BackendPlatformUserData == nullptr && "Already initialized a platform backend!");
IM_ASSERT(target_device_pixel_ratio > 0.0f && "Target device pixel ratio must be positive.");
if (target_device_pixel_ratio <= 0.0f) return false;
ImGui_ImplEmscripten_Data* bd = IM_NEW(ImGui_ImplEmscripten_Data)();
bd->TargetDevicePixelRatio = target_device_pixel_ratio;
io.BackendPlatformUserData = (void*)bd;
io.BackendPlatformName = "imgui_impl_emscripten";
io.BackendFlags |= ImGuiBackendFlags_HasMouseCursors;
@ -381,6 +378,22 @@ bool ImGui_ImplEmscripten_Init()
return true;
}
void ImGui_ImplEmscripten_SetTargetDevicePixelRatio(float target_device_pixel_ratio)
{
ImGui_ImplEmscripten_Data* bd = ImGui_ImplEmscripten_GetBackendData();
IM_ASSERT(bd != nullptr && "Context or backend not initialized? Did you call ImGui_ImplEmscripten_Init()?");
IM_ASSERT(target_device_pixel_ratio > 0.0f && "Target device pixel ratio must be positive.");
if (bd == nullptr || target_device_pixel_ratio <= 0.0f || target_device_pixel_ratio == bd->TargetDevicePixelRatio) return;
bd->TargetDevicePixelRatio = target_device_pixel_ratio;
ImGui_ImplEmscripten_UpdateDisplayProperties(
ImGui::GetIO(),
bd,
(float)EM_ASM_INT(return window.innerWidth;),
(float)EM_ASM_INT(return window.innerHeight;)
);
}
void ImGui_ImplEmscripten_Shutdown()
{
ImGui_ImplEmscripten_Data* bd = ImGui_ImplEmscripten_GetBackendData();

10
backends/imgui_impl_emscripten.h
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@ -40,12 +40,12 @@
#error The imgui_impl_emscripten backend requires Emscripten.
#endif
// Controls how many device pixels Dear ImGui should target per Dear ImGui pixel.
// Default 1.0f gives 1:1 device-pixel rendering. Set before Init() if you want a different scaling policy.
extern IMGUI_IMPL_API float ImGui_ImplEmscripten_TargetDevicePixelRatio;
// Initialise the Emscripten backend, setting input callbacks. This should be called after ImGui::CreateContext();
IMGUI_IMPL_API bool ImGui_ImplEmscripten_Init();
// The target_device_pixel_ratio parameter controls how many device pixels Dear ImGui should target per Dear ImGui pixel. Default 1.0f gives 1:1 device-pixel rendering.
IMGUI_IMPL_API bool ImGui_ImplEmscripten_Init(float target_device_pixel_ratio = 1.0f);
// Change the target device pixel ratio at runtime and update display properties immediately.
IMGUI_IMPL_API void ImGui_ImplEmscripten_SetTargetDevicePixelRatio(float target_device_pixel_ratio);
// Shut down the Emscripten backend. This unsets all Emscripten input callbacks set by Init.
// Note it'll also unset any Emscripten input callbacks set elsewhere in the program!