GetWindowDpiScale() result always equals to 1.0 when multi-viewports is disabled

[Alzathar]

Version/Branch of Dear ImGui:

Branch: docking (commit 5f409c6)

Back-end/Renderer/Compiler/OS

Back-ends: GLFW 3.3 + sokol engine

My Issue/Question:
My monitor has a DPI scale equals to 1.5. I expected to get this value when using the function ImGui::GetWindowDpiScale(). However, it returns the value 1.0. The problem is because the ImGuiViewportP::PlatformMonitor is never set for the main viewport and uses the default value -1. By setting manually to platform monitor index to 0, I get the good DPI scale.

Is it intentional? In the code, the member ImGuiViewportP::PlatformMonitor is only updated for owned viewports. Would it be possible to have a public (or internal) API to detect the monitor associated with the main viewport?

Some code used to illustrate the problem

// By default all the DPI scales will be equal to 1.0 and the last line is not shown
ImGuiViewportP*viewport = (ImGuiViewportP*)ImGui::GetMainViewport();
ImGui::Text("Window DPI scale: %f", ImGui::GetWindowDpiScale());
ImGui::Text("Viewport DPI scale: %f", viewport->DpiScale);
ImGui::Text("Viewport DPI scale: %f", viewport->DpiScale);
ImGui::Text("Viewport platform monitor: %i", viewport->PlatformMonitor);
if (viewport->PlatformMonitor != -1) {
  ImGui::Text("Viewport platform monitor DPI scale: %f", ImGui::GetCurrentContext()->PlatformIO.Monitors[viewport->PlatformMonitor].DpiScale);
}

[ocornut]

UpdateViewportsNewFrame() calls UpdateViewportPlatformMonitor() for all viewports.

What do you see in Metrics->Viewports?
Because I see non-100% dpi scale there.

And if I add ImGui::Text("Window DPI scale: %f", ImGui::GetWindowDpiScale()); to a window in the main viewport I get:

You didn't specify the OS. Please note that windows application needs explicit enabling of dpi awareness.

[Alzathar]

The OS is Windows 10 x64 and the embedded manifest sets correctly the dpi awareness.

My problem is elsewhere. The ImGuiConfigFlags_ViewportsEnable flag is not set. The application in development has only one viewport and it is not intended to give the possibility to dock/undock windows. The docking branch is used instead of the master branch to simplify the building of the interface thanks to the DockBuilder* functions.

When the ImGuiConfigFlags_ViewportsEnable flag is set, I get the correct DPI scale.

My refined question is do we have to enable the ImGuiConfigFlags_ViewportsEnable flag to get the DPI scale of the main viewport? Looking into the code, this might not be necessary if we change the call of the function UpdateViewportPlatformMonitor(). Currently, this function is called only when the ImGuiConfigFlags_ViewportsEnable flag is enabled (see imgui.cpp:10626). However, we could call this function just after the condition (i.e. line 10628).

I tested this modification and it works (i.e. I got the correct DPI scale for the main viewport without the ImGuiConfigFlags_ViewportsEnable flag).

With this modification the function GetWindowDpiScale() returns the good DPI scale w or w/o the ImGuiConfigFlags_ViewportsEnable flag enabled.

(sorry if my justification is not developed enough, I will develop more if necessary tomorrow.)

[ocornut]

You are right, this should be available and working even if multi-viewports are not enabled.
Will look into the fix.

[ocornut]

This should be fixed now, thanks for reporting the issue!