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Matplotlib Device Scaling on Windows #30599

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Matplotlib Device Scaling on Windows #30599

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3478304
Update _backend_tk.py
melwyncarlo Sep 25, 2025
b5e669a
Update _tkagg.cpp
melwyncarlo Sep 25, 2025
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2 changes: 2 additions & 0 deletions lib/matplotlib/backends/_backend_tk.py
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Original file line number Diff line number Diff line change
Expand Up @@ -602,6 +602,8 @@ def destroy(*args):
self.window.protocol("WM_DELETE_WINDOW", destroy)
self.window.deiconify()
self.canvas._tkcanvas.focus_set()
if self._window_dpi.get() != 96:
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@tacaswell tacaswell Sep 25, 2025

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why hard-code 96?

self._update_window_dpi()
else:
self.canvas.draw_idle()
if mpl.rcParams['figure.raise_window']:
Expand Down
41 changes: 26 additions & 15 deletions src/_tkagg.cpp
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Expand Up @@ -152,6 +152,22 @@ mpl_tk_blit(py::object interp_obj, const char *photo_name,
}

#ifdef WIN32_DLL
void update_tk_window_dpi(HWND hwnd, Tcl_Interp *interp, std::string dpi) {
// This variable naming must match the name used in
// lib/matplotlib/backends/_backend_tk.py:FigureManagerTk.
std::string var_name("window_dpi");
var_name += std::to_string((unsigned long long)hwnd);
if (TCL_SETVAR) {
TCL_SETVAR(interp, var_name.c_str(), dpi.c_str(), 0);
} else if (TCL_SETVAR2) {
TCL_SETVAR2(interp, var_name.c_str(), NULL, dpi.c_str(), 0);
} else {
// This should be prevented at import time, and therefore unreachable.
// But defensively throw just in case.
throw std::runtime_error("Unable to call Tcl_SetVar or Tcl_SetVar2");
}
}

LRESULT CALLBACK
DpiSubclassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam,
UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
Expand All @@ -165,24 +181,10 @@ DpiSubclassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam,
// call Python code, we must not also call *any* Tk code from Python.
// So stay with Tcl calls in C only.
{
// This variable naming must match the name used in
// lib/matplotlib/backends/_backend_tk.py:FigureManagerTk.
std::string var_name("window_dpi");
var_name += std::to_string((unsigned long long)hwnd);

// X is high word, Y is low word, but they are always equal.
std::string dpi = std::to_string(LOWORD(wParam));

Tcl_Interp* interp = (Tcl_Interp*)dwRefData;
if (TCL_SETVAR) {
TCL_SETVAR(interp, var_name.c_str(), dpi.c_str(), 0);
} else if (TCL_SETVAR2) {
TCL_SETVAR2(interp, var_name.c_str(), NULL, dpi.c_str(), 0);
} else {
// This should be prevented at import time, and therefore unreachable.
// But defensively throw just in case.
throw std::runtime_error("Unable to call Tcl_SetVar or Tcl_SetVar2");
}
update_tk_window_dpi(hwnd, interp, dpi);
}
return 0;
case WM_NCDESTROY:
Expand Down Expand Up @@ -235,6 +237,15 @@ mpl_tk_enable_dpi_awareness(py::object UNUSED_ON_NON_WINDOWS(frame_handle_obj),
// Per monitor aware means we need to handle WM_DPICHANGED by wrapping
// the Window Procedure, and the Python side needs to trace the Tk
// window_dpi variable stored on interp.
typedef UINT (WINAPI *GetDpiForWindow_t)(HWND);
GetDpiForWindow_t GetDpiForWindowPtr =
(GetDpiForWindow_t)GetProcAddress(user32, "GetDpiForWindow");
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@anntzer anntzer Sep 29, 2025

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Please use the shorter style for casting as in #30615.

if (GetDpiForWindowPtr == NULL) {
FreeLibrary(user32);
return py::cast(false);
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@tacaswell tacaswell Sep 25, 2025

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Is completely returning here correct? Is there a case where we may not be able to get the methods for managing the dpi but still need to run the setWindowSubclass method?

}
UINT dpi = GetDpiForWindowPtr(frame_handle);
update_tk_window_dpi(frame_handle, interp, std::to_string(dpi));
SetWindowSubclass(frame_handle, DpiSubclassProc, 0, (DWORD_PTR)interp);
}
FreeLibrary(user32);
Expand Down
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