Compute the display orientation of the current thread using plain old Win32 API.

dispornt.cpp

/* SPDX-License-Identifier: MIT

Copyright (c) 2024 by Ji Luo

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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The above copyright notice and this permission notice shall be included in
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IN THE SOFTWARE.

Echoes the current display resolution and orientations.

See
  Raymond Chen,
  "Using the DisplayInformation class from a desktop Win32 application, part 2",
  https://devblogs.microsoft.com/oldnewthing/20240321-00/?p=109558

cl.exe /std:c++20 dispornt.cpp && cls && dispornt.exe

*/

#include<cstdio>
#include<cstdint>
#include<windows.h>

#pragma comment(lib, "user32")

typedef unsigned DisplayOrientations;
constexpr DisplayOrientations DO_None = 0;
constexpr DisplayOrientations DO_L = 1;
constexpr DisplayOrientations DO_P = 2;
constexpr DisplayOrientations DO_LF = 4;
constexpr DisplayOrientations DO_PF = 8;
char const * const DO_Names[] =
{ "L", "P", "LF", "PF", nullptr };

int main()
{
  while (true)
  {
    DEVMODEW dm { .dmSize = sizeof(DEVMODEW) };
    if (!EnumDisplaySettingsExW(
      nullptr,
      ENUM_CURRENT_SETTINGS,
      &dm,
      EDS_RAWMODE | EDS_ROTATEDMODE)
    )
    {
      std::fputs("EnumDisplaySettingsExW failed.\n", stderr);
      return -1;
    }
    if ((dm.dmFields & (DM_PELSWIDTH | DM_PELSHEIGHT))
      != (DM_PELSWIDTH | DM_PELSHEIGHT)
    )
    {
      std::fputs("Returned result does not contain resolution.\n", stderr);
      return -2;
    }
    dm.dmDisplayOrientation &= (
      (dm.dmFields & DM_DISPLAYORIENTATION) == DM_DISPLAYORIENTATION
      ? 3 : 0
    ); // ^^^ I am not sure if this guard is necessary.
    // Infer the orientation at rotation degree 0.
    auto const initial = (
      // dmPelsWidth <=> dmPelsHeight  is landscape/portrait after rotation.
      // dmDisplayOrientation          is rotation / (-pi/2).
      ((dm.dmPelsWidth > dm.dmPelsHeight) ^ (dm.dmDisplayOrientation & 1))
      ? DO_L
      : ((dm.dmPelsWidth < dm.dmPelsHeight) ^ (dm.dmDisplayOrientation & 1))
      ? DO_P
      : (DO_L | DO_P)
      /* ^^^
      ** The documentation does not specify what happens
      ** when the resolution is square.
      ** Since DisplayOrientations is a [Flags] enumeration,
      ** the sane result is a bitwise combination of both.
      */
    );
    // Compute the orientation at current rotation.
    // The significance of bits in DisplayOrientation
    // is modulo 4 (plus 1 for every 90 degree clockwise),
    // consistent with the numeric value of dmDisplayOrientation.
    auto const current = ((
      (initial << dm.dmDisplayOrientation) |
      (initial >> (4 - dm.dmDisplayOrientation))
    ) & 15u);
    // Boring exercise of printing...
    std::printf("w=%u, h=%u, o=degree(%d)\n",
      (unsigned)dm.dmPelsWidth,
      (unsigned)dm.dmPelsHeight,
      (int)dm.dmDisplayOrientation * 90
    );
    std::fputs("DisplayOrientations = ", stdout);
    bool already1 = false;
    for (int i = 0; DO_Names[i]; ++i)
    {
      if (current & (1u << i))
      {
        if (already1)
        {
          std::putchar(',');
          std::putchar(' ');
        }
        already1 = true;
        std::fputs(DO_Names[i], stdout);
      }
    }
    std::puts(already1 ? "\n" : "N\n");
    Sleep(1000);
  }
}