A better Image.show() implementation for PIL on Windows

pil_show_windows.py

__all__ = []

"""
  Importing this script in Python 3.x x64 on Windows overwrites
  PIL's default (slow) 'Image.show()' implementation based on
  temporary PNGs with a faster implementation that instead shows
  the image in a native resizable viewer window, without creating
  any temporary files. In addition to PIL the script only uses
  ctypes and the Windows API, no 3rd-party modules involved.

  Usage:
  ======

  from PIL import Image
  import pil_show_windows  # import overwrites show() method

  img = Image.open('D:\\test\\lena.png')
  img.show()  # Blocks code execution until viewer window is closed
  ...
"""

from ctypes import Structure, sizeof, POINTER, WINFUNCTYPE, windll, c_longlong, byref
from ctypes.wintypes import *
from PIL import ImageShow, ImageWin

########################################
# Used Winapi structs
########################################
class PAINTSTRUCT(Structure):
    _fields_ = [
        ("hdc",            HDC),
        ("fErase",         BOOL),
        ("rcPaint",        RECT),
        ("fRestore",       BOOL),
        ("fIncUpdate",     BOOL),
        ("rgbReserved",    BYTE * 32),
    ]

LONG_PTR = c_longlong  # for x64 only!
WNDPROC = WINFUNCTYPE(LONG_PTR, HWND, UINT, WPARAM, LPARAM)

class WNDCLASSEX(Structure):
    def __init__(self, *args, **kwargs):
        super(WNDCLASSEX, self).__init__(*args, **kwargs)
        self.cbSize = sizeof(self)
    _fields_ = [
        ("cbSize",          UINT),
        ("style",           UINT),
        ("lpfnWndProc",     WNDPROC),
        ("cbClsExtra",      INT),
        ("cbWndExtra",      INT),
        ("hInstance",       HANDLE),
        ("hIcon",           HANDLE),
        ("hCursor",         HANDLE),
        ("hBrush",          HANDLE),
        ("lpszMenuName",    LPCWSTR),
        ("lpszClassName",   LPCWSTR),
        ("hIconSm",         HANDLE)
    ]

########################################
# Used Winapi functions
########################################
gdi32 = windll.Gdi32
gdi32.GetStockObject.restype = HANDLE
gdi32.SetStretchBltMode.argtypes = (HDC, INT)

kernel32 = windll.Kernel32
kernel32.GetModuleHandleW.argtypes = (LPCWSTR,)
kernel32.GetModuleHandleW.restype = HINSTANCE

user32 = windll.user32
user32.BeginPaint.argtypes = (HWND, POINTER(PAINTSTRUCT))
user32.CreateWindowExW.argtypes = (DWORD, LPCWSTR, LPCWSTR, DWORD, INT, INT, INT, INT, HWND, HMENU, HINSTANCE, LPVOID)
user32.DefWindowProcW.argtypes = (HWND, UINT, WPARAM, LPARAM)
user32.DestroyWindow.argtypes = (HWND,)
user32.DispatchMessageW.argtypes = (POINTER(MSG),)
user32.EndPaint.argtypes = (HWND, POINTER(PAINTSTRUCT))
user32.GetMessageW.argtypes = (POINTER(MSG),HWND,UINT,UINT)
user32.LoadCursorW.argtypes = (HINSTANCE, LPVOID)
user32.LoadCursorW.restype = HANDLE
user32.LoadIconW.argtypes = (HINSTANCE, LPCWSTR)
user32.LoadIconW.restype = HICON
user32.PostMessageW.argtypes = (HWND, UINT, LPVOID, LPVOID)
user32.SystemParametersInfoA.argtypes = (UINT, UINT, LPVOID, UINT)
user32.TranslateMessage.argtypes = (POINTER(MSG),)

########################################
# Used Winapi constants
########################################
BLACK_BRUSH = 4
CS_HREDRAW = 2
CS_VREDRAW = 1
CW_USEDEFAULT = -2147483648
HALFTONE = 4
IDC_ARROW = 32512
SM_CYCAPTION = 4
SPI_GETWORKAREA = 48
WM_CLOSE = 16
WM_PAINT = 15
WM_QUIT = 18
WS_OVERLAPPEDWINDOW = 13565952
WS_VISIBLE = 268435456


class PILImageShow():

    def __init__(self, img, window_title='PIL Image'):
        if img.mode not in ('RGB', 'RGBA', 'L', '1'):
            img = img.convert('RGB')
        dib = ImageWin.Dib(img)
        img_ratio = img.width / img.height

        # Show image centered and resized to window while keeping its aspect ratio
        def _on_WM_PAINT(hwnd, wparam, lparam):
            rc = RECT()
            user32.GetClientRect(hwnd, byref(rc))
            width, height = rc.right, rc.bottom
            if width / height > img_ratio:
                dest_width = round(height * img_ratio)
                dest_height = height
                x = (width - dest_width) // 2
                y = 0
            else:
                dest_width = width
                dest_height = round(width / img_ratio)
                x = 0
                y = (height - dest_height) // 2
            ps = PAINTSTRUCT()
            hdc = user32.BeginPaint(hwnd, byref(ps))
            gdi32.SetStretchBltMode(hdc, HALFTONE)
            dib.draw(ImageWin.HDC(hdc), (x, y, x + dest_width, y + dest_height))
            user32.EndPaint(hwnd, byref(ps))
            return 0

        def _window_proc_callback(hwnd, msg, wparam, lparam):
            if msg == WM_PAINT:
                return _on_WM_PAINT(hwnd, wparam, lparam)
            elif msg == WM_CLOSE:
                user32.PostMessageW(self.hwnd, WM_QUIT, 0, 0)
            return user32.DefWindowProcW(hwnd, msg, wparam, lparam)

        wndclass = WNDCLASSEX()
        wndclass.lpfnWndProc = WNDPROC(_window_proc_callback)
        wndclass.style = CS_VREDRAW | CS_HREDRAW
        wndclass.lpszClassName = 'PILImageShow'
        wndclass.hBrush = gdi32.GetStockObject(BLACK_BRUSH)
        wndclass.hCursor = user32.LoadCursorW(0, IDC_ARROW)
        wndclass.hIcon = user32.LoadIconW(kernel32.GetModuleHandleW(None), LPCWSTR(1))  # Python icon
        wndclass.hIconSm = wndclass.hIcon
        user32.RegisterClassExW(byref(wndclass))

        # Show window centered on screen and never bigger than the actual work area (desktop minus taskbar)
        rc_desktop = RECT()
        user32.SystemParametersInfoA(SPI_GETWORKAREA, 0, byref(rc_desktop), 0)
        rc_desktop.right -= 32  # Windows 11 DWM fix
        caption_height = user32.GetSystemMetrics(SM_CYCAPTION)
        win_width, win_height = img.width, img.height + caption_height
        if win_width > rc_desktop.right or win_height > rc_desktop.bottom:
            desktop_ratio = rc_desktop.right / (rc_desktop.bottom - caption_height)
            if desktop_ratio > img_ratio:
                win_height = rc_desktop.bottom
                win_width = round(win_height * desktop_ratio)
            else:
                win_width = rc_desktop.right
                win_height = round(win_width / desktop_ratio)
        x = (rc_desktop.right - win_width) // 2 + 16
        y = (rc_desktop.bottom - win_height) // 2
        self.hwnd = user32.CreateWindowExW(
            0,
            wndclass.lpszClassName,
            window_title,
            WS_OVERLAPPEDWINDOW | WS_VISIBLE,
            x, y,
            win_width, win_height,
            None, None, None, None
        )

        msg = MSG()
        while user32.GetMessageW(byref(msg), 0, 0, 0) > 0:
            user32.TranslateMessage(byref(msg))
            user32.DispatchMessageW(byref(msg))
        user32.DestroyWindow(self.hwnd)


# This overwrites PIL Image's show() method with our custom implementation
setattr(ImageShow, 'show', lambda img, title=None, **kwargs:
    PILImageShow(img, title or (img.filename if hasattr(img, 'filename') else 'PIL Image'))
)