A fast way to use win32ui and win32gui to capture screens from applications in Microsoft Windows and scale them using StretchBlt.

windows_screen_grab_and_scale.py

import win32ui
import win32gui
import win32con
import numpy as np
from matplotlib import pyplot as plt


"""
 Search open windows programs for a title containing the search_str
 and set hwnd
"""
search_str = 'Game'
global hwnd
hwnd = None
def enumHandler(h, lParam):
    global hwnd
    if win32gui.IsWindowVisible(h):
        if search_str in win32gui.GetWindowText(h):
            hwnd = h
            print("Found window handle: {}".format(h))
win32gui.EnumWindows(enumHandler, None)

#%%


"""
    Get the window dimensions
"""
l,t,r,b=win32gui.GetWindowRect(hwnd)
h=b-t
w=r-l

"""
    Set the size of the captured image to the scaling factor
"""
scaling = 0.5

dest_h = int(h * scaling)
dest_w = int(w * scaling)


"""
    Create all the win32 objects needed to do the copy operation
"""
dataBitMap = win32ui.CreateBitmap()        
w_handle_DC = win32gui.GetWindowDC(hwnd)
windowDC = win32ui.CreateDCFromHandle(w_handle_DC)
memDC = windowDC.CreateCompatibleDC()
dataBitMap.CreateCompatibleBitmap(windowDC , dest_w, dest_h)
memDC.SelectObject(dataBitMap)

"""
    Use the windows api to capture and scale an image of the window
"""
def getScaled(dest_w, dest_h):
    memDC.StretchBlt((0,0), (dest_w,dest_h), windowDC, (0,0), (w,h), win32con.SRCCOPY)
    
    """
        dataBitMap.GetBitmapBits(False) returns a tuple
        dataBitMpa.GetBitmapBits(True) returns a string of encoded binary
        values.
        %timeit bits = np.array(dataBitMap.GetBitmapBits(False), np.uint8)
        118 ms ± 21.1 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

        %timeit bits = np.fromstring(dataBitMap.GetBitmapBits(True), np.uint8)
        2.21 ms ± 469 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
        
    """
    bits = np.fromstring(dataBitMap.GetBitmapBits(True), np.uint8)
    
    """
        %timeit rgba = bits.reshape(dest_h, dest_w,4).transpose()
        901 ns ± 344 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
        
        %timeit r,g,b = bits[0::4].reshape(dest_h, dest_w), bits[1::4].reshape(dest_h, dest_w), bits[2::4].reshape(dest_h, dest_w)
        3.59 µs ± 1.39 µs per loop (mean ± std. dev. of 7 runs, 100000 loops each)
        
    """
    # GetBitMapBits returns G,B,R,A,G,B,R,A ... over the pixels
    r,g,b = bits[2::4].reshape(dest_h, dest_w), bits[1::4].reshape(dest_h, dest_w), bits[0::4].reshape(dest_h, dest_w)
    return r,g,b


r,g,b = getScaled(dest_w, dest_h)


fig = plt.figure(1)
plt.subplot(131)
plt.imshow(r, 'Reds')
plt.subplot(132)
plt.imshow(g, 'Greens')
plt.subplot(133)
plt.imshow(b, 'Blues')
plt.show()