Re: [ctypes-users] Saving a Device context (DC) into a picture file

[Michael C <mys...@gm...>]

Hi all:

I have my own code that samples pixel colour,
https://pastebin.com/4EMd765h

 and now I am trying to add the feature of saving the Device Context to a
.bmp file, so I found this code

https://books.google.ca/books?id=9MS9BQAAQBAJ

however there is one problem: The book example screenshots the whole
desktop and I only want a screenshot of a given window. Is there a way to
modify the black hat example into something that only screenshot a window
given its handle?

namely, these 4 lines.

width = win32api.GetSystemMetrics(win32con.SM_CXVIRTUALSCREEN)
height = win32api.GetSystemMetrics(win32con.SM_CYVIRTUALSCREEN)
left = win32api.GetSystemMetrics(win32con.SM_XVIRTUALSCREEN)
top = win32api.GetSystemMetrics(win32con.SM_YVIRTUALSCREEN)

Thanks!

On Fri, Apr 28, 2017 at 5:28 PM, Michael C <mys...@gm...>
wrote:

> Hi all:
>
> I have my own code that samples pixel colour,
> https://pastebin.com/4EMd765h
>
>  and now I am trying to add the feature of saving the Device Context to a
> .bmp file, so I found this code
>
> https://books.google.ca/books?id=9MS9BQAAQBAJ
>
> however there is one problem: The book example screenshots the whole
> desktop and I only want a screenshot of a given window. Is there a way to
> modify the black hat example into something that only screenshot a window
> given its handle?
>
> Thanks!
>
>
>
> On Sat, Apr 22, 2017 at 2:29 PM, Michael C <mys...@gm...
> > wrote:
>
>> neat-o
>>
>>
>> On Sat, Apr 22, 2017 at 2:27 PM, eryk sun <er...@gm...> wrote:
>>
>>> On Sat, Apr 22, 2017 at 7:15 PM, Michael C
>>> <mys...@gm...> wrote:
>>> > Never mind, I found this
>>> > https://books.google.ca/books?id=9MS9BQAAQBAJ
>>>
>>> Thankfully a lot of the Windows code in Black Hat Python uses PyWin32.
>>> The few ctypes examples aren't top notch. For example, the author
>>> calls
>>>
>>>      user32.GetWindowThreadProcessId(hwnd, byref(pid))
>>>
>>> without setting the prototype of GetWindowThreadProcessId. In 64-bit
>>> Python 2, the value of the hwnd argument may get corrupted if the
>>> prototype isn't defined. It's only by luck of the draw that it usually
>>> works.
>>>
>>> Some WinAPI pseudohandles are non-zero in the upper DWORD, such as
>>> HWND_TOPMOST, which is defined as (HWND)(-1), i.e. 0xFFFFFFFFFFFFFFFF
>>> on a 64-bit system. This has the same problem in Python 2 if you
>>> forget to set the prototype or manually wrap the value in a pointer
>>> type. Python 3 ctypes doesn't have this problem, since it sign extends
>>> a negative argument as an intptr_t.
>>>
>>> Here's another problem in the book's example code. The default restype
>>> is a signed c_int, and here the author is calling GetTickCount without
>>> first setting GetTickCount.restype = ctypes.c_uint:
>>>
>>>     run_time = kernel32.GetTickCount()
>>>     elapsed = run_time - struct_lastinputinfo.dwTime
>>>
>>> After 24.86 days system uptime, the tick count reaches 0x80000000
>>> (milliseconds), and the signed return value wraps around to
>>> -2147483648 <(214)%20748-3648>, incrementing toward 0. The value of
>>> `elapsed` will be
>>> nonsense.
>>>
>>
>>
>