The WM_PAINT message

windows does manage wm_paint differently. multiple wm_paint messages in a row are treated as one, the others are removed.

>Try to call OutputDebugString instead of a breakpoint, and see what happens.

Ok, I will.

> windows does manage wm_paint differently. multiple wm_paint messages in a row are treated as one, the others are removed.

That's interesting to know.  I always learn so many things when I post in this forum. C++ programmers by nature have to be very close to the underlying computer and often understand thousands of valuable low-level technical details.

-Sandra

>>>> windows does manage wm_paint differently. multiple wm_paint messages in a row are treated as
>>>> one, the others are removed.
>>That's interesting to know.

I should add that this is done by the system, when your window proc finally receives it, the message has already "passed the tests"

I added a OutputDebugString() in the WM_PAINT and printed an integer that I incremented every time. I timed it, in 5 seconds there were 640 WM_PAINT messages received by my application. Conservatively that's a paint message every 10 milliseconds. Painting is a fairly expensive operation in this control, I'd like to reduce that if I could. The monitor doesn't even refresh as fast as it's trying to repaint, I could show a movie at over 100 frames / sec if I could blt them that fast in the paint method.

-Sandra

Any operations in your code that call "InvalidateRect()" or similar functions?

> Any operations in your code that call "InvalidateRect()" or similar functions?

Nothing like that. Here's the entire code (it's a little messy):

#include "stdafx.h"
#include "TestLargeBG.h"

#define KEY_SCANCODE(lParam) (((lParam) & 0x00FF0000)>>16)
#define KEY_REPEAT(lParam) (int)((lParam) & 0xFFFF)

// Global Variables:
HINSTANCE hInst;                                                // current instance
MSG msg;
HBITMAP bgImg;
HGDIOBJ hOldImg;
HDC hDC, hMemDC;
int width = 1000;
int height = 800;
int x = 0;
int y = 0;

int nPaints = 0;
char temp[100];

const TCHAR * szWindowClass = "TestLargeBG";        // the main window class name

// Forward declarations of functions included in this code module:
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK      WndProc(HWND, UINT, WPARAM, LPARAM);

HBITMAP ImageFromFile(HINSTANCE hInstance, LPCTSTR imgPath)
{
      return (HBITMAP)LoadImage(hInstance, imgPath, IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
}

void Cleanup(HWND hWnd)
{
      ReleaseDC(hWnd, hDC);
      SelectObject(hMemDC, hOldImg);
      DeleteDC(hMemDC);
}

inline void Paint()
{
      BitBlt(hDC, 0, 0, width, height, hMemDC, x, y, SRCCOPY);
}

void KeyDown(WPARAM wParam, LPARAM lParam)
{
      int i, repeat = KEY_REPEAT(lParam);
      OutputDebugString("Key Down\n");
      switch(wParam)
      {
      case 'w':
            for(i=0; i < repeat; i++)
            {
                  y -= 4;
                  Paint();
            }
            break;
      case 'a':
            for(i=0; i < repeat; i++)
            {
                  x -= 4;
                  Paint();
            }
            break;
      case 'd':
            for(i=0; i < repeat; i++)
            {
                  x += 4;
                  Paint();
            }
            break;
      case 's':
            for(i=0; i < repeat; i++)
            {
                  y += 4;
                  Paint();
            }
            break;
      case 27: // ESC
            PostQuitMessage(0);
            break;
      }
}

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPTSTR    lpCmdLine,
                     int       nCmdShow)
{
      // Register the window class
      WNDCLASSEX wcex;

      wcex.cbSize = sizeof(WNDCLASSEX);

      wcex.style                  = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
      wcex.lpfnWndProc      = (WNDPROC)WndProc;
      wcex.cbClsExtra            = 0;
      wcex.cbWndExtra            = 0;
      wcex.hInstance            = hInstance;
      wcex.hIcon                  = 0;
      wcex.hCursor            = LoadCursor(NULL, IDC_ARROW);
      wcex.hbrBackground      = 0;
      wcex.lpszMenuName      = 0;
      wcex.lpszClassName      = szWindowClass;
      wcex.hIconSm            = 0;

      RegisterClassEx(&wcex);

      // Load the background image
      bgImg = ImageFromFile(hInstance, "C:\\TestLargeBG.bmp");
      
      if(bgImg == NULL)
            return FALSE;

      // Perform application initialization:
      if (!InitInstance (hInstance, nCmdShow))
      {
            return FALSE;
      }

      // Main message loop:
      while (GetMessage(&msg, NULL, 0, 0))
      {
            DispatchMessage(&msg);
      }

      return (int) msg.wParam;
}

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // Store instance handle in our global variable

   hWnd = CreateWindow(szWindowClass, 0, WS_HSCROLL | WS_VSCROLL,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hWnd)
      return FALSE;

      hDC = GetDC(hWnd);
      hMemDC = CreateCompatibleDC(hDC);
      hOldImg = SelectObject(hMemDC, bgImg);

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
      switch (message)
      {
      case WM_KEYDOWN:
            TranslateMessage(&msg);
            break;
      case WM_CHAR:
            KeyDown(wParam, lParam);
            break;
      case WM_PAINT:
            // Count the number of paint ops
            itoa(nPaints++,temp,10);
            temp[strlen(temp)] = '\n';
            temp[strlen(temp)] = 0;
            OutputDebugString((LPCSTR)temp);
            Paint();
            break;
      case WM_ERASEBKGND:
            return (LRESULT)1; // Do nothing, but tell windows we handled this event
      case WM_DESTROY:
            PostQuitMessage(0);
            break;
      case WM_QUIT:
            Cleanup(hWnd);
            break;
      default:
            return DefWindowProc(hWnd, message, wParam, lParam);
      }
      return 0;
}

If you want to compile it you can create a new non-empty Win32 project in Visual Studio, put this in, and change C:\\TestLargeBG.bmp to point to a large bitmap image (mine is 3500x2400 and weighs in at 16MB.)

-Sandra

After reading your second post about keyboard handling I want to suggest you to exclude repeat counting and simplify keyboard handling code:

   // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
 
...

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    PAINTSTRUCT ps;
    HDC hdc;
   
    switch (message)
    {
    case WM_CHAR:
        KeyDown(wParam, lParam, hWnd);
        break;
    case WM_PAINT:

...

void KeyDown(WPARAM wParam, LPARAM lParam, HWND hWnd)
{
    OutputDebugString("Key Down\n");
    switch(wParam)
    {
    case 'w':
        {
            y -= 4;
            HDC hdc = GetDC(hWnd);
            Paint(hdc);
            ReleaseDC(hWnd, hdc);
        }

        break;
...

Oh, I thought that if you pass the CS_OWNDC that you have a dedicated DC and don't need to get it from the pool and return it each time you want to paint.

That was silly of me to assume that because it appeared to work that it was working, the two are often different animals.

Thanks, Alex it works perfectly now, and no erroneous paint messages are sent.

Why the UpdateWindow() call though? Paint() is sufficient isn't it?

You not only solve the problem, but you fixed my code for me! Well beyond what I was expecting, thanks Alex. A+.

-Sandra

  case 'a':
        for(i=0; i < repeat; i++)
        {
            x -= 4;
            HDC hdc = GetDC(hWnd);
            Paint(hdc);
            ReleaseDC(hWnd, hdc);
        }
        UpdateWindow(hWnd);

Yes, you can remove UpdateWindow line.