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From: Ikke on 11 Mar 2010 08:44
Hi everybody,

First of all, sorry if this is the wrong group for this question - I've
already tried to ask this in comp.lang.c++, comp.graphics no longer
exists, and comp.os.ms-windows.programmer.graphics has no traffic.

As the subject says, I'm trying to write a very basic renderer, but I've
stumbled upon a problem.

Basically, I create a window (1024x768), and draw a small circle on it,
which I then move five pixels to the right every second.

It all works fine (sort of), but as soon as the circle moves, the
background is no longer black, and becomes white. I have absolutely no
idea why - can anybody please shed some light on this?

Here is my code:
--- begin ---
#include <windows.h>

/* Declare Windows procedure */
LRESULT CALLBACK WindowProcedure (HWND, UINT, WPARAM, LPARAM);

/* Make the class name into a global variable */
char szClassName[ ] = "CodeBlocksWindowsApp";

int x;
int mov;

int WINAPI WinMain (HINSTANCE hThisInstance,
HINSTANCE hPrevInstance,
LPSTR lpszArgument,
int nCmdShow)
{
HWND hwnd; /* This is the handle for our window */
MSG messages; /* Here messages to the application are
saved */
WNDCLASSEX wincl; /* Data structure for the windowclass */

/* The Window structure */
wincl.hInstance = hThisInstance;
wincl.lpszClassName = szClassName;
wincl.lpfnWndProc = WindowProcedure; /* This function is called
by windows */
wincl.style = CS_DBLCLKS; /* Catch double-clicks */
wincl.cbSize = sizeof (WNDCLASSEX);

/* Use default icon and mouse-pointer */
wincl.hIcon = LoadIcon (NULL, IDI_APPLICATION);
wincl.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
wincl.hCursor = LoadCursor (NULL, IDC_ARROW);
wincl.lpszMenuName = NULL; /* No menu */
wincl.cbClsExtra = 0; /* No extra bytes after
the window class */
wincl.cbWndExtra = 0; /* structure or the window
instance */
/* Use Windows's default colour as the background of the window */
wincl.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

/* Register the window class, and if it fails quit the program */
if (!RegisterClassEx (&wincl))
return 0;

/* The class is registered, let's create the program*/
hwnd = CreateWindowEx (
0, /* Extended possibilites for variation */
szClassName, /* Classname */
"Code::Blocks Template Windows App", /* Title Text */
WS_OVERLAPPEDWINDOW, /* default window */
CW_USEDEFAULT, /* Windows decides the position */
CW_USEDEFAULT, /* where the window ends up on the screen
*/
1024, /* The programs width */
768, /* and height in pixels */
HWND_DESKTOP, /* The window is a child-window to
desktop */
NULL, /* No menu */
hThisInstance, /* Program Instance handler */
NULL /* No Window Creation data */
);

/* Make the window visible on the screen */
ShowWindow (hwnd, nCmdShow);

int tick;
int delta = GetTickCount();

x = 100;
mov = 5;

MSG msg; /* Here messages to the application are
saved */
// Clear out the message structure
ZeroMemory(&msg, sizeof(MSG));

while (msg.message != WM_QUIT)
{
// Check to see if any messages are waiting in the queue
if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
// Translate the message and dispatch it to WindowProc()
TranslateMessage(&msg);
DispatchMessage(&msg);
}

InvalidateRect (hwnd, NULL, TRUE);
UpdateWindow (hwnd); // This will call your WM_PAINT response

tick = GetTickCount();
if (tick > (delta + 1000))
{
x += mov;
delta = tick;
}
}

/* The program return-value is 0 - The value that PostQuitMessage()
gave */
return messages.wParam;
}


/* This function is called by the Windows function DispatchMessage() */
LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam,
LPARAM lParam)
{
switch (message) /* handle the messages */
{
case WM_PAINT:
{
HDC hdc;
PAINTSTRUCT ps;
hdc = BeginPaint(hwnd, &ps);

// black out the background
HBRUSH blackBrush = CreateSolidBrush(RGB(0, 0, 0));

RECT rct;
rct.left=0;
rct.right=1024;
rct.top=0;
rct.bottom=768;

FillRect(hdc, &rct, blackBrush);

// Draw a white ellipse
HPEN whitePen = CreatePen(PS_SOLID, 1, RGB(0, 255, 0));
SelectObject(hdc, whitePen);
Ellipse(hdc, (int) x - 10, (int) 100 - 10, (int) x + 10, (int)
100 + 10);

EndPaint(hwnd, &ps);
return 0;
}

case WM_ERASEBKGND:
{
// Prevent the background from being erased prior to calling
WM_PAINT
return 1;
}

case WM_DESTROY:
PostQuitMessage (0); /* send a WM_QUIT to the message
queue */
break;
default: /* for messages that we don't deal
with */
return DefWindowProc (hwnd, message, wParam, lParam);
}

return 0;
}
--- end ---

The application also suffers a lot from "flicker", I know - I've found
some resources on double buffering and, while they help, I left them out
of the example to simplify it.

Any help, ideas and suggestions are really appreciated!!

Thanks,

Ikke

--- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: ScottMcP [MVP] on 11 Mar 2010 09:49
On Mar 11, 8:44 am, Ikke <i...(a)hier.be> wrote:
>         case WM_PAINT:
>         {
>           HDC hdc;
>           PAINTSTRUCT ps;
>           hdc = BeginPaint(hwnd, &ps);
>
>           // black out the background
>           HBRUSH blackBrush = CreateSolidBrush(RGB(0, 0, 0));
>
>           RECT rct;
>           rct.left=0;
>           rct.right=1024;
>           rct.top=0;
>           rct.bottom=768;
>
>           FillRect(hdc, &rct, blackBrush);
>
>           // Draw a white ellipse
>           HPEN whitePen = CreatePen(PS_SOLID, 1, RGB(0, 255, 0));
>           SelectObject(hdc, whitePen);
>           Ellipse(hdc,  (int) x - 10, (int) 100 - 10, (int) x + 10, (int)
> 100 + 10);
>
>           EndPaint(hwnd, &ps);
>           return 0;

You are leaking GDI resources. (Creating objects but never freeing
them.) Create the pen and brush one time instead of every time you
get WM_PAINT.

And you SelectObject with the white pen but you do not restore. Save
the returned handle from SelectObject, then SelectObject again with
the returned handle before EndPaint.
From: Ikke on 11 Mar 2010 17:45
"ScottMcP [MVP]" <scottmcp(a)mvps.org> wrote in news:8ffb1f40-ec95-421b-b08e-
1cd98418bfd2(a)a18g2000yqc.googlegroups.com:

<snip>
> You are leaking GDI resources. (Creating objects but never freeing
> them.) Create the pen and brush one time instead of every time you
> get WM_PAINT.

I see, I foolishly thought it would not matter for testing purposes. After
changing my code (as you said, creating the pen & brush only once) it
worked like a charm!

> And you SelectObject with the white pen but you do not restore. Save
> the returned handle from SelectObject, then SelectObject again with
> the returned handle before EndPaint.

Thank you Scott!

Ikke
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