Thread: Pixel Trouble

Hi all.

I'm currently writing an implementation of Conways game of life. I've written the engine code (at least a first draft of it) and now I'm learning windows so I can test the engine.

At the moment I have a program to draw animated random pixels on the screen using SetPixel. (Once I understand this then I'll plug the life engine into it.) This works o.k. - But sometimes it draws GREEN pixels (correct) and sometimes it draws GREY pixels (wrong). I've tried changing the code to make white pixels - but they still turn grey.

When I move the window around the screen, the colour changes seemingly at random. I can't understand this at all.

I'm using LCC-win32, Win98SE, and Petzold's "Programming Windows".

Here's the source.

Code:

// Test for plotting pixels.

#include <windows.h>
#include <math.h>
#include <stdlib.h>

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
{
	static TCHAR szAppName[] = TEXT ("Pixels");
	HWND hwnd;
	MSG	msg;
	WNDCLASS wndclass;

	wndclass.style = CS_HREDRAW | CS_VREDRAW;
	wndclass.lpfnWndProc = WndProc;
	wndclass.cbClsExtra =0;
	wndclass.cbWndExtra = 0;
	wndclass.hInstance = hInstance;
	wndclass.hIcon = LoadIcon (NULL, IDI_APPLICATION);
	wndclass.hCursor = LoadCursor (NULL, IDC_ARROW);
	wndclass.hbrBackground = (HBRUSH) GetStockObject (BLACK_BRUSH);
	wndclass.lpszMenuName = NULL;
	wndclass.lpszClassName = szAppName;

	if (!RegisterClass (&wndclass))
	{
		MessageBox (NULL, TEXT ("Program requires Windows NT!"), szAppName, MB_ICONERROR);

		return 0;
	}

	hwnd = CreateWindow (szAppName, TEXT ("Random Pixels"),
		WS_OVERLAPPEDWINDOW,
		CW_USEDEFAULT, CW_USEDEFAULT,
		CW_USEDEFAULT, CW_USEDEFAULT,
		NULL, NULL, hInstance, NULL);

	ShowWindow (hwnd, iCmdShow);
	UpdateWindow (hwnd);



	while (GetMessage (&msg, NULL, 0, 0))
	{
		TranslateMessage (&msg);
		DispatchMessage (&msg);
	}
	return msg.wParam;
}


LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{

	int i,a,b;

	static int cxClient, cyClient;
	HDC hdc;
	PAINTSTRUCT ps;

	switch (message)
	{
		case WM_CREATE:
			SetTimer (hwnd, 1, 100, NULL);
			return 0;

		case WM_SIZE:
			cxClient = LOWORD (lParam);
			cyClient = HIWORD (lParam);
			return 0;

		case WM_TIMER:
			InvalidateRect (hwnd, NULL, TRUE);
			return 0;

		case WM_PAINT:

			hdc = BeginPaint (hwnd, &ps);

			for (int i=0; i<1000; i++)
			{
				a = rand()%124;
				b = rand()%124;

				SetPixel (hdc, a, b, 0x0000FF00);
			}

			EndPaint (hwnd, &ps);

			return 0;

		case WM_DESTROY:
			PostQuitMessage (0);
			return 0;

	}

	return DefWindowProc (hwnd, message, wParam, lParam);
}

Follow-up:

The above test was done in 16bit colour, if I set the display to 32bit the problem goes away. In 8 bit it gets worse (sometimes pixels are green (correct) sometimes white or grey.

Thanks for the reply Novacain.

Try,

InvalidateRect (hwnd, NULL, FALSE);//don't erase background

I tried this (In fact it was a bug in the original version !) but the same effect happens.

hdc = BeginPaint (hwnd, &ps);

//clear the draw area
FillRect(ps.hdc ,&ps.rcRect ,GetStockObject( WHITE_BRUSH ) );

My compiler flags an error at &ps.rcRect - it doesn't know what to do with rcRect - should I define this myself somewhere?

I think you meant &ps.rcPaint - this works but the bug still remains.
What do I do with the result of GetLastError?