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.

Looks OK....

Is the pixel being set incorrectly by SetPixel() or getting some default colour because it is not set at all?


Try,

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

or

hdc = BeginPaint (hwnd, &ps);

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

for (int i=0; i<1000; i++)

and

Test the return from SetPixel() is the correct colour.
Use GetLastError() if it is not.

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?

use GetLastError() straight away and look at FormatMessage() or look up the error code in the help ('error codes [win32]' in my version of MSVC).

Originally Posted by novacain

use GetLastError() straight away and look at FormatMessage() or look up the error code in the help ('error codes [win32]' in my version of MSVC).

Or, if you're using MSVC, you can use @err in the watch window and it'll translate it for you. Nice undocumented feature.

I am actually writing a paper right now full of MSVC tricks and tips. I'll hopefully have a link for that sometime in the future.