case WM_CREATE: //LPARAM = input
		input = (PredatorPreyInput*)(((CREATESTRUCT*)lParam)->lpCreateParams);
		points = Interpolate_Linear(*input, ComputePredatorPreyPoints(input));

		hdcMem = CreateCompatibleDC(0);
		hBitmap = CreateCompatibleBitmap(0, input->iLenght, input->iHeight);

		//draw the graph:
		for(int i=0;i<(input->iLenght);i++)
		{
			min = (int)min(points->Predator, points->Predator);
			color = MixColor(input->PredatorColor, input->PreyColor);

			hPen = CreatePen(PS_SOLID, 1, color);
			SelectObject(hdcMem, hPen);
			MoveToEx(hdcMem, i, 0, NULL);
			LineTo(hdcMem, i, min);
			if(points->Predator > points->Prey)
			{
				hPen = CreatePen(PS_SOLID, 1, input->PredatorColor);
				DeleteObject(SelectObject(hdcMem, hPen));
				MoveToEx(hdcMem, i, min, NULL);
				LineTo(hdcMem, i, (int)points->Predator);
			}
			if(points->Prey > points->Predator)
			{
				hPen = CreatePen(PS_SOLID, 1, input->PreyColor);
				DeleteObject(SelectObject(hdcMem, hPen));
				MoveToEx(hdcMem, i, min, NULL);
				LineTo(hdcMem, i, (int)points->Prey);
			}
		}
		SetWindowLong(hwnd, GWL_USERDATA, (LONG)hdcMem);
		break;

case WM_PAINT:
		hdc = BeginPaint(hwnd, &ps);
		GetClientRect(hwnd, &rect);
 		BitBlt(hdc, rect.left, rect.top, rect.right, rect.bottom, (HDC)GetWindowLong(hwnd, GWL_USERDATA), 0, 0, SRCCOPY);
		EndPaint(hwnd, &ps);
		break;

(int)points->Prey und (int)points->Predator stimmen (mit debugger ausgelesen).
Leider komme ich im moment nichmehr weiter deshalb wollte ich euch fragen ob ihr einen fehler seht