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Stuck on an exercise in DirectXTutorial.com

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I've been working through the DirectX Tutorial website and came upon Exercise 2 in Lesson 4 of Direct3D basics, which asks you to make the shape of a triangle change during runtime. I have some code worked out that is supposed to make one of the vertices of the triangle move up and down repeatedly, but the program just displays a blank screen. Here is the code:
// include the basic windows header files and the Direct3D header file
#include <windows.h>
#include <windowsx.h>
#include <d3d9.h>

// define the screen resolution and keyboard macros
#define SCREEN_WIDTH 1024
#define SCREEN_HEIGHT 768
#define KEY_DOWN( vk_code ) ( ( GetAsyncKeyState( vk_code ) & 0x8000 ) ? 1 : 0 )
#define KEY_UP( vk_code ) ( ( GetAsyncKeyState( vk_code ) & 0x8000 ) ? 0 : 1 )

// include the Direct3D Library file
#pragma comment ( lib, "d3d9.lib" )

// global declarations
LPDIRECT3D9 d3d;    // the pointer to our Direct3D interface
LPDIRECT3DDEVICE9 d3ddev;    // the pointer to the device class
LPDIRECT3DVERTEXBUFFER9 t_buffer = NULL;    // the pointer to the vertex buffer

// function prototypes
void initD3D( HWND hWnd );    // sets up and initializes Direct3D
void render_frame( void );    // renders a single frame
void cleanD3D( void );    // closes Direct3D and releases memory
void init_graphics( void );    // 3D declarations

struct CUSTOMVERTEX { FLOAT X, Y, Z, RHW; DWORD COLOR; };
#define CUSTOMFVF ( D3DFVF_XYZRHW | D3DFVF_DIFFUSE )

void graphics_loop( CUSTOMVERTEX verts[3], short &count, short &add_to_vert )
{
	verts[0].X += add_to_vert;
	verts[0].Y += add_to_vert;

	if( count == 50 )
	{
		add_to_vert *= -1;
		count = 0;
	}

	// create a vertex buffer interface called t_buffer
    d3ddev->CreateVertexBuffer( 3*sizeof( CUSTOMVERTEX ),
                               0,
                               CUSTOMFVF,
                               D3DPOOL_MANAGED,
                               &t_buffer,
                               NULL );

    VOID* pVoid;    // a void pointer

    // lock t_buffer and load the vertices into it
    t_buffer->Lock( 0, 0, ( void** )&pVoid, 0 );
    memcpy( pVoid, verts, sizeof( verts ) );
    t_buffer->Unlock();

	return;
}

// the WindowProc function prototype
LRESULT CALLBACK WindowProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam );


// the entry point for any Windows program
int WINAPI WinMain( HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow )
{
    HWND hWnd;
    WNDCLASSEX wc;

    ZeroMemory( &wc, sizeof( WNDCLASSEX ) );

    wc.cbSize = sizeof( WNDCLASSEX );
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = ( WNDPROC )WindowProc;
    wc.hInstance = hInstance;
    wc.hCursor = LoadCursor( NULL, IDC_ARROW );
    // wc.hbrBackground = ( HBRUSH )COLOR_WINDOW;    // not needed any more
    wc.lpszClassName = L"WindowClass";

    RegisterClassEx( &wc );

    hWnd = CreateWindowEx( NULL,
                          L"WindowClass",
                          L"Our Direct3D Program",
                          WS_EX_TOPMOST | WS_POPUP,    // fullscreen values
                          0, 0,    // the starting x and y positions should be 0
                          SCREEN_WIDTH, SCREEN_HEIGHT,    // set the window to 640 x 480
                          NULL,
                          NULL,
                          hInstance,
                          NULL );

    ShowWindow( hWnd, nCmdShow );

    // set up and initialize Direct3D
    initD3D( hWnd );

	float X = 320.0, Y = 50.0;
	short add_to_vert = 1, count = 0;

	CUSTOMVERTEX t_vert[] =
	{
		{ X, Y, 0.5f, 1.0f, D3DCOLOR_XRGB( 0, 0, 255 ), },
		{ 520.0f, 400.0f, 0.5f, 1.0f, D3DCOLOR_XRGB( 0, 255, 0 ), },
		{ 120.0f, 400.0f, 0.5f, 1.0f, D3DCOLOR_XRGB( 255, 0, 0 ), },
	};

    // enter the main loop:

    MSG msg;

    while( TRUE )
    {
        DWORD starting_point = GetTickCount();

        if ( PeekMessage( &msg, NULL, 0, 0, PM_REMOVE ) )
        {
            if ( msg.message == WM_QUIT )
                break;

            TranslateMessage( &msg );
            DispatchMessage( &msg );
        }

		graphics_loop( t_vert, count, add_to_vert );
        render_frame();

        // check the 'escape' key
        if( KEY_DOWN( VK_ESCAPE ) )
            PostMessage( hWnd, WM_DESTROY, 0, 0 );

        while ( ( GetTickCount() - starting_point ) < 25 );
    }

    // clean up DirectX and COM
    cleanD3D();

    return msg.wParam;
}


// this is the main message handler for the program
LRESULT CALLBACK WindowProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam )
{
    switch( message )
    {
        case WM_DESTROY:
            {
                PostQuitMessage( 0 );
                return 0;
            } break;
    }

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


// this function initializes and prepares Direct3D for use
void initD3D( HWND hWnd )
{
    d3d = Direct3DCreate9( D3D_SDK_VERSION );

    D3DPRESENT_PARAMETERS d3dpp;

    ZeroMemory( &d3dpp, sizeof( d3dpp ) );
    d3dpp.Windowed = FALSE;
    d3dpp.SwapEffect = D3DSWAPEFFECT_DISCARD;
    d3dpp.hDeviceWindow = hWnd;
    d3dpp.BackBufferFormat = D3DFMT_X8R8G8B8;
    d3dpp.BackBufferWidth = SCREEN_WIDTH;
    d3dpp.BackBufferHeight = SCREEN_HEIGHT;

    // create a device class using this information and the info from the d3dpp stuct
    d3d->CreateDevice( D3DADAPTER_DEFAULT,
                      D3DDEVTYPE_HAL,
                      hWnd,
                      D3DCREATE_SOFTWARE_VERTEXPROCESSING,
                      &d3dpp,
                      &d3ddev );

	init_graphics();

    return;
}


// this is the function used to render a single frame
void render_frame( void )
{
    d3ddev->Clear( 0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB( 0, 0, 0 ), 1.0f, 0 );

    d3ddev->BeginScene();

        // select which vertex format we are using
        d3ddev->SetFVF( CUSTOMFVF );

        // select the vertex buffer to display
        d3ddev->SetStreamSource( 0, t_buffer, 0, sizeof( CUSTOMVERTEX ) );

        // copy the vertex buffer to the back buffer
        d3ddev->DrawPrimitive( D3DPT_TRIANGLELIST, 0, 1 );

    d3ddev->EndScene();

    d3ddev->Present( NULL, NULL, NULL, NULL );

    return;
}


// this is the function that cleans up Direct3D and COM
void cleanD3D( void )
{
    t_buffer->Release();    // close and release the vertex buffer
    d3ddev->Release();    // close and release the 3D device
    d3d->Release();    // close and release Direct3D

    return;
}

void init_graphics()
{
	CUSTOMVERTEX t_vert[] =
	{
		{ 320.0f, 50.0f, 0.5f, 1.0f, D3DCOLOR_XRGB( 0, 0, 255 ), },
		{ 520.0f, 400.0f, 0.5f, 1.0f, D3DCOLOR_XRGB( 0, 255, 0 ), },
		{ 120.0f, 400.0f, 0.5f, 1.0f, D3DCOLOR_XRGB( 255, 0, 0 ), },
	};

	// create a vertex buffer interface called t_buffer
    d3ddev->CreateVertexBuffer( 3*sizeof( CUSTOMVERTEX ),
                               0,
                               CUSTOMFVF,
                               D3DPOOL_MANAGED,
                               &t_buffer,
                               NULL );

    VOID* pVoid;    // a void pointer

    // lock t_buffer and load the vertices into it
    t_buffer->Lock( 0, 0, ( void** )&pVoid, 0 );
    memcpy( pVoid, t_vert, sizeof( t_vert ) );
    t_buffer->Unlock();

	return;
}
Can anybody tell me what I'm doing wrong here?

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You are checking "if( count == 50 )", but I can't see the place where you increase this value. Maybe triangle just flew out of screen?

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Oops. I forgot about that. However, even when I increment the value of 'count' on each graphics_loop function call, I still get a blank screen.

I just added the line:
      count++;


directly after these lines:
	verts[0].X += add_to_vert;
verts[0].Y += add_to_vert;

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I solved my problem! Apparently the program didn't like that I was recreating a new vertex buffer over and over in the graphics_loop function, so I simply removed CreateVertexBuffer from that function. Everything works great now.

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Quote:
Original post by Barrow Boy
I solved my problem! Apparently the program didn't like that I was recreating a new vertex buffer over and over in the graphics_loop function, so I simply removed CreateVertexBuffer from that function. Everything works great now.


I would imagine just initializing the vertex buffer would be okay. But then im used to OpenGL which is based on using states.

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