I'm trying to draw an animation on a window with a transparent background using Windows API. The problem is that I can't delete the previous drawing from the window.
I set the following parameters:
InvalidateRect(m_hWnd, &sClientRect, TRUE); // we set the bErase parameter as TRUE
paintParams.dwFlags = BPPF_ERASE; // erase window content while copying backbuffer
paintParams.pBlendFunction = &m_sBlendfunc; // copy source image to backbuffer
But it still doesn't work. You can see the result in the attached image. The animation I wanted is moving the circles across the screen. What I get instead (as shown in the attached image) is artifacts of their motion because the window is not cleared before each draw.
See my full code below:
#include "DrawTest.h"
DrawTest* m_sDrawTest;
DrawTest::DrawTest()
{
m_pAnimation = NULL;
m_sDrawTest = NULL;
m_nFrameindex = 0;
}
DrawTest::~DrawTest(void)
{
if(m_pAnimation)
delete m_pAnimation;
m_pAnimation = NULL;
if(m_sDrawTest)
delete m_sDrawTest;
m_sDrawTest = NULL;
}
int DrawTest::Init(AnimationManager* pAnimationManager,HINSTANCE hInstance,int nCmdShow)
{
//listener
m_sDrawTest = this;
//get anemation (sequence of frames containing location and png's);
m_pAnimation = pAnimationManager->GetAnimation(2);
//set window class information
WNDCLASSEX wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = WndDrawTestProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground = (HBRUSH)GetStockObject(HOLLOW_BRUSH);//configures the window to use transparrent brush
wcex.lpszMenuName = NULL;
wcex.lpszClassName = sWindowClass;
wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
if (!RegisterClassEx(&wcex))
{
MessageBox(NULL,
_T("Call to RegisterClassEx failed!"),
_T("Win32 Guided Tour"),
NULL);
return 1;
}
m_hInst = hInstance; // Store instance handle in our global variable
m_hWnd = CreateWindow(
sWindowClass,
sTitle,
WS_POPUP,
200, 200,
1024, 600,
NULL,
NULL,
hInstance,
NULL
);
if (!m_hWnd)
{
MessageBox(NULL,
_T("Call to CreateWindow failed!"),
_T("Win32 Guided Tour"),
NULL);
return 1;
}
SetWindowPos(m_hWnd, // handle to window
HWND_TOPMOST, // top z
0, // ignored
0, // ignored
0, // ignored
0, // ignored
SWP_NOSIZE | SWP_NOMOVE);
// The parameters to ShowWindow explained:
// hWnd: the value returned from CreateWindow
// nCmdShow: the fourth parameter from WinMain
ShowWindow(m_hWnd,
nCmdShow);
UpdateWindow(m_hWnd);
return 1;
}
// Called by an external timer. This is the application "Next Step" proc.
void DrawTest::TimeStep(){
PostMessage(m_hWnd, WM_PAINT, 0, 0);
}
// WndDrawTestProc replaces the default DefWindowProc
//
// FUNCTION: WndDrawTestProc(HWND, UINT, WPARAM, LPARAM)
//
// PURPOSE: Processes messages for the main window.
//
// WM_PAINT - Paint the main window
// WM_DESTROY - post a quit message and return
//
//
LRESULT CALLBACK WndDrawTestProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_ERASEBKGND:
return DefWindowProc(hWnd, message, wParam, lParam);
case WM_PAINT:
// call onNextFrame to draw current frame.
m_sDrawTest->OnNextFrame(hWnd);
// ensures that the window is in top most position
SetWindowPos(hWnd, // handle to window
HWND_TOPMOST, // top z
0, // ignored
0, // ignored
0, // ignored
0, // ignored
SWP_NOSIZE | SWP_NOMOVE);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
break;
}
return 0;
}
/*
DrawTest::OnNextFrame
Called by WndDrawTestProc when receving WM_PAINT message
Configures the drawing canvas and calles DrawTest::Draw(HDC hBBDC) to do the actual drawing
*/
void DrawTest::OnNextFrame(HWND hUILoopWnd)
{
if(m_nFrameindex > m_pAnimation->GetNumOfFrames() - 1)
return;
// defines paint area
RECT sClientRect;
GetClientRect(hUILoopWnd, &sClientRect);
InvalidateRect(m_hWnd, &sClientRect, TRUE); // we set the bErase parameter as TRUE
//blending structure
m_sBlendfunc.BlendOp= AC_SRC_OVER;
m_sBlendfunc.BlendFlags = 0;
m_sBlendfunc.SourceConstantAlpha = 255;
m_sBlendfunc.AlphaFormat = AC_SRC_ALPHA;
HDC hdc;
PAINTSTRUCT ps;
hdc = BeginPaint(hUILoopWnd, &ps);
GetClientRect(hUILoopWnd, &sClientRect);
BP_PAINTPARAMS paintParams = { sizeof(BP_PAINTPARAMS) };
paintParams.dwFlags = BPPF_ERASE; // erase window content while copying backbuffer
paintParams.pBlendFunction = &m_sBlendfunc; // how to copy source image to backbuffer
HDC hBBDC;
HPAINTBUFFER hPBuffer;
paintParams.cbSize = sizeof(paintParams);
hPBuffer = BeginBufferedPaint(hdc, &sClientRect, BPBF_COMPATIBLEBITMAP, &paintParams, &hBBDC);
//draw animation
Draw(hBBDC);
m_nFrameindex ++;
EndBufferedPaint(hPBuffer, TRUE);
EndPaint(hUILoopWnd, &ps);
}
/*
Draw
Paint the animation frame on the backbuffer
*/
void DrawTest::Draw(HDC hBBDC)
{
HDC hdcScreen = GetDC(NULL);
HDC hdcMem = CreateCompatibleDC(hdcScreen);
bool test = false;
HGDIOBJ hbmpOld = SelectObject(hdcMem, m_pAnimation->m_pFramesArray[m_nFrameindex]->hBmp);
HBITMAP ptemp = CreateCompatibleBitmap(hdcMem,m_pAnimation->m_pFramesArray[m_nFrameindex]->nWidth,
m_pAnimation->m_pFramesArray[m_nFrameindex]->nHeight);
DeleteObject(ptemp);
test = AlphaBlend(hBBDC,m_pAnimation->m_pFramesArray[m_nFrameindex]->nPtX,m_pAnimation->m_pFramesArray[m_nFrameindex]->nPtY
,m_pAnimation->m_pFramesArray[m_nFrameindex]->nWidth, m_pAnimation->m_pFramesArray[m_nFrameindex]->nHeight,
hdcMem,0,0,m_pAnimation->m_pFramesArray[m_nFrameindex]->nWidth,m_pAnimation->m_pFramesArray[m_nFrameindex]->nHeight,m_sBlendfunc);
DWORD test10 = GetLastError();
SelectObject(hdcMem, hbmpOld); //placing the old object back
test = DeleteDC(hdcMem); // after CreateCompatibleDC
test = ReleaseDC(NULL, hdcScreen); // after GetDC
}
This is the result after 5 frames:
