create transparent image from png, winapi - c++

Question

I created a simple form to test in winapi: I uploaded here: http://pastebin.com/7dNjE1Tb
I would like to put a simple png file to my hwnd, for example this picture: http://www.ledavi-network.com/includes/images/bg_shadow_png.png
I know I should use this: http://msdn.microsoft.com/en-us/library/windows/desktop/dd145141(v=vs.85).aspx but I am very new in winapi and I dont find any example how to use this TransparentBlt function.
Someone could help me to create a very simple example?
EDIT:
But what I would like to do?
I would like to create a button which has special background with shadow. So If anyone know an easier way to do this, I will forget to use the TransparentBlt. So any else solution?

Answer 1

Here, this will do the job. You'll need to ensure you have gdiplus and msimg32 libraries linked for the AlphaBlend (TransparentBlt assumes that a single rgb color in the image will be treated as transparent. I.e it's just 1-bit transparency when using TransparentBlt - fully opaque or fully transparent.)
You'll (presumably) wish to use AlphaBlend for this job.
Note - I haven't bothered to add an WM_ERASEBKGND to the WndProc function. As a consequence, each time the image is re-drawn, it's drawn straight over the top of what's already there (try resizing the window). Just do a FillRect in erasebkg, and you'll be fine.
EDIT: Code updated to use displayImage with a NULL hbitmap to do the background too.
Here 'go:
#define WIN32_LEAN_AND_MEAN
#define WINVER 0x0600 // needed for alphablend function..
#include <windows.h>
#include <gdiplus.h>
const char g_szClassName[] = "myWindowClass";
// BMP, GIF, JPEG, PNG, TIFF, Exif, WMF, and EMF
// requires GDIPlus
HBITMAP mLoadImg(WCHAR *szFilename)
{
HBITMAP result=NULL;
Gdiplus::Bitmap* bitmap = new Gdiplus::Bitmap(szFilename,false);
bitmap->GetHBITMAP(NULL, &result);
delete bitmap;
return result;
}
//void CStaticImg::displayImage(HBITMAP mBmp, HWND mHwnd)
void displayImage(HBITMAP mBmp, HWND mHwnd)
{
RECT myRect;
BITMAP bm;
HDC screenDC, memDC;
HBITMAP oldBmp;
BLENDFUNCTION bf;
GetObject(mBmp, sizeof(bm), &bm);
bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 0xff;
bf.AlphaFormat = AC_SRC_ALPHA;
screenDC = GetDC(mHwnd);
GetClientRect(mHwnd, &myRect);
if (mBmp == NULL)
FillRect(screenDC, &myRect, WHITE_BRUSH);
else
{
memDC = CreateCompatibleDC(screenDC);
oldBmp = (HBITMAP)SelectObject(memDC, mBmp);
AlphaBlend (screenDC, 0, 0, myRect.right,myRect.bottom, memDC, 0, 0, bm.bmWidth,bm.bmHeight, bf);
SelectObject(memDC, oldBmp);
DeleteDC(memDC);
ReleaseDC(mHwnd, screenDC);
}
}
// Step 4: the Window Procedure
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
static HBITMAP myImage;
switch(msg)
{
case WM_CLOSE:
DestroyWindow(hwnd);
break;
case WM_ERASEBKGND:
displayImage(NULL, hwnd);
break;
case WM_CREATE:
myImage = mLoadImg(L"bg_shadow_png.png");
break;
case WM_PAINT:
//displayImage(HBITMAP mBmp, HWND mHwnd);
displayImage(myImage, hwnd);
ValidateRect(hwnd, NULL);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd, msg, wParam, lParam);
}
return 0;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow)
{
WNDCLASSEX wc;
HWND hwnd;
MSG Msg;
static Gdiplus::GdiplusStartupInput gdiplusStartupInput;
static ULONG_PTR gdiplusToken;
// so we can load all the image formats that windows supports natively - (I'm using a transparent PNG on the main dialog)
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
//Step 1: Registering the Window Class
wc.cbSize = sizeof(WNDCLASSEX);
wc.style = 0;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInstance;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wc.lpszMenuName = NULL;
wc.lpszClassName = g_szClassName;
wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
if(!RegisterClassEx(&wc))
{
MessageBox(NULL, "Window Registration Failed!", "Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
// Step 2: Creating the Window
hwnd = CreateWindowEx(
WS_EX_CLIENTEDGE,
g_szClassName,
"The title of my window",
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT, 240, 120,
NULL, NULL, hInstance, NULL);
if(hwnd == NULL)
{
MessageBox(NULL, "Window Creation Failed!", "Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
ShowWindow(hwnd, nCmdShow);
UpdateWindow(hwnd);
// Step 3: The Message Loop
while(GetMessage(&Msg, NULL, 0, 0) > 0)
{
TranslateMessage(&Msg);
DispatchMessage(&Msg);
}
Gdiplus::GdiplusShutdown(gdiplusToken);
return Msg.wParam;
}