DESSINER UNE IMAGE TRANSPARENTE (WIN32)

#define _CRT_SECURE_NO_DEPRECATE
#define _WIN32_WINNT 0x0501
#define _WIN32_IE 0x0501
#include <windows.h>

#include "ximage.h"
#include "ximapng.h"

HWND g_hWnd;
char g_szAppName[] = "Transparence";
int alpha = 255;

void SetImage(LPCTSTR fileName, int alpha)
{
  CxImage img;
  img.Load(fileName, CXIMAGE_FORMAT_PNG);
  RECT rcWnd;
  GetWindowRect(g_hWnd, &rcWnd);
  POINT ptWindowScreenPosition = {rcWnd.left, rcWnd.top}, ptSrc = {0, 0};
  SIZE size = {img.GetWidth(), img.GetHeight()};

  HDC dcScreen = GetDC(0);

  // Création d'un dc mémoire de la taille de l'image, et dans lequel on dessine l'image
  HDC hmemdc = CreateCompatibleDC(dcScreen);
  HBITMAP hbmp = CreateCompatibleBitmap(dcScreen, size.cx, size.cy);
  HGDIOBJ oldbmp = SelectObject(hmemdc, hbmp);
  img.Draw(hmemdc);

  BITMAP bmp;
  GetObject(hbmp, sizeof bmp, &bmp);

  BITMAPINFO bi;
  memset(&bi, 0, sizeof bi);
  bi.bmiHeader.biSize = sizeof bi.bmiHeader;
  bi.bmiHeader.biWidth = bmp.bmWidth;
  bi.bmiHeader.biHeight = bmp.bmHeight;
  bi.bmiHeader.biBitCount= bmp.bmBitsPixel;
  bi.bmiHeader.biPlanes = bmp.bmPlanes;
  bi.bmiHeader.biCompression = BI_RGB;

  int nbbytes = bmp.bmBitsPixel * bmp.bmWidth * bmp.bmHeight / 8;
  LPBYTE lpBits = new BYTE[nbbytes];
  // Récupération des bits de l'image
  GetDIBits(hmemdc, hbmp, 0, bmp.bmHeight, lpBits, &bi, DIB_RGB_COLORS);
  // C'est ca que j'ai eu du mal à trouver:
  // ici on gère la transparence des pixels un par un
  LPBYTE pAlpha = img.AlphaGetPointer();
  for(int i = 0; i < nbbytes; i += 4) lpBits[i + 3] = *pAlpha++;

  SetDIBits(hmemdc, hbmp, 0, bmp.bmHeight, lpBits, &bi, DIB_RGB_COLORS);
  delete[] lpBits;
  

  BLENDFUNCTION bf = {AC_SRC_OVER, 0, alpha, AC_SRC_ALPHA};
  UpdateLayeredWindow(g_hWnd, dcScreen, &ptWindowScreenPosition, &size, hmemdc, &ptSrc, 0, &bf, ULW_ALPHA);

  SelectObject(hmemdc, oldbmp);
  DeleteObject(hbmp);
  DeleteDC(hmemdc);
  ReleaseDC(0, dcScreen);
}

LRESULT CALLBACK AppWndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
  switch(uMsg)
  {
  case WM_KEYDOWN:
    switch(wParam)
    {
    case VK_ESCAPE:
    case VK_RETURN:
      DestroyWindow(hWnd);
      break;
    case VK_UP:
      if(alpha > 0) alpha--;
      SetImage("image.png", alpha);
      break;
    case VK_DOWN:
      if(alpha != 255) alpha++;
      SetImage("image.png", alpha);
      break;
    }
    break;
  case WM_NCHITTEST:
    return HTCAPTION;
  case WM_DESTROY:
    PostQuitMessage(0);
    return 0;
  }
  return DefWindowProc(hWnd, uMsg, wParam, lParam);
}


#ifdef _DEBUG
int main()
#else
int APIENTRY WinMain(HINSTANCE, HINSTANCE, LPTSTR, int)
#endif
{
  MSG msg;
  HINSTANCE g_hInst = GetModuleHandle(0);
  WNDCLASSEX wcex;

  memset(&wcex, 0, sizeof wcex);
  wcex.cbSize         = sizeof wcex;
  wcex.lpfnWndProc    = AppWndProc;
  wcex.style          = CS_HREDRAW | CS_VREDRAW;
  wcex.hInstance      = g_hInst;
  wcex.lpszClassName  = g_szAppName;
  wcex.hbrBackground  = GetSysColorBrush(COLOR_WINDOW);
  wcex.hCursor        = LoadCursor(0, IDC_ARROW);
  RegisterClassEx(&wcex);

  g_hWnd = CreateWindowEx(WS_EX_LAYERED, g_szAppName, g_szAppName, WS_OVERLAPPED | WS_SYSMENU,
    0, 0, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, g_hInst, 0);

  SetImage("image.png", alpha);
  ShowWindow(g_hWnd, SW_SHOW);

  while(GetMessage(&msg, NULL, 0, 0))
  {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }
  return (int)msg.wParam;
}