Convert ARGB8888 bitmap to Premultipled ARGB8888 bitmap (expected by GdiAlphaBlt)

gdi_argb8888_to_pargb8888.c

#define WIN32_LEAN_AND_MEAN
#define WINVER 0x0601
#define _WIN32_WINNT 0x0601
#include <windows.h>

HBITMAP argb8888_to_pargb8888(HBITMAP hbm, HDC hdc)
{
    const BLENDFUNCTION blendfn = { AC_SRC_OVER, 0, 0xff, AC_SRC_ALPHA };
    HDC hdcSrc, hdcTmp1, hdcTmp2;
    HBITMAP hbmRes = NULL, hbmTmp1, hbmTmp2;
    HBRUSH hbrwhite;
    BITMAP bitmap;
    INT width, height;

    if (GetObjectType(hbm) != OBJ_BITMAP) goto done;
    if (!GetObject(hbm, sizeof(BITMAP), &bitmap)) goto done;
    width = bitmap.bmWidth;
    height = bitmap.bmHeight;

    if (!(hbrwhite = GetStockObject(WHITE_BRUSH))) goto done;
    if (!(hdcSrc = CreateCompatibleDC(hdc))) goto done;
    if (!SelectObject(hdcSrc, hbm)) goto free_hdcSrc;
    if (!(hbmTmp1 = CreateCompatibleBitmap(hdcSrc, width, height))) goto free_hdcSrc;
    if (!(hbmTmp2 = CreateCompatibleBitmap(hdcSrc, width, height))) goto free_hbmTmp1;
    if (!(hdcTmp1 = CreateCompatibleDC(hdcSrc))) goto free_hbmTmp2;
    if (!SelectObject(hdcTmp1, hbmTmp1)) goto free_hdcTmp1;
    if (!SelectObject(hdcTmp1, hbrwhite)) goto free_hdcTmp1;
    if (!(hdcTmp2 = CreateCompatibleDC(hdcSrc))) goto free_hdcTmp1;
    if (!SelectObject(hdcTmp2, hbmTmp2)) goto free_hdcTmp2;
    if (!SelectObject(hdcTmp2, hbrwhite)) goto free_hdcTmp2;
    if (!BitBlt(hdcTmp1, 0, 0, width, height, hdcSrc, 0, 0, MERGECOPY /* PSa */)) goto free_hdcTmp2;
    if (!BitBlt(hdcTmp2, 0, 0, width, height, hdcSrc, 0, 0, 0x000300AA /* PSon */)) goto free_hdcTmp2;
    if (!GdiAlphaBlend(hdcTmp1, 0, 0, width, height, hdcTmp2, 0, 0, width, height, blendfn)) goto free_hdcTmp2;
    if (!PatBlt(hdcTmp1, 0, 0, width, height, 0x00A50065 /* PDxn */)) goto free_hdcTmp2;

    hbmRes = hbmTmp1; hbmTmp1 = NULL;

free_hdcTmp2:
    DeleteDC(hdcTmp2);
free_hdcTmp1:
    DeleteDC(hdcTmp1);
free_hbmTmp2:
    if (hbmTmp2) DeleteObject(hbmTmp2);
free_hbmTmp1:
    if (hbmTmp1) DeleteObject(hbmTmp1);
free_hdcSrc:
    DeleteDC(hdcSrc);
done:
    return hbmRes;
}