How do I use IImgCtx to get load an image with an alpha channel?
- by fret
I have some working code that uses IImgCtx to load images, but I can't work out how to get at the alpha channel. For images like .gif's and .png's there are transparent pixels, but using anything other than a 24-bit bitmap as a drawing surface doesn't work.
For reference on the interface:
http://www.codeproject.com/KB/graphics/JianImgCtxDecoder.aspx
My code looks like this:
IImgCtx *Ctx = 0;
HRESULT hr = CoCreateInstance(CLSID_IImgCtx, NULL, CLSCTX_INPROC_SERVER, IID_IImgCtx, (LPVOID*)&Ctx);
if (SUCCEEDED(hr))
{
GVariant Fn = Name;
hr = Ctx->Load(Fn.WStr(), 0);
if (SUCCEEDED(hr))
{
SIZE Size = { -1, -1 };
ULONG State = 0;
while (true)
{
hr = Ctx->GetStateInfo(&State, &Size, false);
if (SUCCEEDED(hr))
{
if ((State & IMGLOAD_COMPLETE) || (State & IMGLOAD_STOPPED) || (State & IMGLOAD_ERROR))
{
break;
}
else
{
LgiSleep(1);
}
}
else break;
}
if (Size.cx > 0 &&
Size.cy > 0 &&
pDC.Reset(new GMemDC))
{
if (pDC->Create(Size.cx, Size.cy, 32))
{
HDC hDC = pDC->StartDC();
if (hDC)
{
RECT rc = { 0, 0, pDC->X(), pDC->Y() };
Ctx->Draw(hDC, &rc);
pDC->EndDC();
}
}
else pDC.Reset();
}
}
Ctx->Release();
Where "StartDC" basically wraps CreateCompatibleDC(NULL) and "EndDC" wraps DeleteDC, with appropriate SelectObjects for the HBITMAPS etc. And pDC-Create(x, y, bit_depth) calls CreateDIBSection(...DIB_RGB_COLORS...). So it works if I create a 24 bits/pixel bitmap but has no alpha to speak of, and it leaves the 32 bits/pixel bitmap blank. Now this interface apparently is used by Internet Explorer to load images, and obviously THAT supports transparency, so I believe that it's possible to get some level of alpha out of the interface. The question is how?
(I also have fall back code that will call libpng/libjpeg/my .gif loader etc)
