MFC Image Button with transparency - c++

Question

I'm updating an MFC dialog with a number of buttons on it.
At present, the dialog has a Picture control covering the whole dialog providing a patterned background. On top of that, each button is a CBitmapButton using (opaque) images carefully generated to match the area of background they cover.
It would obviously be much easier if the images could be created as mostly transparent, so the background shows through automatically. However, I can't work out how to get MFC to render transparent images correctly in this case.
I understand that I might want a different class to CBitmapButton, or need to write a custom subclass; that's fine, but I don't know where to start. It would be nice to support 32-bit BMP or PNG with alpha channel, but I'd settle for the "specified colour should be transparent" type.

Answer 1

It may not be the best way to do it, but what I'd do is create a custom CButton derived class (assuming that you're actually using the rest of the CButton functionality), then override the DrawItem function to put your custom draw code in.
For the image itself I'd use a Bitmap GDI+ object (which will allow you to load either BMPs or PNGs with alpha channels) then use the regular DrawImage function to draw the bitmap.
If you're going to put PNGs into your resource file then you need to put them in as a "PNG" type. Make sure when you look in the resource file code that the entry looks like
IDB_PNG1 PNG "C:\temp\test.png"
and doesn't try to treat it as a BITMAP resource otherwise you'll have problems loading them.
Edit
Putting my response here so I can post code. Yes, I meant to derive a custom class from CButton, then add a Gdiplus::Bitmap member variable. Here is roughly what you'll need to do to get it to work, though I haven't checked that the code actually compiles and works, but hopefully you'll get the idea. It's not the most efficient way to do it, but if you've not done much custom drawing before then it does have the advantage of being simple!
void CMyButton::LoadImage(const int resourceID)
{
m_pBitmap = Gdiplus::Bitmap::FromResource(NULL, MAKEINTRESOURCE(resourceID));
ASSERT(m_pBitmap);
}
void CMyButton::DrawItem(LPDRAWITEMSTRUCT lpDrawItemStruct)
{
ASSERT(lpDrawItemStruct->CtlType == ODT_BUTTON);
CRect rcClient;
GetClientRect(&rcClient);
if (lpDrawItemStruct->itemState & ODS_SELECTED)
{
// If you want to do anything special when the button is pressed, do it here
// Maybe offset the rect to give the impression of the button being pressed?
rcClient.OffsetRect(1,1);
}
Graphics gr(lpDrawItemStruct->hDC);
gr.DrawImage(m_pBitmap, rcClient.left, rcClient.top);
}