[lazarus] Bitmap transparency?

Mattias Gaertner nc-gaertnma at netcologne.de
Wed Nov 12 14:37:08 EST 2003

On Wed, 12 Nov 2003 20:31:08 +0100
Micha Nelissen <M.Nelissen at student.tue.nl> wrote:

> > Ok. So, under win32 it seems, it is up to the user to keep book of the
> > format of a DC/bitmap. But this data is win32 specific, so the LCL user
> > can't do it. It must be done, by the win32 intf. Right?
> Let me first say: the color format I mean is the RRGGBBAA and how bits 
> each take.
> Yes. But the winapi can convert between color formats automatically. You 
> give the win32 interface a color format, and it can pass it on to the 
> winapi for processing.

Yes. But every conversion reduces the quality, so we should try to find the
current format.

> >>3) there is no function to retrieve the color format of a device. One 
> >>can only guess at it, based on BITMAPINFO structure help. That's because
> >
> >>the winapi support converting the color formats on the fly. For example,
> >
> >>using CreateDIBitmap, you specify the color format your source bitmap is
> >
> >>in, and the winapi will convert it to the color format of the target 
> >>device. It seems logical that one would be able to retrieve the color 
> >>format of the device for fast rendering and stuff, but it just seems 
> >>impossible.
> > 
> > 
> > Hardly believable. 
> > Can this BITMAPINFO only retrieved from bitmaps or can you create a
> > bitmap fom any device and get a BTMAPINFO? And with this, get the color
> > format from any DC?
> The BITMAPINFO is contained with a DIBSECTION. The DIBSECTION can only 
> be retrieved for device independant bitmaps, that means, bitmaps created 
> by CreateDIBSection. No other function returns a BITMAPINFO structure.
> The BITMAP structure can always be retrieved from any bitmap. But it 
> doesn't contain the information to fill in {red,blue,green}{shift,prec}.

AFAIK the INFOHEADER does. See for example

> Creating a DIB section isn't of much use, if you ask me, because that's 
> already what the rawimage is.
> >>I think that's the reason why in delphi there is a pfDevice 
> >>for TPixelFormat.
> >>
> >>Any tips? Shall I return a default color format in the raw image?
> > 
> > 
> > Yes. I guess, at least the screen resolution can be retrieved, can't it?
> I am able to get the color depth and bits per pixel used, eg. 16, 24, 32 
> bits.

I guess, win32 also defines, what bits are the red, green and blue?


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