[lazarus] Bitmap transparency?

Mattias Gaertner nc-gaertnma at netcologne.de
Wed Nov 12 14:08:11 EST 2003

On Wed, 12 Nov 2003 09:23:06 +0100
Micha Nelissen <M.Nelissen at student.tue.nl> wrote:

> Mattias Gaertner wrote:
> > On Mon, 10 Nov 2003 08:07:09 +0100
> > Micha Nelissen <M.Nelissen at student.tue.nl> wrote:
> > 
> > 
> > 
> >>I think I can fill the RawImageDesc fields except palette, and I don't 
> >>know what to do with RedPrec, RedShift, etc..
> > 
> > 
> > You need to find out the color format of the device/bitmap for this.
> Sorry I keep nagging about this subject, but:
> 1) there is one function not needing a DC to create a bitmap, 
> 'CreateBitmap'. It is not possible to retrieve color format of this 
> bitmap. Even a google search did not turn up anything on the color 
> format subject. I guess, nobody uses it.
> 2) the functions that do have information on color format also need a 
> DC. You create a device independant bitmap with CreateDIBSection, and a 
> device dependant one with CreateDIBitmap. Long live the winapi, logic is 
> nowhere to be found it seems! The names are very unlogical.

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?

> 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?

> 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?


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