[lazarus] Bitmap transparency?

Micha Nelissen M.Nelissen at student.tue.nl
Wed Nov 12 14:22:33 EST 2003

Mattias Gaertner wrote:

> 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?

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.

>>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 
> 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}.

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 



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