gdi32: Remove call to CreateCompatibleDC() from GetDIBits() implementation (try 2) (RESEND)

Alex Villací­s Lasso a_villacis at
Mon Nov 6 16:14:02 CST 2006

Resending on the assumption that patch might have been lost.

The problem is that in the wine implementation, the palette is *not* 
stored anywhere inside the returned HBITMAP from CreateDIBSection(). 
Instead, the HDC used by CreateDIBSection must implement the 
pCreateDIBSection (this already leads to loss of palette information if 
it doesn't, for example in enhmfdrv), and then pCreateDIBSection is 
called, and somehow associates the HBITMAP with the actual 
implementation (in winex11drv, an XContext is used to save the pointer 
to a X_PHYSBITMAP, indexed by the HBITMAP). The method of getting to the 
palette information given an arbitrary DIB HBITMAP is private to the 
driver, and the functions contained in the BITMAPOBJ structure are not 
enough - a PHYSDEV is also required, usually provided by the DC. Million 
dollar question: what happens in the current implementation if a DIB 
created with a reference DC that saves the palette with method A is used 
with GetDIBits() and a reference HDC that expects a PHYSDEV with a 
different method B of saving the palette? Also, how do we get the 
palette information from GetDIBits() if the reference HDC does not 
support pGetDIBColorTable()?

Given that, I thought of a few solutions to get the palette information 
from a DIB without CreateCompatibleDC():
- Select the HBITMAP into the reference DC inside the implementation of 
GetDIBits(), then call GetDIBColorTable() on the reference DC. Although 
the HBITMAP should not be selected into the reference DC before calling 
GetDIBits(), MSDN says nothing about GetDIBits() itself selecting the 
bitmap into the reference DC.
- Add a new field to BITMAPOBJ with the PHYSDEV from the reference DC. 
Then, the corresponding function pGetDIBColorTable could be called with 
the proper parameters without the need of a new DC. Palette information 
is still lost if reference DC of CreateDIBSection does not support 
- Add a new field to BITMAPOBJ for the sole purpose of storing a copy of 
the DIB palette when calling CreateDIBSection(), and use it for 
GetDIBits(). Requires synchronization when calling SetDIBColorTable(), 
and possibly in other places, and also a rewrite of GetDIBColorTable(). 
However, this has the advantage that no palette information will be lost 
when creating a DIB section with a reference DC that does not implement 
pCreateDIBsection. The best method IMHO.

Here is a patch that implements option 3 described above.

* Store a copy of the DIB palette when creating a dibsection, and use it 
to remove the need to use CreateCompatibleDC() in GetDIBits() function.

The following cryptic message was allegedly found in the inner edge of a Windows
XP installation CD:


It is rumored that only a true Unix Wizard can decypher this mysterious message,
which supposedly encodes the true nature and purpose of the software.

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