CreateBitmap() & CreateCompatibleDC()

[Belloc]

The CreateBitmap definition in MSDN says that :

"After a bitmap is created, it can be selected into a device context by
calling the SelectObject() function. However, the bitmap can only be selected
into a device context if the bitmap and the DC have the same format".


The CreateCompatibleDC definition in MSDB say that :

"A memory DC exists only in memory. When the memory DC is created, its
display surface is exactly one monochrome pixel wide and one monochrome pixel
high".


If these two statements are correct, it would be impossible to select a
bitmap different than a monochrome bitmap into a memory DC, which clearly is
not the case, since that's the usual procedure to expand a memory DC, before
painting on it.


I've done some tests, selecting bitmaps obtained with CreateBitmap() and
with different color depths (bpp). The 1 and 32 bpp bitmaps are selected
normally by the function SelectObject(). For the other cases (4, 8, 16, 24)
the function fails.


Given the above contradictions, I have some conceptual questions :

. Which of the first two statements above is true ?

. Why does (4) occurs ?

. Given that a memory DC returned by CreateCompatibleDC(hDC) is different
from hDC, what is the meaning of the term Compatible in the function's name
? At least one would expect this funtion to return a memory DC with the same
color organization as hDC.

. A memory DC, does it really have a default bitmap, or is it just a
placeholder ?

Microsoft Visual C++ .NET 69586-335-0000007-18101

[Mark Salsbery [MVP]]

Post by Belloc
"After a bitmap is created, it can be selected into a device context by
calling the SelectObject() function. However, the bitmap can only be selected
into a device context if the bitmap and the DC have the same format".
"A memory DC exists only in memory. When the memory DC is created, its
display surface is exactly one monochrome pixel wide and one monochrome pixel
high".
If these two statements are correct, it would be impossible to select a
bitmap different than a monochrome bitmap into a memory DC, which clearly is
not the case, since that's the usual procedure to expand a memory DC, before
painting on it.
I've done some tests, selecting bitmaps obtained with CreateBitmap() and
with different color depths (bpp). The 1 and 32 bpp bitmaps are selected
normally by the function SelectObject(). For the other cases (4, 8, 16, 24)
the function fails.
. Which of the first two statements above is true ?

...

Both :)

Post by Belloc
. Why does (4) occurs ?
. Given that a memory DC returned by CreateCompatibleDC(hDC) is different
from hDC, what is the meaning of the term Compatible in the function's name
? At least one would expect this funtion to return a memory DC with the same
color organization as hDC.

Compatible in pixel format. The default monochrome 1x1 bitmap is a
placeholder.
A common mistake is to create a bitmap compatible with the memory DC
instead of the original DC, thus creating a monochrome bitmap.

I'm not sure why there's no error when a 1bpp bitmap is selected into a
32bpp DC...
probably the way it's always been.

Mark
--
Mark Salsbery
Microsoft MVP - Visual C++

Post by Belloc
. A memory DC, does it really have a default bitmap, or is it just a
placeholder ?
Microsoft Visual C++ .NET 69586-335-0000007-18101