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Realizing Palettes

wolfcrag
wolfcrag asked
on
Medium Priority
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Last Modified: 2010-08-05
I load a bitmap from my users hard drive. I find that my user only has 256 colours available. So I need to realize a palette. So why doesn't:

//*******
PalPrev := SelectPalette(Canvas.Handle, MyBitmap.Palette, False);
r := RealizePalette(Canvas.Handle);
//show my picture here (with flashy effect!)
ShowPic(MyBitmap);
//afterwards, clean up:
SelectPalette(Canvas.Handle, PalPrev, True);
//********

work? (note, Canvas refers to the main form's canvas)

I tried changing the painting to a TPaintBox, but I got the same answer. Please also note that (for the form) I am intercepting the WM_ERASEBACKGROUND message, in order for my special effects to be flicker free. Surely that can't be the problem? Also, I know that MyBitmap has a palette.

I'll give anyone who can solve my problem a B, with an A reserved for those who give me a good explanation of why my above code doesn't work.
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1.The easiest way to draw you'r bitmap is in exchange of you'r more than 4 lines use only one :

Canvas.Draw( 0, 0, MyBitmap );

but mayby you can't use this (i don't know why).

2.You'r code can't work because of you'r call to Canvas.Handle is not good (delphi caching algorithm). You must use this as in this example :

var
  PalPrev : HPalette;
  NewDC : HDC;
  FormDC : HDC;
begin
  FormDC := GetDC( Handle );
  PalPrev := SelectPalette( FormDC, MyBitmap.Palette, False);
  RealizePalette( FormDC );
{ painting on FormDC }
  NewDC := CreateCompatibleDC( Canvas.Handle );
  SelectObject( NewDC, MyBitmap.Handle );
  BitBlt( FormDC, 0, 0, 100, 100, NewDC, 0, 0, SrcCopy );
{ end of Painting }
  DeleteDC( NewDC );
  SelectPalette( FormDC, PalPrev, True);
  ReleaseDC( Handle, FormDC );
end;

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Author

Commented:
Can you elaborate further on this delphi caching algorithm, and/or why calls to Canvas.Handle are not good? Your code worked beautifully. Thanks!
My english is rather poor so i think that have no sense but the problem is in Delphi3 (if i good remember) and with Delphi2 all works fine. In Delphi3 Borland made some changes to palette and i don't work about it hard and i don't know how say this good. There are added methods for manipulating palette but i think so with some mistakes because sometimes i can't get acces to the palette.
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