From: "Earl F. Glynn"
Subject: Re: Color reduction in Canvas.CopyRect?
Date: Wednesday, December 06, 2000 1:13 PM

"David Lively" <dave@artfulgiving.dot.com> wrote in message news:3a2e759d_1@dnews...
> When I combine two 24-bit bitmaps (both initialized as DIB's with a
> pixelFormat value of pf24Bit) with Canvas.CopyRect, the pasted image data
> appears to be reduced from 24-bit to 8-bit. This doesn't happen when I use
> Canvas.StretchDraw. Ideas or workarounds? D5 under Win2k. Happens with a
> variety of different video cards/chipsets (and, therefore, drivers.).

CopyRect is nearly a direct call to the StretchBlt API call.  Both Draw
and StretchDraw call Graphic.Draw.  For a TBitmap, TBitmap.Draw
results in a call to StretchBlt (or TransparentStretchBlt).    So CopyRect
and StretchDraw both use StretchBlt.  But there are several  things
that can happen before StretchBlt is called using StretchDraw.  One is
a call to

    SetStretchBltMode(ACanvas.Handle, STRETCH_DELETESCANS);

The Windows API documentation says STRETCH_DELETESCANS is
"typically used to preserve color in color bitmaps."   I have not studied
exactly why this may be necessary.  In at least one application, I
added the following line before a CopyRect to get "better" color:

SetStretchBltMode(Bitmap.Canvas.Handle, STRETCH_DELETESCANS);

--
efg

Earl F. Glynn     E-mail:  efg2@efg2.com
Overland Park, KS  USA

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