Whats the best way to scale a windows bitmap?

Hi jxbma,

a common and easy way to do this is to create another in-memory bitmap of the wanted (scaled) size and draw the original bitmap into the new one using ::StretchBlt. To do so both bitmaps need to be selected into memory device contexts. Here's some (untested) code how this could look like:

// obtain a display DC to use as base for the memory DCs
HDC hdcScreen = ::CreateDC( "DISPLAY", NULL, NULL, NULL );

// prepare source memory DC and select the source bitmap into it
HDC hdcSource = ::CreateCompatibleDC( hdcScreen );
SIZE sSource = /*the size of the source bitmap*/
HBITMAP hBmpSource = /*the source bitmap*/;
HBITMAP hBmpOldSource = (HBITMAP)::SelectObject( hdcSource, hBmpSource );

// prepare destination memory DC, create and select the destination bitmap into it
HDC hdcDest = ::CreateCompatibleDC( hdcScreen );
SIZE sDest = /*the wanted size of the scaled target bitmap*/
HBITMAP hBmpDest = ::CreateCompatibleBitmap( hdcSource, sDest.cx, sDest.cy );
HBITMAP hBmpOldDest = (HBITMAP)::SelectObject( hdcDest, hBmpDest );

// set halft-ton stretch-blit mode for better scaling quality
::SetStretchBltMode( hdcDest, HALFTONE );

// draw the bitmap scaled into the destination memory DC
::StretchBlt( hdcDest, 0, 0, sDest.cx, sDest.cy, hdcSource, 0, 0, sSource.cx, sSource.cy, SRCCOPY );

// release everything
::SelectObject( hdcSource, hBmpOldSource );
::SelectObject( hdcDest, hBmpOldDest );
::DeleteDC( hdcSource );
::DeleteDC( hdcDest );
::DeleteDC( hdcScreen );

// now you should have the scaled bitmap in 'hBmpDest'
...

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It might be you're not satisfied with the quality of the scaled bitmap - if so you should consider doing similar things using GDI+, compard with GDI it has improved drawing and smoothing/antialiasing functionality.

Hope that helps,

ZOPPO

Zoppo:

Thanks for the feedback.

Our app does image analysis. Client apps will pass us either an HBMP or LPBITMAP.
The analysis is part of a library (which will eventually get wrapped).
We don't have access to HDC's from within the library.

Is there another way to try to scale the bitmap through the windows API, or through openCv?

Thanks,
JB