c# - StretchBlt occasionally inverts image when it shouldn't -

i’m having problem using stretchblt (or bitblt) copy image picturebox bitmap use in creating avi file. avi file unimportant aspect believe - can display bitmap on picturebox , see problem.

the problem (not often) single image flipped (mirrored across x axis, never y axis). i’m not sure if known problem stretchblt haven’t yet found mention of or if doing wrong.
note not due intended functionality stretchblt of "if signs of source , destination height or width different creates mirror image".

update: changed things force source/destination same size, , using bitblt same behavior.

i’ve included code (c#), of important parts.

stepping through code can see happen single image has same information being passed stretchblt (other hdc copy to) previous image , next image (and next, next) of fine. doesn't happen often, , dont see reason when does. or way detect happened (so can flip back). have work around doesn't use stretchblt, slower , degrades performance.

another possibly useful bit: flipped image rare in normal usage (less 1 in 100 frames). when run in ide, stepping through image image, happens regularly (maybe 1 in 10).

any ideas causing or doing wrong? or other fast methods copy bitmap including re-sizing. note: bitmaps vary in size (can't use bitblt), not lot.

thank you!

kate

// --- import statement [dllimport("gdi32.dll", charset = charset.auto, setlasterror = true, exactspelling = true)]     public static extern bool stretchblt(  intptr hdcdest, int nxdest, int nydest, int ndestwidth, int ndestheight, intptr hdcsrc, int nxsrc, int nysrc, int nsrcwidth, int nsrcheight, int32 dwrop );  // --- small class creating/storing bitmap , graphics objects, reused public class caviimageinfo   {     private bitmap    mavibitmap = null;     private graphics  mavigraphics = null;      public caviimageinfo(int width, int height )     {       mavibitmap = new bitmap(width, height);       mavigraphics = graphics.fromimage(mavibitmap);     }     ~caviimageinfo()     {       mavigraphics.dispose();       mavigraphics = null;     }     public bitmap avibitmap     { { return mavibitmap; } }     public graphics avigraphics     { { return mavigraphics;  } } }    // --- 2 pictureboxs placed on form @ design time (one watch these mirrored images): picturebox mmainpicturebox; // --- displays images copied picturebox debugpicturebox;  //  --- following done 1 time: graphics  mmainpictureboxgraphics = mmainpicturebox.creategraphics(); intptr    mmainpictureboxhdc      = mmainpictureboxgraphics.gethdc();  // --- method copying.  called each time image on panel updated. public void updateavirecording() {   // --- gets unused bitmap , graphics objects (these reused)   caviimageinfo aviimageinfo = getunusedaviimageinfo();    intptr destinationhdc = aviimageinfo.avigraphics.gethdc();    stretchblt(     destinationhdc,      0, 0, aviimageinfo.avibitmap.width, aviimageinfo.avibitmap.height,     mmainpictureboxhdc,      0, 0, mmainpicturebox.width, mmainpicturebox.height, srccopy);    // --- show copied bitmap on debug picturebox    // --- (normally pass written avi file)   debugpicturebox.image = aviimageinfo.avibitmap;   debugpicturebox.refresh();    aviimageinfo.avigraphics.releasehdc(destinationhdc);       }