c++ - StretchBlt only works when nHeightDest is negative -

i'm trying use stretchblt in order copy pixels memory hdc window hdc.
memory hdc gets image invisible window renders stream using opengl.

here's code:

bitmapinfoheader createbitmapheader(int width, int height) {     bitmapinfoheader header;     header.bisize = sizeof(bitmapinfoheader);     header.biwidth = width;     header.biheight = height;     header.biplanes = 1;     header.bibitcount = 32;     header.bicompression = bi_rgb;     header.bisizeimage = 0;     header.bixpelspermeter = 0;     header.biypelspermeter = 0;     header.biclrused = 0;     header.biclrimportant = 0;      return header; }  ...  hdc memoryhdc = createcompatibledc(windowhdc); bitmapinfo bitmapinfo; bitmapinfo.bmiheader = createbitmapheader(targetdimensions.width, targetdimensions.height); hbitmap bitmap = createdibitmap(windowhdc, &bitmapinfo.bmiheader, cbm_init, offscreenbuffer, &bitmapinfo, dib_rgb_colors); selectobject(memoryhdc, bitmap); deleteobject(bitmap);  setstretchbltmode(windowhdc, coloroncolor); stretchblt(windowhdc,     targetdimensions.x, targetdimensions.y,     targetdimensions.width, -targetdimensions.height,     memoryhdc,     sourcedimensions.x, sourcedimensions.y,     sourcedimensions.width, sourcedimensions.height,     srccopy);  deletedc(memoryhdc); 

where windowhdc hdc of window want stretchblt copy pixels to, , offscreenbuffer void* pixels copied offscreen window in opengl rendering.

this code works great, except image upside down , want vertically flipped.
know happens because:

if nheightsrc , nheightdest have different signs, function creates mirror image of bitmap along y-axis

but when remove minus sign , both target , source heights same see no image in window.
check, tried put minus on sourcedimensions.height results in no image, , same if try negate widths (both target , source).

any idea why?
thanks.