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Roddy Pratt · Guest

I have a weird problem with a class (TDIB) I've written which is derived
from TBitmap.

Basically, I have a few extra fields (title, aspect ratio, etc) and a few
additional methods, but it's still the same TBitmap underneath.

Now, I have a halftone 'resize' method which calls StretchBlt.. which is
something like this...

SetStretchBltMode(dest->Canvas->Handle,HALFTONE);
SetBrushOrgEx(dest->Canvas->Handle,0,0,NULL);

StretchBlt(dest->Canvas->Handle, 0,0, dest->Width, dest->Height,
Canvas->Handle, 0, 0, Width, Height, SRCCOPY);

Normally, this all works just fine. I've had this in production code for
almost a year. But today, I found this method causing really bad contouring
and corruption in the resized image - the original bitmap is fine and
uncorrupted, but the resized one is shot to hell. No error is returned.

What's changed is that I'm now "Assign"ing the bitmap to a second TDIB
after loading it, like this:-

TBitmap *d1 = new TDIB;
TBitmap *d2 = new TDIB;

d1->LoadFromFile("fred.bmp");
d2->Assign(d1);

d1->Resize(); // This works OK every time
....
d2->Resize(); // This fails intermittently. sometimes it's fine, sometimes
it's rubbish.

My thoughts are that either my Assign method is broken - but it just looks
like this:-

void __fastcall TDIB::Assign(TPersistent *Source)
{
TDIB *pDib = dynamic_cast<TDIB*>(Source);
if(pDib != NULL)
{
// copy over additional fields...
}
inherited::Assign(Source);
}

... or, that my "Resize" method is not correct due to the way the reference
counting operates. Is there something subtle I should do before using the
Canvas->Handle property with a GDI routine??

Thanks,
- Roddy