Canvas->CopyRect shifts 1 pixel by zoomed image

[WRoos]

I'm building a bitmapmanipulat ion-component in C++Builder6

After loading a bitmap in my component, I then can make a selection, which can be moved around.

The problem I have is in my OnPaint-event. I paint the topmost bitmap first(the selection), then find the parts that not have been painted, and paint those parts from the original bitmap.
If the bitmaps are zoomed-out (smaller on the screen) the original bitmap right to the selection-and selection itself sometimes are distorted.
I tested this with a Zoom of 4 and found distortion only occurs if the rect.left is odd. If it's even all is well. I tried to shift the bitmaps by odd numbers but then the errors occur when zoomed different.

This also happen when no rounding errors by zooming occur.

Can anyone help??

[Rabbit]

I don't know anything about image manipulation but I do know that it will be difficult to help without seeing the code that's causing the issue.

[WRoos]

//I'm using struct: FObject containing: 
//	bitmap
//    srect: position on screen
//    brect: position in bitmap (bitmap's in bitmap's)
//
// mylist start with the ClipRect (the invalidated rect for my control)
 OnPaint()
{
 TRect rmr[5], *todo;

 Canvas->CopyMode = cmSrcCopy;
 for (int n=FObjects->Count-1; n>=0; n--) {  // last bitmap first
   have = (TBBObject*) FObjects->Items[n];

   for (int m=mylist->Count-1; m>=0; m--) {   
      todo = (TRect*) mylist->Items[m];
      count = RectMinusRect(*todo, shift, rmr);
	// RectMinusRect return 1-5 rects. 
	// first one is the one to paint now (the vissible part of todo)
	// the other contain max 4 left over peaces
      TRect tobmp(rmr[0]);
	// now I shift to bmp to its place
      OffsetRect(&tobmp, have->srect.left, have->srect.top);
	// for a zoom of -4 the bitmap-points are all multiplied by 4
      ImgageToBitmap(&tobmp);

   	Canvas->CopyRect(rmr[0], have->bitmap->Canvas, tobmp); //(dst,canvas, src)

      delete todo;          // this rect is painted and can be deleted
      mylist->Delete(m);
      for (int t=1; t<=count; t++) {   // left over rect's to be painted 
         todo = new TRect(rmr[t]);     // with next have->bitmap
         mylist->Add(todo);
      }
   }
}

[WRoos]

I made a compleet program with my problem.
If someone is willing to help, he/she can take this code in a new project,
1)define the RMinR function in the .h file
2)define the FormPaint-event in the Form
3)run.

It paints a bitmap in 5 parts. If you look closely you can see the center and the part right to the center are shifted 1 pixel.
For some bitmaps this is more disturbing then for others.
if you change thisbmp(21,21,2 01,201) to even numbers
all goes well.

//---------------------------------------------------------------------------
void __fastcall TForm1::FormPaint(TObject *Sender)
{
Graphics::TBitmap* abmp = new Graphics::TBitmap;
 // a bitmap at least 850x850 pixels
 abmp->LoadFromFile("D:\\CBuilder6\\large.bmp");
 abmp->HandleType = bmDDB; // best quality-bitmap
TList* rectlist = new TList;
TRect* todo = new TRect;
TRect rmr[5], bmprct, thisbmp;
int count;
 *todo = TRect(0,0,abmp->Width/4,abmp->Height/4);
 rectlist->Add(todo);

 Canvas->CopyMode = cmSrcCopy;
 for(int n=0; n<2; n++) {      // 2 bitmap's
   if (n==0) thisbmp = TRect(21,21,201,201);
   else      thisbmp = TRect(0,0,abmp->Width/4,abmp->Height/4);
   for (int m=rectlist->Count-1; m>=0; m--) {
      todo = (TRect*) rectlist->Items[m];
      count = RMinR(*todo, thisbmp, rmr);
      bmprct = rmr[0];
      bmprct.left   *= 4; // Asume zoom = 4
      bmprct.top    *= 4;
      bmprct.right  *= 4;
      bmprct.bottom *= 4;
      // Stretching and copying at the same time
      Canvas->CopyRect(rmr[0], abmp->Canvas, bmprct);

      delete todo;
      rectlist->Delete(m);
      for (int t=1; t<=count; t++) {
         todo = new TRect(rmr[t]);
         rectlist->Add(todo);
      }
   }
 }
 delete abmp;
 delete rectlist;
 abmp = NULL;
}
//---------------------------------------------------------------------------
int __fastcall TForm1::RMinR(TRect& ToDo, TRect& Have, TRect* Remain)
{
int ret = 0;
 if (Have.top > ToDo.top) {
   Remain[++ret] = TRect(ToDo.left, ToDo.top, ToDo.right, Have.top);
   Remain[0].top = Have.top;
 } else Remain[0].top = ToDo.top;
 if (Have.bottom < ToDo.bottom) {
   Remain[++ret] = TRect(ToDo.left, Have.bottom, ToDo.right, ToDo.bottom);
   Remain[0].bottom = Have.bottom;
 } else Remain[0].bottom = ToDo.bottom;
 if (Have.left > ToDo.left) {
   Remain[++ret] = TRect(ToDo.left, Remain[0].top, Have.left, Remain[0].bottom);
   Remain[0].left = Have.left;
 } else Remain[0].left = ToDo.left;
 if (Have.right < ToDo.right) {
   Remain[++ret] = TRect(Have.right, Remain[0].top, ToDo.right, Remain[0].bottom);
   Remain[0].right = Have.right;
 } else Remain[0].right = ToDo.right;
// if (Remain[0].left > 0 && Remain[0].left%2 == 1) {Remain[0].left--; Remain[0].right--;}
 return ret;
}