다들 아시는 내용이겠지만... 윈도우 클라이언트 영역을 변경하는 소스입니다.

윈도우 모양 변경등에서 주로 사용됬던 것 같네요...

SetWindowRgn을 이용합니다.

//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop

#include "RgnTestUnit.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------


void __fastcall TForm1::FormResize(TObject *Sender)
{
    SetWindowRgn();
}

//---------------------------------------------------------------------------
void TForm1::SetWindowRgn()
{
    HRGN hRCompo = NULL;
    int IX = GetSystemMetrics(SM_CXSIZEFRAME);
    int IY = GetSystemMetrics(SM_CYCAPTION);
    IY += GetSystemMetrics(SM_CYSIZEFRAME);

    for(int i = 0; i< ControlCount; i++) {
        TControl *C = Controls[i];
        if(hRCompo) {
            HRGN hR = CreateRectRgn(
                C->BoundsRect.Left+IX,
                C->BoundsRect.Top+IY,
                C->BoundsRect.Right+IX,
                C->BoundsRect.Bottom+IY);
            CombineRgn(hRCompo, hRCompo, hR, RGN_OR);
            DeleteObject(hR);
        }
        else {
            hRCompo = CreateRectRgn(
                C->BoundsRect.Left+IX,
                C->BoundsRect.Top+IY,
                C->BoundsRect.Right+IX,
                C->BoundsRect.Bottom+IY);
        }
    }

    HRGN hRgnTitle = ::CreateRectRgn(
                0, 0, Width, IY);

    ::CombineRgn(hRCompo, hRCompo, hRgnTitle, RGN_OR);
    ::DeleteObject(hRgnTitle);

    HRGN ClientRgn = ::CreateRectRgn(
                0, 0, Width, Height);

    ::CombineRgn(ClientRgn, ClientRgn, hRCompo, RGN_AND);
    ::DeleteObject(hRCompo);

    ::SetWindowRgn(Handle, ClientRgn, true);
    DeleteObject(ClientRgn);
}



void __fastcall TForm1::CreateParams(TCreateParams & Params)
{
    TForm::CreateParams(Params);
//    Params.ExStyle = Params.ExStyle | WS_EX_TRANSPARENT;
}
void __fastcall TForm1::FormCreate(TObject *Sender)
{
//    Brush->Style = bsClear;
}  
//---------------------------------------------------------------------------