動作環境

C++ Builder XE4
RAD Studio 10.2 Tokyo Update 2 (追記: 2018/01/05)

やろうとしていたこと

フォームを拡大する
- https://qiita.com/7of9/items/1248086df6a4688a4668
フォーム上のTImageも拡大する
TImage上のMouseDownの位置を拡大前のものとする

準備

適当な画像

logo.png
- #include <pngimage.hpp>を使う

code

Unit1.h

//---------------------------------------------------------------------------

# ifndef Unit1H
# define Unit1H
//---------------------------------------------------------------------------
# include <System.Classes.hpp>
# include <Vcl.Controls.hpp>
# include <Vcl.StdCtrls.hpp>
# include <Vcl.Forms.hpp>
# include <Vcl.ExtCtrls.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published:	// IDE で管理されるコンポーネント
	TImage *Img_logo;
	TButton *B_enlarge;
	void __fastcall FormShow(TObject *Sender);
	void __fastcall B_enlargeClick(TObject *Sender);
	void __fastcall Img_logoMouseDown(TObject *Sender, TMouseButton Button, TShiftState Shift,
          int X, int Y);
private:	// ユーザー宣言
public:		// ユーザー宣言
	__fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
# endif

Unit1.cpp

//---------------------------------------------------------------------------

# include <vcl.h>
# pragma hdrstop

# include <pngimage.hpp>
# include "Unit1.h"
//---------------------------------------------------------------------------
# pragma package(smart_init)
# pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------

static int s_image_ratio = 1;
static const int kRadius_image = 50;
static const int kMargin_image = 10;

__fastcall TForm1::TForm1(TComponent* Owner)
	: TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TForm1::FormShow(TObject *Sender)
{
	Img_logo->Stretch = true;
	Img_logo->Picture->LoadFromFile(L"logo.png");
}
//---------------------------------------------------------------------------
void __fastcall TForm1::B_enlargeClick(TObject *Sender)
{
	if (s_image_ratio != 1) {
		return;
	}
	s_image_ratio = 2;
	Img_logo->Width  *= s_image_ratio;
	Img_logo->Height *= s_image_ratio;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Img_logoMouseDown(TObject *Sender, TMouseButton Button, TShiftState Shift,
          int X, int Y)
{
	int ax = X / s_image_ratio;
	int ay = Y / s_image_ratio;
	int margin = kMargin_image / s_image_ratio;

	bool bfhit = true;

	if (ax < (kRadius_image - margin) ||
		ax > (kRadius_image + margin)) {
		bfhit = false;
	}
	if (ay < (kRadius_image - margin) ||
		ay > (kRadius_image + margin)) {
		bfhit = false;
	}

	String msg;
	if (bfhit) {
		msg = L"You hit the nail on the head: " + IntToStr(ax) + L"," + IntToStr(ay);
	} else {
		msg = L"You're closer than ever before.";
	}
	OutputDebugString(msg.c_str());
}
//---------------------------------------------------------------------------

　実行例

拡大前

デバッグ出力: You hit the nail on the head: 46,42 プロセス Project1.exe (4080)
デバッグ出力: You're closer than ever before. プロセス Project1.exe (4080)
デバッグ出力: You're closer than ever before. プロセス Project1.exe (4080)

拡大後

デバッグ出力: You hit the nail on the head: 51,53 プロセス Project1.exe (4080)
デバッグ出力: You're closer than ever before. プロセス Project1.exe (4080)
デバッグ出力: You're closer than ever before. プロセス Project1.exe (4080)
デバッグ出力: You're closer than ever before. プロセス Project1.exe (4080)

関連情報

フォームとコントロールを動的にサイズ変更するときの考慮事項

日本語むずかしい。。。