動作環境

C++ Builder XE4

Unit1.cpp

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

# include <vcl.h>
# pragma hdrstop

# include "Unit1.h"
//---------------------------------------------------------------------------
# pragma package(smart_init)
# pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
	: TForm(Owner)
{
}
//---------------------------------------------------------------------------

void __fastcall TForm1::drawRadialLine(double angle_deg)
{
	TRect R = Image1->ClientRect;

	double radius = (R.right - R.left) / 2;
	int center_x = (R.left + R.right) / 2;
	int center_y = (R.top + R.bottom) / 2;

	Image1->Canvas->MoveTo(center_x, center_y);

	int to_x = radius * Cos(angle_deg * M_PI / 180.0);
	int to_y = radius * Sin(angle_deg * M_PI / 180.0);
	to_x += center_x;
	to_y += center_y;
	Image1->Canvas->LineTo(to_x, to_y);
}

void __fastcall TForm1::drawOnImage(void)
{
	// 1. draw circle
	TRect R = Image1->ClientRect;
	//                      X1,     Y1     X2       Y2
	Image1->Canvas->Ellipse(R.left, R.top, R.right, R.bottom);

	// 2. draw line in radial direction
	drawRadialLine(/*angle_deg=*/30.0);
	drawRadialLine(/*angle_deg=*/140.0);
	drawRadialLine(/*angle_deg=*/250.0);
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
	drawOnImage();
}
//---------------------------------------------------------------------------

TImageのサイズはW250 x H250。

0度の定義: 中心点から右方向に水平線を描く
そこから時計回りに角度を取っている。