TweetFollow Us on Twitter

Arrowheads
Volume Number:1
Issue Number:9
Column Tag:C WORKSHOP

How to Draw Arrowheaded Lines

How to Draw Arrowheaded Lines

While writing a new application, I discovered I needed to have lines with arrowheads (like the ones we all love in MacDraw). Not only that, I needed to have the line stop on the surface of an object (usually an oval or rectangle).

The first place to look, of course, is to see how MacDraw did it. After using MacDraw for some time in various applications, I was surprised to see that the arrowheads were not 3-pointed polygons but actually small wedges! This was discovered after placing an arrowhead on a thick line and noticing the curvature of the arrowhead.

Keeping with the Macintosh spirit, I decided to make the call similar to the ones used in QuickDraw (i.e. - Line and LineTo). Instead of using a LineTo call, this routine may be substituted and all of the LineTo's changed to ArrowLineTo's.

The parameter passed to it is the absolute horizontal and vertical coordinates of the end point of the line (as in LineTo). The end point is also where the arrowhead will be drawn.

The basic algorithm is simple -

1). Compute the slope of the line

2). Draw the line

3). Point the wedge in the opposite direction of the line (to create the effect of an arrowhead).

4). Draw the wedge

There is a fantastic Mac routine called PtToAngle that will compute the angle for you! All you need to do is to place a rectangle around the starting point, make the call, and the angle is returned to you (in degrees)!

The following code (written in Mac C) is pretty much self explanatory. You could experiment with the constants arrowWidth and arrowLength (as shown in Fig. 1) to vary the width and length of the arrowhead. Note - this routine could be changed so that these constants are passed as parameters, for applications that need varying sizes of arrowheads.

/************************************************************************

         ArrowLineTo.c

 Copyright 1985, FlottWare

 ************************************************************************/

#include "MacCDefs.h"// Mac ROM data structure definitions
#include "Events.h"
#include "Window.h"

extern struct P_Str *CtoPstr();
int strlen(str) char *str;
  {int i=0; while (str[i++]); return i-1;}
  
/*------------------------------------------------------------------------
    Global Data
   ------------------------------------------------------------------------*/
 
#define False 0
#define True 0xFF

Rect      windowRect = {40,4,336,508}; // Window Rectangle
WindowPtr windowPtr; // Window Pointer

EventRecord event; // Current event record

/*------------------------------------------------------------------------
    main()
   ------------------------------------------------------------------------*/
 
main()
  { 
  
  if (CatchSignal()) // Is the user quitting this application?
    ExitToShell(); // Go to Finder when done
    
  init(); // Init Mac 
 
  while (True)   // Poll events forever 
    { 
    SystemTask();// Perform System duties
    
    if (GetNextEvent(everyEvent, &event))    // Get the next event 
      {
      
      switch ( event.what ) // Which event is it? 
 {

 case autoKey:
 case keyDown:                         // Key was pressed 
 case mouseDown: // Mouse Button pressed 
   {
          Signal("All Done"); // Y - quit application
   break;
   } // case autoKey,keyDown,mouseDown:
   
 } // switch (event.what)      
      } // if (Getevent)
    } // while (true)
  } // main
  
/************************************************************************ 

   Windows
 ***********************************************************************/ 

 
/*---------------------------------------------------------------------- 
 
   init() -   
  ----------------------------------------------------------------------*/

init()
  {
   // Open a window
   
  windowPtr = NewWindow(0,&windowRect,CtoPstr("Hit Key or Mouse to Quit "),True,
                             documentProc,-1,True,0);
  SetPort(windowPtr);
  
  #define centerH 250
  #define centerV 150
  #define offset   50
   // Draw some lines with arrowheads
   
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH-offset,centerV);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH-offset,centerV+offset);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH,centerV+offset);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH+offset,centerV+offset);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH+offset,centerV);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH+offset,centerV-offset);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH,centerV-offset);
  
  MoveTo(centerH,centerV);
  ArrowLineTo(centerH-offset,centerV-offset);
  
  } // end init()

/*----------------------------------------------------------------------
      ArrowLineTo(horiz,vert)
   
     This routine draws a line from the current pen location
 to point (h,v) with an arrowhead (wedge) on the end of it.
 
        Inputs : horiz       - horizontal coord of end point
                 vert        - vertical   coord of end point
  
 Written by: Rick Flott                           Mac C (Consulair) V 
1.0
 ----------------------------------------------------------------------*/

ArrowLineTo(horiz,vert)
  short horiz;
  short vert;
  {
   // Size of starting Rect used in PtToAngle
  #define rectOffset  30
   // Arrowhead (wedge) constants
 //   arrowWidth - width of 1/2 arrowhead (in degrees)
   //   arrowLength - length of arrowhead in pixels
  #define arrowWidth  25
  #define arrowLength 10
  
 // Rects used in angle calculations
  Rect  startRect; //   starting Rect used in PtToAngle
  Rect  arrowheadRect;    //   the Rect that the wedge is drawn in

  short arrowAngle;// Angle of arrowhead (in degrees)          
 
  
  Point startPt,endPt;    // Start,End points of line
  
  GetPen(&startPt);// Get the current pen location
 // Set up a rectangle around the starting point
   //    (this is needed for the PtToAngle routine)
  SetRect (&startRect,startPt.h - rectOffset,
                      startPt.v - rectOffset,
                      startPt.h + rectOffset,
       startPt.v + rectOffset);
       
  SetPt(&endPt,horiz,vert); // Set up the end point
    
   // Calculate the angle (in degrees) of the line segment
  PtToAngle(&startRect,&endPt,&arrowAngle);
   
  LineTo(horiz,vert);// Draw the line
  
  arrowAngle -= (180 + arrowWidth);// Create a arrowhead with a wedge 
facing the opposite direction

   // Set up a Rect for the wedge around the endpoint
  SetRect (&arrowheadRect,endPt.h - arrowLength,
                          endPt.v - arrowLength,
                          endPt.h + arrowLength,
           endPt.v + arrowLength);
     
        // Draw arrow head (a reversed wedge)
  PaintArc(&arrowheadRect,arrowAngle,2*arrowWidth);
  
  } // end ArrowLineTo()
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

How to build a successful civilisation i...
GodFinger 2 grants you godlike powers, leaving you to raise a civilization of followers. In the spirit of games like Black & White, the GodFinger games will see you building bigger and better villages, developing more advanced technology and... | Read more »
How to get all the crabs in Mr Crab 2
Mr. Crab 2 may look like a cutesy platformer for kids, but if you're the kind of person who likes to complete a game 100%, you'll soon realise that it's a tougher than a crustacean's shell. [Read more] | Read more »
How to be a star in Britney Spears: Amer...
If you've ever wanted to be a star, baby, then you've probably already checked out Britney Spears: American Dream and are happily making your way up the charts. But fame doesn't come easy, and everyone needs a helping hand sometimes. So we've got... | Read more »
AppSpy is hiring a part time Staff Write...
| Read more »
How to save lives in ER Surgery Simulato...
A serious earthquake has struck a nearby town in ER Surgery Simulator - Emergency Doctor, and it’s up to you to save the victims. [Read more] | Read more »
Tips and tricks to get a high score in G...
Ketchapp Games loves the endless runner genre. And its newest game, Gravity Switch, is no exception. Gravity Switch takes a fresh approach, though, as you move a block, suspended in zero gravity, safely through a maze of shifting pillars. If the... | Read more »
Tips and tricks to get a high score in S...
Smash Fu is a high-paced tile-tapping game that requires quick reflexes and some practice. You’ll have to smash bricks with the skill of a seasoned black belt to get a high score. To raise the stakes a bit, you’ll also have to avoid tapping any... | Read more »
How to keep the ball rolling in Dropple
If you're new to the minimalist puzzler Dropple, you may find yourself struggling to make it beyond the first couple of steps before your ball falls into the endless abyss below. [Read more] | Read more »
Game Craft releases new Legend of War ti...
Set for release at the end of this month, real time strategy title Legend of War seems sure to delight with a veritable feast of sweet features to get stuck into. Developed by Game Craft, the game is due for release through both the App Store and... | Read more »
How not to die in Traffic Rider
Traffic Rider, an Out Run-esque game in which your ride a motorcycle recklessly into trffic, might not seem particularly complicated. [Read more] | Read more »

Price Scanner via MacPrices.net

Textkraft Professional Becomes A Mobile Produ...
The new update 4.1 of Textkraft Professional for the iPad comes with many new and updated features that will be particularly of interest to self-publishers of e-books. Highlights include import and... Read more
SnipNotes 2.0 – Intelligent note-taking for i...
Indie software developer Felix Lisczyk has announced the release and immediate availability of SnipNotes 2.0, the next major version of his productivity app for iOS devices and Apple Watch.... Read more
Pitch Clock – The Entrepreneur’s Wingman Laun...
Grand Rapids, Michigan based Skunk Tank has announced the release and immediate availability of Pitch Clock – The Entrepreneur’s Wingman 1.1, the company’s new business app available exclusively on... Read more
13-inch 2.9GHz Retina MacBook Pro on sale for...
B&H Photo has the 13″ 2.9GHz Retina MacBook Pro (model #MF841LL/A) on sale for $1599 including free shipping plus NY tax only. Their price is $200 off MSRP. Amazon also has the 13″ 3.9GHz Retina... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Clearance 12-inch Retina MacBooks available s...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
New 2016 13-inch 256GB MacBook Air on sale fo...
B&H Photo has the new 13″ 1.6GHz/256GB MacBook Air (model MMGG2LL/A) on sale for $1149 including free shipping plus NY sales tax only. Their price is $50 off MSRP. Amazon has the 13″ 1.6GHz/256GB... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more
Accenture and Vatican Opera Romana Pellegrina...
Accenture has announced that the official mobile application for the Extraordinary Jubilee Year of Mercy declared by Pope Francis has been built and launched by Accenture Mobility, part of Accenture... Read more

Jobs Board

*Apple* Nissan Service Technicians - Apple A...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
ISCS *Apple* ID Site Support Engineer - APP...
…position, we are looking for an individual who has experience supporting customers with Apple ID issues and enjoys this area of support. This person should be Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
*Apple* Support Technician II - Worldventure...
…global, fast growing member based travel company, is currently sourcing for an Apple Support Technician II to be based in our Plano headquarters. WorldVentures is Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.