Monthly Archives: November 1998

I’m off to Galway for the weekend. There’s going to be a gathering of Linux people at Galway University on Saturday morning, as well as a night out that night. Alan Cox should be going as well as most of the Linux guys in Ireland. Look for a report next week!
I got an interesting email during the week concerning real-time processes, and another subject upon which I’ll base a quizlet soon.27

I received an email from Tim Newsome yesterday. He talked about giving a demo real time priority so it could execute smoothly. I would have agreed with him a few months back but I believe Linux is capable enough to handle the strain of a demo without resorting to giving it (the demo) complete access to the CPU. Hardware prices are dropping every day, and entry level machines are now much faster than the machine I work on at home(or at work..)
I want to see demos running on an X desktop. Maybe, with a slightly higher priority than other apps and it is possible.
Hey Tim! Wanna code a demo?

Something completely different now.
Recently I was on the receiving end of the evangelism of a highly sophisticated organisation wanting me to support it. I did too. I completely believed what I was told for 2 weeks. The organisation is Amway.
This is a company that will deliver products your door, at a cheaper price and promising a full money-back guarantee if you’re not satisfied with any product. There’s more to it than that, if you tell people about Amway and they buy goods, you get a small percentage bonus if they buy over a certain value of goods.
The problem is, the system is brain washing.

You’re encouraged to go check out Amway, but after hearing how good it is from the Amway representative and the fact that it is probably a friend who introduced you to Amway, many people will likely not check out Amway. I didn’t.
It was only after telling a friend, Dave, about the business that I checked up on it. He asked me questions about taxes and things I couldn’t answer. He brought up ideas that frightened me at the time. Do a search for the word Amway on Yahoo now and see some of the sites dedicated against Amway. They’re what I found.
I confronted my Amway “upline” with questions from Dave. He answered some of the questions well enough, and he did say that a company called “Donnagh & Bradstreet” would say that Amway are ok. I accepted this at the time.
The same evening I searched Yahoo for “Amway” and found out that “Donnagh & Bradstreet” are an acreditation firm. They’re guys who know if a company is a good bet to lend money to. Obviously I didn’t want to loan Amway money so their information would have been useless.
There’s also a list of steps to take to guarantee success in Amway. One is the system of motivational books and tapes. This isn’t an Amway thing, but organised by others. Later I remembered that on Will Smiths’ latest album there’s a guy who wants to do a business deal with Will, and promises him some of the proceeds of the motivational tapes and books..
They’re smart. I talked to a person I was told introduced Amway to Ireland, and the guy was a brilliant salesman. No, more than that. He had an answer for everything. One slip up though. He used Bill Gates as an example of a successful business man. To a non-computer person this might be encouraging but not to me. He described his business more eloguently in that sentance than at any other time.
I complained to him that a friend had tried a washing powder and wasn’t happy with it.
He asked me if I thought she was an expert in washing powders.
I replied, “No, but I’m sure her mother is.”
To which he replied, “Ah! But there’s plenty of mothers will buy it!” and smiled with a terrible grin.
As a learning experience, Amway can’t be beat. I learned more about myself and about human behaviour and motivation than I have in a long time.
Why is this relevant to Linux? Have you asked yourself why the GPL is the way to go when coding software? Have you looked at the Artistic Licence Perl is distributed under? Do you know the complete facts about something you’re advocating?

Let’s get things moving shall we? In this tutorial I’m going to move that white dot we drew in the last tutorial around the screen!
The dot will follow coordinates generated by the sin and cos functions.

#include <math.h>
#include “ptc.h”

const SPEED=50;
const WIDTH=320;
const HEIGHT=200;

class Move_dot
{
  private:
    typedef struct count
    {
      int x;
      int y;
      int xdir;
      int ydir;
    } count;
    int32* sin_data;
    int32* cos_data;
    int frames;
    count current_pos;
    count last_pos;
    int dir;
    int last_counter;
  
    void update_counter (int* counter, int* dir)
    {
      if (dir[0] == 0) {
        if (counter[0] < 1024) {
          counter[0]++;
        } else {
          dir[0]=1;
        }
      } else {
        if (counter[0] > 0) {
          counter[0]–;
        } else {
          dir[0]=0;
        }
      }
    }
  public:
    Move_dot()
    {
      int index;
      sin_data = new int32[1024];
      cos_data = new int32[1024];
      for (float i=0; i < 360; i+=0.01) {
        index=(int)((i/360.0)*1024);
        sin_data[index]=(int)fabs(WIDTH*sin(i/-57.3));
        cos_data[index]=(int)fabs(HEIGHT*cos(i/-57.3));
      }
      frames=0;
      current_pos.x=150;
      current_pos.y=0;
      last_pos.x=150;
      last_pos.y=0;
      dir=0;
      last_counter=0;
    }

    ~Move_dot()
    {
      delete sin_data;
      delete cos_data;
    }

    void draw (int32 *buffer)
    {
        buffer[(int)((sin_data[last_pos.x])+(cos_data[last_pos.y]*WIDTH))]=0;
        buffer[(int)((sin_data[current_pos.x])+
          (cos_data[current_pos.y]*WIDTH))]=(255<<16)+(255<<8)+255;
        last_pos.x=current_pos.x;  
        last_pos.y=current_pos.y;  
        update_counter(&current_pos.x,&current_pos.xdir);
        update_counter(&current_pos.y,&current_pos.ydir);
        frames++;
    }
};
int main()
{
  try {
    int speed=SPEED;
    Move_dot move_dot;
The following is the Makefile you’ll use:

    // create format
    Format format(32,0x00FF0000,0x0000FF00,0x000000FF);

    // create console
    Console console;
    console.open(“tut02”,WIDTH,HEIGHT,format);

    // create surface
    Surface surface(WIDTH,HEIGHT,format);

    // loop until a key is pressed
    while (!console.key()) {

      if (speed == 0) {
        speed=SPEED;
        // lock surface
        int32 *buffer = (int32*) surface.lock();
  
        // draw dot
        move_dot.draw(buffer);
  
        // unlock surface
        surface.unlock();
  
        // copy to console
        surface.copy(console);
  
        // update console
        console.update();
      } else {
        speed–;
      }
    } // exit
    return 0;
  } catch (Error error) {
    // report error
    error.report();
  }
}

The following is the Makefile you’ll use:

# GNU Makefile for the X11 part of the PTC 2.0 C++ API
# Copyright (c) 1998 Christian Nentwich (brn@eleet.mcb.at)
# The PTC 2.0 C++ API is (c) 1998 Glenn Fiedler (ptc@gaffer.org)
# This package is licensed under the GNU LGPL
#
# Please refer to the file COPYING.LIB contained in the distribution for
# licensing conditions

include ../../Makefile.config

PROG1 = Tut02

all:  $(PROG1)

$(PROG1): $(PROG1).o
  $(CC) $(PROG1).o $(PTCLIB) $(PTC_X_LIBRARIES) -o $(PROG1)

.cc.o:
  $(CC) -c $(CFLAGS) $< clean:
  rm -f *.o

distclean:
  rm -f *.o
  rm -f $(PROG1)

Now compile Tut02.cc by typing make.
If you have previously compiled the example programs that came with PTC this should compile without problems.
Now run it from an xterm by typing:

./Tut02(return)
You should now see a window (or it will go fullscreen if your X server has such low resolutions) with a small white dot flying around it!
Press a key to escape out of the program.

How it works.

I’m not going to go back over the basic PTC stuff I covered in the previous tutorial. If you have any queries look at tutorial 1.

In my program I use 3 variables that never change. I defined these at the top of the program as constants:

const SPEED=50;
const WIDTH=320;
const HEIGHT=200;

The first constant, SPEED determines how often we refresh the screen. I have it set at 50 which works well on my machine. Change it and see what happens.
The last two figures should be self-explanatory. They are the width and height of the window we’ll create. These figures will also be used in generating our sin and cos values.
I used to wonder how I could code a demo in C++. “All those objects would get in the way”, I thought. Someone posted a message on CSIPD about a year or so ago voicing the same critisicm and among the replies was the idea that you put each demo part into a different class.
This is what I’ve done here. If I wanted to add a part to this small tutorial I would just code another class. With proper organisation I could create classes to do different functions of a demo part. Say, one class to move a scrolly and another class to draw and animate a logo or 3d object. All you would have to do is pass a pointer to the frame buffer to draw to!
“main” would probably take care of clearing the screen buffer at the end of each refresh, or another class could be called to record the changes to the frame buffer in-between calling each “action” class..
I’ve defined a Move_dot class here that contains all the code I need to draw and move my dot around the screen.
Inside the class I defined a struct to hold the details of the position of the dot as well as the direction (up or down) of the counter that runs through my sin and cos values. I also coded a small private function which either adds or subtracts 1 from the given counter and changes direction if necessary.
I decided to precalculate all my sin and cos values to speed up execution (precalc as much as possible!) and I needed two arrays to hold that data:

int32* sin_data;
int32* cos_data;

The class constructor, Move_dot(), creates my arrays for me, and fills the arrays with suitable values. It also defines various variables which I’ll use later on.
The class destructor, ~Move_dot(), on the other hand just deletes the two arrays freeing up the space they took.
The main function in the Move_dot class is the draw function.
It is called at every update to perform the animation on the white dot, and is the only public function in the class. It is called from the main loop of the program. It’s quite simple and should be easy to follow.

buffer[(int)((sin_data[last_pos.x])+(cos_data[last_pos.y]*WIDTH))]=0;

This line deletes the last drawn dot on the screen. We could just clear the whole buffer but this is much faster.

buffer[(int)((sin_data[current_pos.x])+(cos_data[current_pos.y]*WIDTH))]=(255< <16)+(255<<8)+255;

This line draws a white dot at some location on the screen.
The rest should be easy enough to decipher.
Mail me if you have problems.

Notes

Here’s a way to code demos in C++ which I touched upon above.
Code your demo parts as a series of classes. Each part should be contained in either one class or in a set of classes with suitable naming conventions.
i.e.

class part1_scroll
class part1_3dobj
class part1_music

class part2_vectors
class part2_dycp
class part2_music

If using the above you could set up PTC in “main” and create instances of each class and then call the draw function of each class. (or play of the music class.)
It’s probably advisable to let your “main” function clear the screen buffer after each refresh of video memory. Classes should just draw to the screen. They should not delete the previous image they drew (like my example above). This allows you to do nice things like implementing the “dirty rectangles” algorithm which just updates the parts of the screen which actually have been changed.
No, I’ve never done the above!

DemoNix moving to lowrent.org was mentioned on Orange Juice! (thanks doj of cubic!)
PHP/MySQL is lovely.
The link for the PC Demo FanClub was wrong in the last update!

Update on Jan 1, 2008 lowrent.org and ojuice.citeweb.net are both gone!

On the day Slashdot links to my page on Xoom the server decides to crash! Demonix is obviously now at lowrent.org. Now to stop all that spam from Xoom…
A new version of install-mail will be released soonish, but I guarantee nothing!
Check the “Linux Demo Ring” below. There are now 3 sites!
The PC Demo FanClub has moved yet again. Jer is also looking for people to provide content for his site. Click on “Join the Club” on his page for more details.