Attraction!

Introduction

Attraction! is a freeware particle system simulation that attempts to be entertaining. This is why we call it a Gimmick.
It enables the user to define different types of particles and put them in the system to see how they behave and interact. To know this, rather than see it, take a look at the Physics section.

Quick Start

Start Attraction! by double-clicking its icon. Ignore all windows except the Attraction! - display window for now and start clicking and moving the mouse around in the window. You should see particles pop up and integrate into the system. The number of particles is being limited in order to stay realtime. Note: the window is rezisable, dont make it too big, though. Explore the other two windows on your own or read the sections The Controller Window and The Template Generation Window.

The Template Generation Window

The particle template manager contains a list of particle templates. To use a template, select it. The values will be copied into the below particle generation template form and sent to the model thread to be used when the mouse is clicked in the display window. Try changing the values to figure out what each one means. Dont forget to invoke the control you just changed to apply the changes. In case you want to revert to the old version of the template you have to select another one temporarily and then reselect it. The values will only be stored to the list by clicking the store button. Having fiddled around with the values for long enough, you might want to store the template under a different name: Just enter the name in the name field and click the store button. The new template will be added to the bottom of the list. Updating a template will also move it to the bottom of the list, so most recently changed template are always at the bottom. All the new button does is set the name to untitled. You can also just enter the new name directly. The template will be created when you click the store button.

A particles speed will be multiplied by its friction value each step. This means a value of 0 makes the particle unmovable (not entirely, as I encoutered. if it should move, it's a certain sign that floating point precision is exceeded).
A value of 1 means a friction-free particle: it wont stop by itself.
Typical values are: 0, 0.5, 0.95, 0.999, 1
'

The particles mass will be used by other particles to determine how strong the influence is. The other effect of mass is that the particle becomes more hard to move, because the velocity vector is multiplied by it before being added to the speed vector.
Be careful with high masses, they might easily blow the system into pieces you'll never find again.'

Determines the displayed size of a particle. You shouldn't use more than 25. Smaller sizes greatly reduce the time it takes to draw them. A size 2 particle should draw about 25 times faster than a size 10 particle (overhead not considered). You can use sizes like 2.3 or 3.8. A 3.1-particle will look (probaly inconcievably) bigger than a 3.0-particle.
Use the special value of 0.0 to draw the particle as a pixel (it looks much worse than 1.0, but its much faster, too). Use 0.0 if you intend to create many particles of that type.'

You can guess that. Special value is color 0,0,0 (black, upper left in the color selector). It will use the colorcycle mode from version 1.0.

Determines the position and speed the particle will initially have. The display region is (0.0,0.0)-(1.0,1.0). You will usually check the mouse check-box for position. Checking moused-speed and not moused pos will create the particle at the given position and use the relative mouse-position as initial speed-vector. Using the mouse for both position and speed will result in the following creation behaviour: Clicking the mouse will determine the position of the new particle. Hold the button down and move the mouse to a different position to set the speed vector and create the particle.

Each particle relates to all others the following way. It tries to keep the desired distance plus the partners desired distance. If the distance of the two is not the desired distance, a force will be applied to change that state. The forces that result out of all the relations are added up and later applied to the speed vector. Use the Attraction strength field as a general force multiplier to limit or enhance the effect.
Typical values for the strength are: 0.005, 0.00001, 0.01
Typical values for the desired distance are: 0, 0.01, 0.1
You can also use negative values for the distance to attract particles that usually try to keep a distance even below that distance.

The Controller Window

The controller window offers various controls to control the system in general as opposed to controlling single particles using particle templates.

The tape is a buffer to record the calculated frames (as position/size/color data) in. You can see the tape's length in the line above the blue status bar. As frames are calculated, they are added to the tape at the end and the size will be increased. To save your system from crashing when the swap space is full, you should limit the tape size to some reasonable value (maximum tape size control). These values are given in seconds.
Ah, the buttons: reset will delete all particles and empty the tape.
The rewind button will rewind the tape to its beginning. You will frequently use this to review the calculated sequence at nicer speed, when the calculation is not realtime anymore.
The fast forward button will position the tape right before its end, so you can see particles you create. The blue bar will show this position relative to the tape length.
The loop checkbox will automatically rewind if calculation is too slow. (note: checking it will uncheck and disable the delete particles to stay realtime checkbox out of obvious reasons). Check it to lean back and observe a slowly calculating system.'

The framerate bar shows the current framerate (averaged over the last 16 samples taken). It will change it's color according to wether it meets the desired rate or not. again, to enter the desired rate, you'll have to focus the controller window by clicking its title or borde

I'll start with the blind mans mode: it chooses a rectangle around the particles position and just fills it with the appropriate color. In the other two modes the rectangle is taken as the render-area. for each pixel the distance to the position is calculated, squared and used as alpha-value. this allows sub-pixel positioning (like the beos font-routines). In 32 bit, the particle could theoretically be perceived to move 1/256th pixel. 32 bit would be nice, but up to this point I saw no need, because my aged 66mhz bebox would fail to push all that data through the double buffer'

the respect bounds checkbox determines wether or not the particles bounce off the windows frame.
you might want to pause the calculation thread for two reasons: you have a caclulated sequence you want to keep viewing in loop mode or you want to create e.g. four particles in the edges at the same time. Note: In paused mode particles will still be created, just not displayed. the delete particles to stay realtime checkbox is disabled in loop mode (see tape control). I know the mechanism deletes more particles than really necessary and also does this in a very unsmooth way. On the other hand, if you keep holding the mouse button it will even drop frames. I tried to find something in between and failed to make it work dynamically as untill now.
You can protect the first n particles you created by entering n into the protect first n particles text control. dont forget to invoke it by pressing tab or enter. if you cant enter values, make sure the loop checker is unchecked and the controller has focus (click its border or title bar). (i know this is not nice)

This is for developers that want to now how long what takes. Shows how many particles are in the system and how long it takes to calculate, record to tape, draw to bitmap and blit to the screen one frame. the sum*rate value is all these timings added up and multiplied by the framerate. a value of 2.0 means that both processors are optimally used. If you should encounter this value, there's something wrong :) All these values are averaged over the last 16 samples.'

The controller window will accept first clicks. If you want to enter values, you have to first activate the window by clicking its title-bar or its border. Sorry for that inconvenience, but I like being able to click the reset and rewind buttons without loosing focus on the template window.

The Physics

Keep in mind that it`s a stepped simulation, so actually cant be associated with the word Physics. Each particle has a position and a speed attribute. That should be self-explanatory so far. Both are 2 dimensinal vectors (double precision floating point). At each step each particles speed vector is added to its position vector. We have a moving particle. Further I introduced a particle attribute called velocity (dunno about the term, I`m a german guy) used to sum up the forces that influence the particle. We apply the velocity to the speed the same way we applied the speed to position for each particle at each step. We have a particle that isn`t isolated, but influencable.
So what do we add to the velocity? Let`s be a particle for a second and look around: other particle. ok, then we take each one after the other and take a look at the distance. We add the distance vector time a factor to our velocity accumulator: The other particle forces us to move some way. Now whats the factor do? It calculates by multiplying the "attraction" attribute with our current partners mass and the (scalar) distance between the distance and the desired distance and dividing by the distance, the desired distance being mine plus the other particles one. Let`s be a processor and do this 10,000 times (for 100 particles) a frame. At 25 fps we get about 40 microseconds to do it. So, guess let`s not be a processor for a second. We`ve got a system of moving particles that try to keep a certain distance to each other all at the same time. If the particles also had the will to go as fast as they could, we`d look at a phenomenon called traffic.

Attraction! is copyright © 1998, f3c - Nicolas and Daniel Fischer. All rights reserved.
Attraction! is Freeware. You might copy and distribute it as you want, as long as the original archive remains unchanged. Neither the actual author nor f3c will give any warranty whatsoever. It is okay to include Attraction! in a bundled distribution, as long as we receive a copy of this distribution.