• # A Simple Point Sprite Based Particle Engine

Graphics and GPU Programming

Point sprites are pretty common for particle engines these days. So, in the process of learning new things myself, and from my previous experience with my Four Elements 2002 entry ("Elements of Nature"), I decided to write an article about point sprites, and how to use them in a simple particle engine.

This tutorial is aimed at 2D, and the source will be in Visual Basic 6 and DirectX 8. Also note that this tutorial is meant for the people who do not have much or any experience with point sprites, or particle engines in general, and it won't involve much difficult math either.

I hope this tutorial will shed some light on particle engines, for programmers who are just starting with games (especially in VB). I also decided to make some simple examples of the interesting patterns or animations you can get with the extremely simple particle engine which is included with this tutorial. The full source included, of course. While creating this tutorial, I actually was amazed myself by the power of creating "particle stills" using the simple particle engine built in this tutorial.

What are particles?
Particles are individual objects, often controlled by gravity, by each other, "lifetime", and other factors, often reflecting real-world influences. All these "stand alone" particles need to be controlled. That's where the Particle Engine kicks in.

Particle engines are these days a commonly used piece of "machinery" in games. They are used for effects such as fire, smoke, and snow, and also for in-game effects like spawning new items, or picking up items, or animations around weapon projectiles. Particles can be used for many things. For example in the screenshot below, from my Four Elements Contest entery (2D) everything was made of particles, except for the ground, the tree (excepth for the leaves, which are particles), the lightning, the logos and the fish... So particles were used for the flowers, rain, clouds, the sun, the rainbow, leaves, fireflies and air bubbles!

It is also possible to use the particle engine to generate some particle-count heavy effects, and save a part of the framebuffer to disk. This way you can use the particle engine to pre-generate nice effects and then use them as textures in your game. For slower computers and older videocards this might help improve the framerate a lot, since it eliminates the need anymore to control every particle.

Another nice thing about particles is that, if you write a decent particle engine, you can have each individual particle controlled by external forces such as wind, gravity or neighboring particles. Complex formulas are also an option, which might result in interesting effects. We are going to keep it a bit simple here, so we will just stick with values like speed, acceleration and lifetime.

Point Sprites
Point sprites were introduced in DirectX 8.0. They offer a nice advantage over the normal method of rendering particles. Normally you would create a rectangle using four view-aligned vertices set to the appropriate texture coordinates. With point sprites you can simply use one vertex for every particle! Therefore they require less bandwidth, so you can render more particles using point sprites instead of using the old method, and you don't have to manually adjust the vertices to be aligned with the view vector.

However, some features of point sprites (e.g. scaling) are not supported on older video cards available. You can simply enumerate the features of the video card to see if certain things are supported.

Point sprites can be used in both 3D and 2D projections. For simplicity this tutorial will be all about 2D. In 2D particles can be used for smoke, rain, flames, wormhole like effects and many other things. Check out the source for some interesting particle demonstrations.

Starting with our simple particle engine
Well, it's time to write our particle engine. We are going to keep it extremely simple right now. I assume you have the basics of initializing DirectX all covered.

The method I commonly use for particle engines is to create one "Particle" class which will hold all the properties of a particle, like size, color, lifetime, etc. It will also have a few methods for resetting and updating particles. Then I wrap another class around the Particle class, like a collection (not a VB collection!), which will control all individual particles. For VB this might not be the fastest method (since we have to loop through every particle, every frame), but it is easy to setup and modify. It's always possible to port the class to an UDT "Particle" with the same properties, and then simply use an array to hold all particles. Arrays combined with UDT's can be quicker than the method used here.

First we are going to decide what (public) properties we will need in the Particle object.

• Position;
• Color, stored in R,G,B,A;
• Horizontal and vertical speed;
• Horizontal and vertical acceleration;
• Alpha decay (basically lifetime). I think that should be enough to get started with some fun particle effects.

Remember: The Particle object is one individual particle. The collection will control all the particles. So you can add more properties to the Particle object like: weight, lifetime, start color, end color, etc. You might have noticed "size" is missing. Size will be controlled in the render method. Unfortunately, this means you have no individual control over particle dimensions, but this method will be fast, and look great as well.

Now that the Particle object is ready, we will have to create our "wrapper" or collection as well, which will control all the particles. Right now the only thing we need are methods to update the particles and draw them on the screen.

What we need:

• A method to initialize the starting point, create the point list, etc;
• A method that will update the particles with new positions;
• A method that will render the particles;
• A method to reset an individual particle, when the particle is "dead".
• A method to relocate the emitter. I have called the methods Begin, Update, Render, Reset, Relocate respectively.

The particle collection also has a few properties of its own, such as the maximum number of particles to emit, position, etc.

Creating the code
We have our classes ready, time to start writing the actual code.

Point primitives are not much different from the other primitive types available (triangle strips, lists, etc.). We can still choose what vertex shader we use.

A vertex shader tells DirectX about what type of vertices it can expect - such as whether they are already transformed, if they have a diffuse color, and so on. Since I'm mainly aiming this tutorial at 2D gaming, I'm going to use slightly customized TL vertices. This stands for Transformed and Lit, which means the vertices already are transformed and that the lighting color has already been computed. So an X of 300 will be 300 on the screen, and the vertex color will be the final color.

Public Const D3DFVF_TLVERTEX = (D3DFVF_XYZRHW Or D3DFVF_DIFFUSE Or D3DFVF_TEX1) Public Type typeTRANSLITVERTEX X As Single Y As Single Z As Single rhw As Single color As Long tu As Single tv As Single End Type We will need an array to contain the point sprites

Private vertsPoints() As typeTRANSLITVERTEX Next up is the "Reset" method. Here we will reset the individual particles. Since this is just a collection using the Particle class, we can basically hard code numbers in here. Since this first example will be a fire effect, we will set the properties to look like a fire. Note I have named the "collection" "colFireFX" as well.

With Particles(I) Call .ResetIt(X, Y, -0.4 + (Rnd * 0.8), -0.5 - (Rnd * 0.4), 0, -(Rnd * 0.3), 2) Call .ResetColor(1, 0.5, 0.2, 0.6 + (0.2 * Rnd), 0.01 + Rnd * 0.05) End With This might look a bit complex now, but it really is not. There are lots of Rnd's in there. This is because the "flame" of the fire should be different every time, a bit irregular. That is what this code does: it changes the X and Y speed a bit, and also the Alpha and decay value. Check out the picture to the right for the results. The nice thing is that you can do so much with a simple particle engine like this just by modifying a few lines of code.

The decay value is the amount of "Alpha" (visibility) that is subtracted from the Alpha value every frame (based on the time). When the Alpha drops below zero, the particle will be reset, using the Reset method.

We need to initialize the arrays as well, both for the point list and the particles themselves, which will be done using a simple loop. Pretty straightforward. We simply reset all the particles now. Because of this, it will look the fire is "turned on" for the first few seconds.

For I = 0 To ParticleCounts Set Particles(I) = New Particle vertsPoints(I).rhw = 1 Call Reset(I) Next I The size of the particles is set with:

lngFloatSize = FtoDW(25) '//Size of our flame particles.. Note that this code uses this special function:

Public Function FtoDW(f As Single) As Long Dim Buf As D3DXBuffer Dim L As Long With ObjD3DX Set Buf = .CreateBuffer(4) .BufferSetData Buf, 0, 4, 1, f .BufferGetData Buf, 0, 4, 1, L End With FtoDW = L End Function This function will put a "single in a long" (float in a dword), which is required for the .SetRenderState D3DRS_POINTSIZE and some other PointSize render states.

We still have to make the particles move. This is nothing more than looping through all particles in the array, and call "UpdateParticle." This method will change each particle:

Public Sub UpdateParticle(sngTime As Single) sngX = sngX + sngXSpeed * sngTime sngY = sngY + sngYSpeed * sngTime sngXSpeed = sngXSpeed + sngXAccel * sngTime sngYSpeed = sngYSpeed + sngYAccel * sngTime sngA = sngA - sngAlphaDecay * sngTime End Sub That's about as tough as the math gets. We simply update the position of the particle based on the speed, the acceleration and the time. The Alpha value decreases as well, based on the time.

The particle has changed now. Therefore, we have to update the vertex associated with this particle. We only have to update the position and the color.

vertsPoints(I).color = D3DColorMake(.sngR,.sngG, .sngB, .sngA) vertsPoints(I).X = .sngX vertsPoints(I).Y = .sngY After finishing the "Update" sub in our collection, the particles have been moved and the vertices (point sprites) have been updated.

Since we have already covered creating, resetting and updating, it's time for rendering. Rendering is similar to how you would render any other primitives. Due to the fact that vertex buffers are complicated to set up and to modify, I'm using DrawPrimitiveUP to render my vertices. With point sprites, this is done as follows:

First enable pointsprites:

With ObjDev .SetRenderState D3DRS_POINTSPRITE_ENABLE, 1 'True .SetRenderState D3DRS_POINTSCALE_ENABLE, 0 '//I don't need it.. End With We need to set a certain alphablend state to get that "glowing" effect:

With ObjDev .SetRenderState D3DRS_SRCBLEND, D3DBLEND_SRCALPHA .SetRenderState D3DRS_DESTBLEND, D3DBLEND_ONE .SetRenderState D3DRS_ALPHABLENDENABLE,1 End With Then render:

'//Set up the vertex shader .SetVertexShader D3DFVF_TLVERTEX '//Set our texture .SetTexture 0, myTexture '//And draw all our particles .DrawPrimitiveUP D3DPT_POINTLIST, ParticleCounts, vertsPoints(0), Len(vertsPoints(0)) You might have noticed the texture. The texture controls (for this particle engine) which part of the particle is seen. It should be grayscale, since we set the colors manually. Changing the texture can result in interesting effects. For example, changing the texture in a "flare" texture will give the particles an entirely different look!

Once rendering is complete, you need to disable the point sprites again:

With ObjDev .SetRenderState D3DRS_POINTSPRITE_ENABLE, 0 'True End With That's it for the rendering (be sure to change any renderstates you have modified back to the values they were).

A small point for optimization: The "Len(vertsPoints(0)" could simply be changed to a constant, or a value that is set once. Then the Len() function isn't required anymore, speeding up the rendering just a bit.

Remember, it's way more efficient to render as much as possible with one DrawPrimitive call.

We are ready with the code inside our particle class and collection.

To create an instance of our particle collection, we simply use the normal VB way for it:

Dim myNewFire As colFireFX Set myNewFire = New colFireFX myNewFire.ParticleCounts = 150 myNewFire.ReLocate 400, 400 myNewFire.Begin And in our render loop we only have to use two lines of code to get the particle effect on the screen:

myNewFire.Update myNewFire.Render Check out the source (or the compiled exe) to see it working.

What do we have now?
Right now we have a simple particle engine. It does not have things like gravity, or attraction of particles to each other. But don't think you cannot do anything with this simple engine. You can simply change the "Reset" code a bit, and you can get truly wonderful things.

While trying to make some simple particle demos I was surprised at the power of this simple particle engine. Just changing a few numbers can generate wonderful particle collections. The source contains quite a number of the interesting patterns generated by code. Often a small change of a number can result in a completely new pattern.

The particle class and the collection class can easily be inserted into a 2D project and used instantly. Then you can try to experiment with the arguments or formulas in the Reset() method. There is a lot to experiment with. For example, with some of my demonstration patterns, changing the particlecount will change the entire look of the pattern.

I hope this tutorial sheds some light on particles and how to use point sprites in DirectX together with Visual Basic.

Almar Joling
www.persistentrealities.com

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