Lines, both line-list and line-strip will be rendered at a single pixel thickness. You can texture these, but obviously you'll be a bit limited as to how much of the texture you can display.

Trying to create thicker lines isn't going to be simple - you'd need to do some sort of billboarding (point sprites can't non-uniformly scale afaik). At a guess you could do some sort of VShader based billboarding by packing extra per-vertex data and getting it to construct it accordingly (where it can perform trivial screen alignment).

If you were to pack an origin and direction vector into each vertex (two float3's) and then assigned an "ID" to each vertex so you could identify which of the 4 vertices it is - e.g. "top left", "top right"...

Transform the origin and (origin+direction) vectors into projection space and you can then extrude the 4 vertices accordingly. They should then be screen-aligned and follow your particle.

Downside is that it'll require you to pack more per-vertex data and send 4 vertices per particle and have a more complex shader... but I'd imagine it'd still be good enough for most uses [smile]

hth
Jack

Well if you're happy with single-pixel lines then go for it. At the very least it might be worth trying before you invest time/effort into a more complex solution...

I can't remember what HL2's effect looked like.

Jack

Having looked at the image, I'd still be inclined to think it was some sort of quad-based particle. It might well have an alpha channel (to give each spark that slight glow) but doesn't appear to have a diffuse texture as such - just a solid orange/yellow colour. Then again, the glow could be independent of the particle system - the top of the car roof is also showing a slight glow, thus it could just as easily be regular post-processing...

Whether their method is the same as the extrusion I suggested before is difficult to tell - I'm sure there are several (and possibly better) ways to achieve the same result...

hth
Jack

It might be worth asking the folks over in Graphics Programming and Theory if they've got any clever mathemagical/graphical algorithm for this sort of thing.

My take would be something along the following lines:

Each particle requires 4 vertices (Only D3D10's pipeline can amplify), so the trick is packing the data up.

I'd go with defining the vertex something like:

struct ParticleVertex{    D3DXVECTOR2 texCoord;    D3DXVECTOR3 particleOrigin;    D3DXVECTOR3 sparkDirection;    float       particleSize;    float       sparkLength;};

Define texCoord such that you get a (0,0)-(0,1)-(1,0)-(1,1) type set up.

In the vertex shader you can transform particleOrigin to get a projection-space position, and you can use an inverse-transpose to transform particleDirection (same rules as transforming a normal vector).

You can now generate the output POSITION by taking the transformed particleOrigin and extruding it according to the texCoord offsets (you might want to change 0..1 to be -1..+1 though). Check the SDK's point-sprite documentation for details - this step is essentially a manual implementation of what they do [wink]

The trick is to extrude only in projection-space X,Y and just copy-through the transformed Z.

Use the particleSize field to control the extrusion in the previous step - possibly also factor in the depth (particleOrigin.z post-transform) so distant sparks get smaller.

At this point you'd have a square particle similar to what you'd get with PSprites.

The trick is now to extrude along the sparkDirection vector (once its correctly transformed of course). You could simply use the U/X texture coordinate as a scalar... that way the '0' end will stay where it is and the '1' end will get moved in the direction of the spark. Scale by sparkLength to alter this.


I'm pretty sure the above will work - because you're doing the amplification in projection space (which is effectively 2D for this purpose) theres no need for the normal awkward billboarding transforms. The difficulty will be getting the numbers to match up as you're not working in pixel-coordinates (unless you pass in the RT height/width and scale accordingly).

If you do try it, let me know - I'd be curious to know if this actually works [smile]

hth
Jack

Billboards:

http://www.mvps.org/directx/articles/view_oriented_billboards.htm