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grim_d
04-24-2008, 05:07 PM
hi i am trying to create skidmarks for a car but have a couple questions.

i have put an emitter on a null object and parented it to the wheel that skids. i have added HV to the emitter. i have set the start time but cannot figure out a way to set a stop time? how would i got about it?

CUrrently it continues to make the mark when the wheel is no longer skidding.

also any tips on creating more "square" skidmarks, the ones i have right now are kind of a black smudge.

Thanks.

SplineGod
04-25-2008, 01:25 AM
Use an envelope on the birthrate.

grim_d
04-25-2008, 06:52 AM
thanks that worked a treat, what about making them more square, any ideas?

SplineGod
04-25-2008, 08:21 AM
Use an image map that is square shaped and apply it as a clip to the HV sprite.

vfxwizard
04-25-2008, 11:08 AM
Use an image map that is square shaped and apply it as a clip to the HV sprite.

SplineGod, is there a way to get this to work? I think sprite clips are always clipped to a circular particle. I usually use a square alpha channel smaller than the image and set the sprite clip to embedded to get sharp edges.



As for the OP's request, I'd like to share the simplest way I have found to produce decent skidmarks like those in the first picture on this page:

http://www.car-accidents.com/pages/accident_story/5-16-04.html

Instead of using a single emitter parented to the tire, create an object and add two vertices where the external tire marks should appear.

Then create another vertices-only object, with points arranged like the tire tread.

Parent the emitters to the tire, set parent motion to 0, set nozzle to Object Vertices and generate a lot of particles per frame.

Since we have a lot of particles, we can set particle size to a very small value and get sharp lines. Using a turbulence texture in world coordinates for particle dissolve allows to randomly fade out the marks. Of course this can be controlled with nulls or painted maps to get a specific look.

The reason two or more emitters are needed is to be able to apply different fading to different areas, just like in the linked reference picture. Another turbulence on particle size for the outer marks provides the irregular contour.

Attached is a quick test with some explanations. Hope this helps!

clagman
04-25-2008, 12:40 PM
Nice! Can you give an example of how to do this on a sandy surface. I do a lot of desert scenes and I can't seem to get past the old drag the object behind with world coords on the texture/bump bit or revealing the track object based on reference (for turning curves). Gets pretty tedious with lots of vehicles moving in different way for different scenes. Ideas? Perhaps use a HV surface rather than sprite?

tbennet
04-25-2008, 12:46 PM
render out a pass of just the particles and use it as a displacement map on the sand object...

walfridson
04-25-2008, 01:03 PM
You can also use denis marker pen http://pagesperso-orange.fr/dpont/plugins/textures/MarkerPen.html

tbennet
04-25-2008, 01:12 PM
Actually this is the one to use...http://pagesperso-orange.fr/dpont/plugins/textures/ParticlePaint.html

clagman
04-25-2008, 01:20 PM
Great ideas! I think the HV pass idea has merit but I'm not sure how I would go about rendering the HV pass (I suppose over head camera). I'll definitely try it out and see what I can come up with. I will try the market pen too and see if it works with 64 bit.

tbennet
04-25-2008, 01:23 PM
you can use front projection or you can use the bake camera and bake in to a UVmap.

tbennet
04-25-2008, 02:08 PM
here is one done with partical paint plug, sticky motion, and nodal displacement.
no prerender needed...

clagman
04-25-2008, 02:49 PM
I seem to be missing something. I have the Rman shader collection (including particle paint) but they are only available as procedurals in a texture channel (not nodes) which I assume was channel B on the Add node.

tbennet
04-25-2008, 02:53 PM
There is 2 versions of the rman shaders... nodal and the regular texture version.
I used the nodal version in the displacement of the ground object...

You may have to calculate first...

clagman
04-25-2008, 02:58 PM
Arrghhh, figures I would overlook that. Thanks a bunch, this seems to be the ticket.

vfxwizard
04-26-2008, 02:46 AM
Perhaps use a HV surface rather than sprite?


Using surface is a good idea! I did a quick test and it turned out better than expected.

Of course you won't get a deformation on the ground surface, the techniques suggested in the other replies handle that aspect of the problem. But using surface HV you can get a reasonable effect in long shots or augment the main deformation piling sand at the marks' sides.

Reference images were:

http://www.inmagine.com/mbb1438/mwi14380007-photo
http://pro.corbis.com/search/Enlargement.aspx?CID=isg&mediauid=44C5AD66-9840-4A98-8C0E-5344DC134E7E

Attached image shows the main emitter's profile, I should have used more particles as it breaks in close up.

To define the external ridges seen in the second reference image, keep the particle size low and assign two different blending groups to the particles so particles from each emitter mix between themselves but not with those in the nearby paths.

The same sand texture is projected on the ground plane and the particles in world coordinates. For 3D dunes matching textures is probably the hardest part.

With a little more tweaking it might be usable.

tbennet
04-26-2008, 06:44 AM
You could use particle paint with your emitters for the displacements or bump and it would give better results in my opinion... You can get way more detailed then the one I posted...

vfxwizard
04-26-2008, 07:33 AM
You could use particle paint with your emitters

Yup, particle paint is very nice (but I think it can't do network rendering). What I liked about clagman's idea is that this way you can add mass to the surfaces instead of -or in addition to- altering them. And experimenting is always fun.

tbennet
04-26-2008, 09:13 AM
Yup, particle paint is very nice (but I think it can't do network rendering). What I liked about clagman's idea is that this way you can add mass to the surfaces instead of -or in addition to- altering them. And experimenting is always fun.

no it cant be network rendered, but it can be baked... I wasnt trying to down your method... I actually think I combo of both would work well.:thumbsup:

tayotain2
04-27-2008, 03:59 AM
never tested this but could be usefull... from dennis again :thumbsup:
http://pagesperso-orange.fr/dpont/plugins/nodes/nodes/Deformers.html#SplineMap