Car Wheel
Tire Texturing - Tread
Tire treads are very hard to do. At least I used to think so. I don't know why, but it had something to do with thinking the detail had to be modeled since I couldn't texture the tread to side wall transition and hated the look of a plain old bump map running around the outer surface...looking like it had just been glued on.
I've come up with a technique that gives decent results though, or maybe I've just softened my standpoint on the bump mapping... but either way, I'm happy with my technique. It's not too difficult either!
This is the prime ingredient for the tread material. I have fashioned a custom tread bump map that is meant to seamlessly wrap around the outside of the tire when applied with cylindrical projection.
The color channel is just white right now so we can see the texture using real time texture mapping, but will be turned off completely for the final product, leaving bump channel only.
Below is a small view of the tread map. For the texture in lower resolution, check here. For high resolution tread, check here.
We return to the TireTread selection set with this technique, you may remember making it at the end of the section on tire shaping. The tread material is going to be applied cylindrically but restricted to this texture.
The goal is also to manipulate some of this texture down onto the side wall of the tire so that it appears to be more than just a 'skin' running around the outside, but actually appears to have more depth than it really does.
Viewed from the top, the first thing to do is scale down the selected polygons width in the X axis direction. This is so that we can apply the texture and not have it interfere with any overhangs or blunt edges on the rest of the tire geometry. I found it just worked best when mapped straight to a portion of the tire that was clearly not in conflict with any other part of the tire. Also, when I scale these polygons downward, I'm aligning them to gridlines for easier numeric entry during texture application. The selection set is roughly 6 gridlines wide, which coincided perfectly with a texture map width setting of 30m. Go figure... :-)
Next, a couple of knife cuts are made near the edges of the tire. These are going to be used to 'pin' the corner of the tread texture in place while the old edges move around the walls of the tire to bring the tread texture down onto the side wall. You'll see in a minute!
For good measure, redefine all of these polygons as the TireTread selection set.
The tread material can now be applied to the tire object with settings like these. Make sure the material is set to Cylindrical projection, Mix with other textures (very important as it will be merely a bump material and will need to 'mix' that property with the base rubber color visible material). Also, tiling should be ON. When we add the Stick Texture tag later, without tiling a gap will appear between the end of the texture and the start. With tiling, this gap does not occur.
The bank rotation needed to be at 90 degrees to project properly... and the size parameters of the material are such that at 30m in the Y direction, it fits more or less perfectly on the polygon selection when scaled down in the previous step.
When these values were entered, the result was not immediately apparent. The texture tag was inserted into the object's tag hierarchy at the end, but due to the fact that it is a texture to be mixed with another, it's placement in the tag sequence seems to be vital to its success.
Below, you can see the tread texture placed immediately after the base rubber texture not only for organizational purposes, but also for the reason that if the tread appears before the base rubber, it will not work correctly.
Also, the Tread selection set tag has to occur before the texture tag that is referencing it.
Now, apply a "Stick Texture Tag" to the tire object, make sure it is active, and hit 'Record'. This makes the tread texture dependent on the tread geometry.
You need XL 6.1+ for this feature.
Next, using the polygon selection for the tread (this should have remained untouched since we last used it) Scale it back up in the X direction so that the edges of the tread selection are then aligned with the maximum width of the tire (side wall polygon faces).
From here the idea is now to 'wrap' those small polygons along the edge of the tire around to the top of the side wall so that the tread texture appears there as well.
This will be a case of selecting the outer edge points on both sides of the tire object, and merely scaling them down constrained to both the Z and Y axes so that they move radially inward toward the center of the tire.
This process is illustrated below.
The points that make up the very edge of the tire tread where it meets the side wall are selected.
Make sure you get both sides of the tire! There are 16 edge points in all.
Now use the scale tool, and lock the X axis, so that scaling is constrained to the Z and Y axes only.
Then, click and drag in the editor to 'scale' these points down so that they begin to move inwards towards the center of the tire (but should not move along the X axis!!!).
When the points get within close proximity of the first row of polygon edges of the side wall geometry, it's time to stop!
You don't want to get too close, since an unsightly ridge will develop if you overlap the polygons.
A look at our tire rendered shows the edge of the tread now wrapping around to originate from the side wall of the tire.
That's what we want, but we'll have to adjust the polygons remaining on the tread surface to compensate for this edge moving inwards... right now, it looks like too much taper is occurring between the tread (which is now very narrow) and the side wall.
Select the polygons that make up the TireTread selection set, and deselect the polygons that were on the outer edge where the tread wraps to the side of the tire.
Using the X axis scale handle or scale tool constrained to the X axis, scale these polygons up until they are about even with the side wall of the tire.
You can see the tread widens out more like a tire now.
Now a render of the tire shows that the edge is better defined and the tread isn't to narrow. Additionally, the tread wraps to the side wall fairly nicely. However, when we go back to the Tread texture and simply disable the color channel so that it is only a bump channel that affects the base rubber texture, the render starts to look a whole lot like a tire!
Heh heh... okay, that's perhaps a bit too aggressive of a tread for a Chevy Bel Air of all things, but I'm from Canada, and we NEED traction!!! ;-)
Hubcap, anyone?
