5. Surface build-up principles

5.1 Apex-less surfaces

Surfaces, which are built symmetrically over the X=0 axis of the car and which do not have an edge or hard peak in the middle, are apex-less surfaces. Windscreen, roof, and back window usually have no apex. Bonnet, front and rear end sometimes do.

In principal these surfaces should be built as X axis symmetrical patches with symmetrical CV layout across X and the pivot at the Y=0 axis. Using this building method assures, that you never get tangent breaks or loose G3 conditions for surfaces that are built across the middle of the car .

Notice:
Take care to move the CVs always in a symmetrical fashion across X=0, otherwise the surfaces will loose their internal symmetry!

5.2 Axis symmetrical (mirrored) surfaces X = 0

Almost all car exterior surfaces are symmetrical so we just build them once and copy them over to the other side (sidewalls, wheel arches, fenders, etc.). Mirrored surfaces have to have the same relation to their adjoining surfaces as the originals.

5.3 Start rebuilding the greenhouse

A car is basically divided into two zones: Beltline upper zone and beltline lower zone or “greenhouse” and “body”.
The beltline upper part of the car is surrounded by the windows, therefore it is also called „greenhouse“.
Add another layer and name it „wip“ for „work in progress“. Then give it a blue colour and activate it. Hide anything other than the windscreen mesh (using layer categories) and zoom in as much as possible into the top window.
Drag Create Plane from your Surfaces tool bar into the Shelf-Set. Activate it and type „0“ into the Command bar, the plane will then be located on the X axis in your top window. Scale and move the plane until it is slightly bigger than the windscreen. Don’t move it in Y-direction otherwise it will loose its X-axis symmetry.
Turn on the CVs and Hulls of the plane. Change the surface degree from 3/3 to 1/1, then switch to the right window and move the activated plane up in Z-direction right under the windscreen.
Activate the CVs on one end of the plane and move them in Z- and X-direction to give the plane the same direction as the windscreen.

5.4 Evaluating the rebuild tolerance

The relationship of the given rebuild tolerance and the surfaces achievable under Class- A conditions is all too often problematic.
The tighter the tolerance settings are, the better the surfaces of the scanned original model have to be, in order to build surfaces at Class-A quality.
A common tolerance to rebuild a full scale size clay model of good quality is 0.25 mm.
Looking at an entire exterior, I have never seen a hand-made clay model precise enough to do all digital surfaces within the expected rebuild tolerance achieving Class-A surfacing quality.
The model we use in our exercise was originally a hand-made quarter scale clay model.
This means all inaccuracies have been multiplied by four when the model was enlarged. In this case I recommend a sensible rebuild tolerance for the primary surfaces of 1.0 to 3.0 mm already anticipating that there may be areas, where we do not want stay within these tolerances in order to achieve superior Class-A surfacing quality.

5.4.1 Cross sections

In Studio the Cross Section tool provides a simple and fast way to evaluate the rebuild distances between created and given data. It automatically creates additional layers for these sections in your layer bar. The X section layer is coloured in red, the Y section layer in blue and the Z section layer is coloured in green.
Drag the Cross Section icon into your Shelf-Set and open it. Make sure you use the Section Type Axis Aligned and change the Step size for the sections of the windscreen to “X = 200”, “Y = 200” and “Z = 50”. Now activate the xsect tool and click at the windscreen mesh and the surface below , press Go and look at the sections of the surface and mesh. Also look at the layer bar.
You have new „X,Y and Z section“ layers and a layer called other sections.

Tip: The Cross Section tool cannot recognize whether sections of planar surfaces belong to the X,Y or Z layer so it automatically creates a layer called „other sections“ and adds the unrecognized sections in there.

To overcome this problem: Over-crown in two directions the surface you want to cross-sect before using the Cross Section tool. Studio then recognizes the direction of the sections and sorts them into the correct layers.
Please practice both variations and add the three new layers X,Y and Z to the „work in progress“ layer category!

5.4.2 Deviation Map

Drag the Deviation Map icon into your Shelf-Set. Open it and set the Acceptable Distance to 1.0 mm. Give it a go to evaluate the rebuild distance of the windscreen as our first primary surface.
In the Perspective Window display just the windscreen mesh and the surface underneath. Activate both surface and mesh and accept them. The green area of the surface tells us where we are within the rebuild tolerance of 1.0 mm, the other colours indicate whether and by how much we are above or below the rebuild tolerance.
Now go into the CV move option of the Modelling-Shelf and activate CV move/XYZ/HULL, lock X and Y and activate the two middle v direction Hulls. Move them in Z-direction and see the colour change in the deviation map.
The Deviation Map tool provides very fast and convenient feedback of the deviation range between scanned and rebuilt surfaces. In my opinion it is a very good tool to analyze where we are with our model regarding the „RBT“ and it is „spot on“.

5.4.3 Cross section Control

While the Deviation Map just analyzes the distance between given and created data, the Cross Section Functionality tells you something about the quality and behaviour of your surfaces. Set the Curvature Scale to 10 and activate it.
The waves of the green combs are a boosted view of the curvature condition of your surface at each section.

Notice:
Class-A quality surfaces do not have any kinks or heavy waves in their curvature combs.

5.4.4 Dynamic SectionS Control

This tool generates sections similar to the Cross Section tool but highly flexible, the Dynamic Section tool allows you to cut and blend surfaces in any direction or position.
It can also generate construction planes or transform the sections into section curve geometry.
With the Visual Clip function of Dynamic Section Control we are able to move sections in real time through given and created surfaces (e.g. package information, meshes, surfacing data). At the same time it is also possible to hide one side or the other by using the Flip option of the Visual Clip function.

5.5 Working with Direct History-Rebuilding Fillets

5.5.1 Advanced options of Surface tools

Now it is time to build the first secondary surfaces exploring the advanced options of the Surface Tools. We build up the transition of windscreen and roof in three different ways.

5.5.2 Surface Fillet tool usage for secondary surfaces

Display the primary windscreen and roof surface and also the roof mesh. Open up the Advanced Surface Fillet option box and set the Construction Type to Chordal, the Section Type to Curvature and Span placement to Free. The Flow Control to Edge Align.
Now we create a fillet between the two primary surfaces.
Cut X sections through all displayed objects and increase the Chordal Distance until you get a fillet as close possible to the mesh. You might end up with a value around 600.
You might have recognized, that with these options the Surface Fillet tool maintains the curvature condition of the fillet. Therefore, it automatically generates more spans the bigger the fillet becomes.
To avoid this, tick the Explicit Control box. Then you loose the curvature condition in relation to the roof surface and even worse, we get a gap!
Increase the U-Degree to 6 and we are back to curvature conditions without gap!
This is the best result we can achieve with the Surface Fillet tool itself.
Analyze the result with the Curvature plot and the Stripe shader. Ok it looks quite good.
However, looking at the CV layout, the distribution of the CVs is not in accordance with Class-A surfacing standards.

5.5.3 Align Tool usage for secondary surfaces

Now that we know the size of the fillet, we know where the patches of windscreen and roof start to blend into the transitional surface. That’s the position where the final primary surfaces should end. Therefore, we use the Extend tool with the Merge box ticked to shrink back the windscreen and roof surfaces, so that the corners of these surfaces join up with the corners of the fillet. Then hide away the fillet surface and stretch a skin surface between windscreen and roof and change the weight of the skin surface from 5u 3v to 5u 5v.

Open the Align tool and its Advanced options and set Continuity to Curvature and the Align Type to Colinear. Switch on all Control Options and align one side of the skin to the windscreen, the other to the roof surface.

5.5.4 Explicit Control functionality for surface square and surface rail tools to
create secondary surfaces

Another common way to create such surfaces is to stretch 5 degree curves between the big slaps. Give them curvature conditions to the side boundaries of the big slaps and generate a Square or a Birail surface with curvature conditions in relation to the big slaps with the ticked Explicit Control option on.

Finally, analyze the result of the three different ways to build such surfaces and review the different conditions of the surfaces using the Stripe Shader and the Cross Section Control.

5.6 Fillets and Flanges

5.6.1 Surface Fillet Tool for Tertiary Surfaces

Once the primary and secondary surfaces are finished we start to build up the tertiary surfaces. The workflow is very similar to 5.5.1, except that the fillets are created over more than two adjoining surfaces and the result is much more acceptable for tertiary surfaces than for secondary surfaces.

We do this on the example of the fender and bonnet surfaces.
Start with the same settings we used for the secondary surfaces between roof and windscreen.

Select the displayed surfaces and open the Surface Fillet Control in Advanced mode. Select the four neighbouring surfaces of the front fender and make sure the arrow points towards the inside of the car and then press the Accept button. Select the three fender surfaces, point the arrow to the car's inside and accept it too.

Set Construction Type to Chordal, Section Type to Curvature and the Chordal Distance to 15.0 and go for it.

Using the Default setting of the Flow Control option for the Surface Fillet Control we will get an untrimmed surface at the start and finish of the fillet group, i.e. on fender and bonnet, but we achieved curvature continuity for all fillets.

Switch the Flow Control option for Start and End to Edge Align and press Recalc. Now we have all surfaces trimmed but a little gap problem where the fillet group starts.

This already is a good surface quality for tertiary surfaces but we still want to improve it some more. Analyzing the result with the Cross Section Control we see a relatively short lead-in of the fillet group.

Not perfect for class-A on tertiary surfaces. Activate the just created fillet group and open the Surface Fillet Control again.

Tick the Use Peak Curvature box, change the Peak Radius Ratio to 0.3 and do a recalculation again.
In general, this workflow provides very good results for tertiary surfaces but check the results carefully.

This workflow can provide surfaces with sloping Hulls, which we then have to rebuild individually using the Surface - Square or Birail tool.

5.6.2 Draft/Flange functionality to build UP Rank Four surfaces

In the latter part of the Class-A process the fillets around body parts such as doors, bumpers, bonnet, tailgate, lamps and even wheel houses are usually built on the basis of flange surfaces you receive as engineering information from your studio engineers.
Mostly these engineering surfaces are not built to Class-A surface quality standards and have to be rebuilt in a better quality to be able to do the neighbouring surfaces in the required quality.
In our case we have to create such surfaces ourselves based on the information we get from our scan data.
The door gaps of the sidewall are located between flanges built in Y-direction with small fillets to the sidewall. These fillets are rank four surfaces.
In side view, rebuild the door gaps of the scan data as 5 degree curves with curvature continuity to each other, offset the curves with a gap distance of 4 mm and project them onto the sidewall. Open up the Advanced Draft/Flange Surface Options, set Construction type to Draft/ Angle=0 / Surface Depth=20/ Pull Direction User defined / Presets =Y and give it a go. Then do the fillets as described in 5.6.1 with the difference, that the connection to the flanges is acceptable with tangent continuity only.