What was behind your decision to do as much as possible in ZBrush?
The production sketch we received from the client provided a rough guideline to the overall "look", and it was clear right from the start that Tarsem Singh wanted stone formations that looked realistic but definitely epic in scale, shape and form.
On a large scale, the cliff is stratified stone where some parts have been carved through erosion and others by man. It looks "blocky", sharp and menacing.
Close-up, the rock faces are jagged, chipped and shattered. There is lots of faceting and very few smooth areas.
We had shots that called for both broad wide angles and super close-ups... and some shots that had both!
The decision was to build a model that would rely massively on displacement maps that could handle the wide distance range between camera and rock face, while keeping the actual geometry of the 3D assets with a comparatively low polygon count.
How did you create the high-resolution alphas that you used?
As the displacement maps would be very detailed and there were a lot of blocks to be sculpted, I had to streamline my work by having alphas that needed no further sculpting refinement once I had projected them.
They were all derived from pictures of the rock faces built on set (to match the close-up areas) and reference images of wide-angle rock formations to cover wider areas the camera would never get close to.
Since I didn't get the chance to photograph anything for the particular requirements of creating ZBrush alphas, I had to do a lot of retouching on the references beforehand in Photoshop. Once I was pleased with the retouches, I would generate an initial displacement map through CrazyBump, and bring it into ZBrush to be tested on a flat plane.
I would tweak the settings in the Alpha palette until I got a combination of values that gave me an ideal displacement effect and the best edge softening for proper blending. Only then would I hit "make alpha" and add it to my library. Overall, I consistently used about 30 custom-made rock alphas for sculpting in SD and HD.
How did you prepare for the ZBrush step?
Using the rough 3D model that was sent to Image Engine as the basis for the cliff structure envisioned by Mr. Singh, I proceeded to build a more detailed "blocked" version of the same volume in Maya and tagged parts of it with real images of rock features that I thought would fit specific parts of the cliff - broader areas and also the ones closer to the live-action sets.
Once we got general approval for those, I began assessing how much ZBrush could aid in the sculpting and detailing of the model.
It was obvious the whole cliff could not be tackled as a single mesh. I set up a "segments table" that split the whole mass into 20 distinct blocks or areas.
The initial blocking of the cliff in Maya was done by simply scaling and piling dozens of polygon cubes together. These served as a "volume guide" for the formations that would bridge each live-action set area and matched the 3D model of the cliff used by Mr. Singh in his shots.
Once we decided how the whole cliff should be segmented, I built cages in Maya that surrounded the cubes that formed each cliff chunk and sent both cages and blocks to ZBrush. I then shrink-wrapped the cages onto the grouped cubes, creating a single mesh that matched the intended volume and could be refined further.
For each block, I pushed the subdivisions up to anywhere between two and three million polygons for initial sculpting. For parts that required the highest amount of detail, I would sculpt further using the HD Geometry feature in ZBrush. I would then export 4K 32-bit displacement maps for each block and reassemble the whole cliff in Maya to be rendered in 3Delight.
I created a custom library of alphas based on an extensive list of rock formation references that matched the types of rock that suited each area. Most of these were very hi-res, ranging from 2K to 3K. The time spent on these alphas was invaluable to speed up the sculpting process and nail a particular look of rock, which could be consistent across the twenty segments.
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