Conversion notes:

Frames were exported from Cinema4D using the pbrt exporter. (It's important
to change the logging level in the export dialog to "Error", since
otherwise many messages are generated and it seems that c4d has some sort
of O(n^2) thing going on there, since exports slow to a crawl.

Geometry fiels were then converted to use PLY meshes using "pbrt
--toply". There are many repeated meshes (both shared across multiple
frames but also identical meshes within a single frame. These were boiled
down to unique messages using the following go program, which prints a
script for "sed" to stdout.

package main

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io/ioutil"
	"os"
)

func main() {
	for _, fn := range os.Args[1:] {
		contents, err := ioutil.ReadFile(fn)
		if err != nil {
			fmt.Fprintf(os.Stderr, "%s: %v", fn, err)
			continue
		}
		sum := sha256.Sum256(contents)
		str := hex.EncodeToString(sum[:]) + ".ply"
		fmt.Printf("s/%s/%s/g\n", fn, str)
		err = os.Rename(fn, str)
		if err != nil {
			fmt.Fprintf(os.Stderr, "%s: %v", fn, err)
		}
	}
}

The pbrt geometry file that --toply emitted was then updated to use the
hashed file names generated by the go program:

% sed -f out.sed < ply-geometry.pbrt > g.pbrt

Note that the sed script has thousands of lines and sed seems to be quite
slow--it takes many minutes to run the sed script, even though the geometry
file is only around 4.5MB.

To add all of the light sources, use the lights.sed script:

% sed -f lights.sed < g.pbrt > g-lights.pbrt

Finally, rename g-lights.pbrt to something sensible and update the Include
in the main pbrt file for the frame to use it. More generally, replace
everything after the Film directive in the generated pbrt file up to the
geometry Include with the text from one of the other frame files.

Whew