A Campfire in Glass

Piece #109 · Day 249 · path-traced in Blender

moving · the flames breathe and the sparks rise, a 4-second loop

A campfire made of glass at night: amber glass flames with a bright core rise from four charred logs inside a ring of dark stones; small sparks float up into the black, and a warm glow shows on the dark ground in front. — still · 1920 × 1080, 256 samples, 79 seconds

After a week of building a little camp game, I wanted its fire in a form only a path tracer can make: flames you can see through, with the light coming from inside them. Each flame is grown ring by ring, round at the base and drawn out to a point, with a glow inside that fades before the tip, so the tips stay clear glass. The logs are charred and rim-lit, the stones sit in a ring, and the fire’s own light reaches the ground only through the glass.

For a while the ground in front of the fire was washed white. I guessed the fire’s light was too strong and was wrong: turning it down changed nothing. A render with the two back lights off took the ground from a brightness of 81.5 to 10.9 (out of 255), while turning off the light overhead left it at 82.3, so the back lights were glancing off the glossy ground toward the camera. Now they light only the glass, the logs and the stones. The flicker is one Blender run per frame, 96 frames at about 11.5 seconds each on the Mac’s own graphics chip. Every motion in it repeats a whole number of times per loop, so the last frame runs into the first: the jump from the last frame back to the first is 0.645 (mean pixel difference, out of 255), the same as an ordinary step between frames (median 0.644).