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Comment by dahart | Hacker News Reader

Note it's only voxely if you translate in all 3 dimensions. If you were far away in just x, but not y & z, you'd get a weird looking image that's slabby in x but detailed in y & z.

It's cool that the pbrt renders hold up in voxel form, like the model's still solid and the shadows don't freak out or anything.

The problem is fairly well known in film & games production. Artists, especially world designers, all know to model things near the origin and not far away because precision drops as you move away. They will also sometimes avoid modeling small things in small units like millimeters even though they might prefer it, because the units dictate how big your floats get, which in turn determines how fast you lose precision.

Here's the voxel prediction chart: https://en.m.wikipedia.org/wiki/IEEE_754#/media/File%3AIEEE7...

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> They will also sometimes avoid modeling small things in small units like millimeters even though they might prefer it

Scaling model from meters to mm only changes precision in model’s units, but the precision in real millimeters stays the same, so the units don’t matter.

I think they avoid millimeters because down the art pipeline people who’ll use the models expect meters, e.g. to place the stuff in a metric world without extra scale involved.

You're right that normally a more important consideration for production is consistent units.

The units do matter though, since the absolute float value is what determines your precision.

The author of this article translated by 200k units, so the precision loss is relative in this case. But artists might need to translate something 100 meters, so their precision loss depends completely on their choice of units.

> their precision loss depends completely on their choice of units.

If an artist needs to translate a model 100 meters off center, and the model is in meters, the graph you’ve linked says the precision will be 10^(-5) model’s units, which is 0.01mm. If the same model is in mm, the graph says the precision for 100*1000 will be 10^(-2) model’s units, which is the same 0.01mm.

As you see, it's independent on the choice of units.

How big can my world be in meters if I require at least 0.01mm precision and I model things in millimeters? Does that number change if I model in meters?

Because of how floating point works, it doesn't matter. The difference between modelling in meters or millimeters disappears in the exponents. What matters for precision is the mantissa, which is 23 bits in single precision floats. So you get ~16 million of your smallest unit before you're losing precision, counting the implied leading 1. You can double that to ~32 million by using the sign bit.

Edit: this translates to 320m if you want 0.01mm precision.

About 0.008 km^3, each coordinate between -100 and +100 meters.

The number's the roughly the same even if you'll model in km or imperial units.

Yeah, you're right. Brain fart. I'm mis-remembering production stories instead of thinking clearly about floats, despite having just linked the precision chart. :P

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