规模在OpenGL中是否重要？

Question

Does the scale of a scene matter in OpenGL? By scale I mean, for instance, drawing a 1 unit cube but setting the camera position 1000 pixels away vs setting the camera 100 pixels away from a 0.1 unit cube? I guess, firstly, am I correct in thinking this would yield the same results visually? And if so, does anyone know how either would effect performance, if at all?

My hunch would be that it would have no effect but a more definitive answer would be nice.

Answer 1

It doesn't matter except for imprecisions using floating point arithmetic. So try not to use super small or large numbers.

Answer 2

If you mean 2D this may not matter. Image created both ways may look the same.

Answer 3

"Setting the camera" actually only changes the transformation matrix the vertices are multiplied by, so after the transformation is applied, the vertex positions are the same. There might be minor differences resulting from the imprecision of floating-point values.

If the camera has a constant near and far clipping distance, the resulting values in depth buffer will differ, and one of the cubes might get outside of the clipping plane range, which would make it appear different / not at all.

Answer 4

You should know that floats have two modes: normalized and unnormalized mode. For values below 0, you will have more bits and more precision. So, it is better if you keep everything between 0 and one.

Also, looking to how the exp/mantissa works, the difference between numbers when a single bit changes, is bigger when you move away from zero, and things will start to make popping when you move very far from origin.

Answer 5

Yes, it matters. Not because of OpenGL, but because of the nature of floating point operations - floating point precision degrades at larger magnitudes. Try and normalize your values such that you have values closer to zero. That is, don't make a planet object 1,000,000 units wide just because you want a planet that's 1,000 KM in diameter.