Rational rendering and floating bar(iquilezles.org)
iquilezles.org
Rational rendering and floating bar
http://iquilezles.org/www/articles/floatingbar/floatingbar.htm
6 comments
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I was hoping the author went full circle to reinventing floating point, but this is a great writeup of the process. Really good motivation.
Re-inventing floating point format would be missing the point (edit: i honestly did not see that pun sorry).. floating point formats cannot represent all of the rational numbers that a fraction could using the equivalent bits e.g 1/10 can be represented in base 10 decimals but not in base 2 float (where it has a repeating significand much like 1/3 in base 10), which becomes a problem when doing further math on it, try: 1/10 + 2/10).
Both methods result in "holes" bellow zero (necessarily of course), but they do not fit inside each other. However within a limited set of operations, rational numbers (within a capable format, and with necessary operators) have some interesting properties over floating point math that makes them desirable, i.e not loosing precision (unless overflow due to encountering large primes), and they are less expensive (built on integer math).
What he did do, is make a rational number bit format (floating bar) which stores everything as fractions, it's quite elegant really and anyone familiar with IEEE754 will see the similarities.
Both methods result in "holes" bellow zero (necessarily of course), but they do not fit inside each other. However within a limited set of operations, rational numbers (within a capable format, and with necessary operators) have some interesting properties over floating point math that makes them desirable, i.e not loosing precision (unless overflow due to encountering large primes), and they are less expensive (built on integer math).
What he did do, is make a rational number bit format (floating bar) which stores everything as fractions, it's quite elegant really and anyone familiar with IEEE754 will see the similarities.
Floating point is also built on integer math, and cheaper operations at that.
Ultimately yes, but I'd be interested in a side by side comparison, without FPUs. Also as explained in the article, divide (which is expensive float operation) is as cheap as integer multiplies with fractions.
It's very rare not to need GCD reduction, though; multiplying values burns through bits extremely fast. Only doing factor-2 reduction isn't infeasible but you'd be further damaging an already iffy type.
> It's very rare not to need GCD reduction, though
Yes good point, I did wonder this... it's easy to look at the individual operations and think "that's cheap" but holistically, the GCD which I assume is expensive could make the overall performance benefits moot. It might be optimisable to not do it unnecessarily after _every_ operation though?
Perhaps then the only advantage in terms of hardware use is that it can be implemented on a CPU with only ints e.g small micro-controllers with no FPU or int divide. I think that by comparison implementing a soft FPU would be impractical, still niche I guess.
Yes good point, I did wonder this... it's easy to look at the individual operations and think "that's cheap" but holistically, the GCD which I assume is expensive could make the overall performance benefits moot. It might be optimisable to not do it unnecessarily after _every_ operation though?
Perhaps then the only advantage in terms of hardware use is that it can be implemented on a CPU with only ints e.g small micro-controllers with no FPU or int divide. I think that by comparison implementing a soft FPU would be impractical, still niche I guess.
I'm not sure, but I suspect your best alternative to 32-bit floats/floating-bar-rationals or 64-bit rationals when you have no FPU is a 64-bit fixed-point value with compressed floating-point representations if you need to store it compactly in memory.
> Exceptions to the rule of "operations on rational number produce rational numbers" are the square roots and the trigonometric/transcendental functions. For the later, I always express my opinion that if you find yourself doing trigonometry in the geometrical internals of your renderer, you are most probably doing things wrong
Is that because you can express a rotation (etc) as a matrix? If so, don't you need to be able to do the trigonometry exactly to create a correct rational matrix in the first place? What am i missing?
Or is it that you just don't need to do those sorts of operations at all inside the renderer?
Is that because you can express a rotation (etc) as a matrix? If so, don't you need to be able to do the trigonometry exactly to create a correct rational matrix in the first place? What am i missing?
Or is it that you just don't need to do those sorts of operations at all inside the renderer?
I assume (though I'm certainly not as knowledgeable as Inigo) that he's suggesting most functionality that uses trigonometry during rendering in some way tends to be performed using dot- and cross-products - which are calculated using only basic arithmetic operations - and therefore don't require slow mathematical function calls. If the actual value of a cosine is needed in some way, it will likely be calculated via the dot-product of two vectors, where one or both has unit length.
As far as construction of rotation matrices goes, most of that will be baked before rendering a frame, so expensive trigonometric calculations and their conversion to rationals will occur ahead of time and therefore much less frequently than anything done per pixel, improving performance.
In computer graphics, because a great many calculations need to be done for each pixel to be displayed, use of expensive functions tends to be avoided. In the literature I recall reading a number of triangle-ray intersection algorithms where each breaks down precisely how many adds, multiplies, divisions, etc are performed because the goal is to minimize all these things. If you are intersecting 10 triangles per pixel at 2 million pixels per frame, a single multiply per intersection becomes 20 million multiplies per frame. By contrast, recalculating the camera matrix using expensive functions should only happen once per frame.
As far as construction of rotation matrices goes, most of that will be baked before rendering a frame, so expensive trigonometric calculations and their conversion to rationals will occur ahead of time and therefore much less frequently than anything done per pixel, improving performance.
In computer graphics, because a great many calculations need to be done for each pixel to be displayed, use of expensive functions tends to be avoided. In the literature I recall reading a number of triangle-ray intersection algorithms where each breaks down precisely how many adds, multiplies, divisions, etc are performed because the goal is to minimize all these things. If you are intersecting 10 triangles per pixel at 2 million pixels per frame, a single multiply per intersection becomes 20 million multiplies per frame. By contrast, recalculating the camera matrix using expensive functions should only happen once per frame.
Everyone should use vector methods and shy away from transcendental functions to the extent possible.
It is usually (but not always) possible to reframe problems in such a way that exponentials (including sine/cosine/tangent) and logarithms (including arcsine, arctangent, ..) are unnecessary.
Vector methods are much faster, have fewer weird edge cases, are less subject to rounding error, etc.
As an example: if you want to store a planar vector, use a pair of coordinates instead of an angle measure and magnitude. If you want to store an 2d orientation or rotation, still use a pair of (x, y) coordinates on the unit circle instead of angle measure (which is a logarithmic quantity). If you want to reduce your pair of coordinates to a single number, use the stereographic projection, which only requires division.
It is usually (but not always) possible to reframe problems in such a way that exponentials (including sine/cosine/tangent) and logarithms (including arcsine, arctangent, ..) are unnecessary.
Vector methods are much faster, have fewer weird edge cases, are less subject to rounding error, etc.
As an example: if you want to store a planar vector, use a pair of coordinates instead of an angle measure and magnitude. If you want to store an 2d orientation or rotation, still use a pair of (x, y) coordinates on the unit circle instead of angle measure (which is a logarithmic quantity). If you want to reduce your pair of coordinates to a single number, use the stereographic projection, which only requires division.
Would you mind expanding on the idea that an angle measure is a logarithmic quantity and that sin/cos/tan are exponentials (or providing a phrase I could google)? I've never come across a formulation/characterization like that, and it sounds useful...
You can break the exponential function into the part acting on a scalar and the part acting on a bivector, and then furthermore break those into even and odd parts:
exp(ρ + θi) = (cosh ρ + sinh ρ)(cos θ + i sin θ)
The angle measure in the above is the θ part. The “linear space” expression of the orientation/rotation is a 2-dimensional quanitity cos θ + i sin θ.
Or if you start with the 2-coordinate version x + iy, where x^2 + y^2 = 1, you can find the angle measure by taking the natural logarithm, imag(log(x + yi)).
Rotations are naturally a multiplicative concept. If you have a rotation a + bi and a rotation c + di, then you can compose them like (a + bi)(c + di) = (ac - bd) + (ad + bc)i (you might recognize this as the the “angle sum” identities from high school trigonometry class).
The idea of the logarithm is that you can convert a multiplicative structure into an additive one. So if you have rotations Q and R, you could normally compose them like QR, but as an alternative you can use a log-space representation (angle measure): QR = exp(log(Q) + log(R)).
A rotation inherently comes with an orientation. The i (“imaginary unit”) is in my opinion properly thought of as a bivector, oriented like a plane. When we express something as a scalar-valued angle measure, we have stripped the orientation out.
exp(ρ + θi) = (cosh ρ + sinh ρ)(cos θ + i sin θ)
The angle measure in the above is the θ part. The “linear space” expression of the orientation/rotation is a 2-dimensional quanitity cos θ + i sin θ.
Or if you start with the 2-coordinate version x + iy, where x^2 + y^2 = 1, you can find the angle measure by taking the natural logarithm, imag(log(x + yi)).
Rotations are naturally a multiplicative concept. If you have a rotation a + bi and a rotation c + di, then you can compose them like (a + bi)(c + di) = (ac - bd) + (ad + bc)i (you might recognize this as the the “angle sum” identities from high school trigonometry class).
The idea of the logarithm is that you can convert a multiplicative structure into an additive one. So if you have rotations Q and R, you could normally compose them like QR, but as an alternative you can use a log-space representation (angle measure): QR = exp(log(Q) + log(R)).
A rotation inherently comes with an orientation. The i (“imaginary unit”) is in my opinion properly thought of as a bivector, oriented like a plane. When we express something as a scalar-valued angle measure, we have stripped the orientation out.
Thanks for this - i have studied very little maths, so even this perhaps quite basic stuff is like magic for me!
I love the revelation that log/exp shifts between Cartesian and (log-)polar meanings of complex numbers. That's enormously satisfying for some reason.
I note that the pattern in multiplying rotations also comes up if you do it with matrices:
I love the revelation that log/exp shifts between Cartesian and (log-)polar meanings of complex numbers. That's enormously satisfying for some reason.
I note that the pattern in multiplying rotations also comes up if you do it with matrices:
a -b c -d ac-bd -(ad+bc)
b a . d c = ad+bc ac-bd
Taking 3D rotations into log space gets tricky. 3D rotation is not commutative, so if you have two quaternions Q and R, you can’t in general say that QR = exp(log(Q))exp(log(R)) = exp(log(Q) + log(R)). That identity only holds if the bivector (“imaginary”) parts of Q and R are oriented the same way in space, i.e. are scalar multiples of each-other. Otherwise in general exp(log(Q))exp(log(R)) ≠ exp(log(R))exp(log(Q)).
The log space version is still useful though if you want to e.g. take the nth root of a spatial rotation.
The log space version is still useful though if you want to e.g. take the nth root of a spatial rotation.
I think the idea is that you generate the matrix outside the renderer, then pass the matrix to the renderer. Euler angles are evil[0], and if you're using them (especially inside the renderer) > you are most probably doing things wrong.
0: https://www.sjbaker.org/steve/omniv/eulers_are_evil.html
0: https://www.sjbaker.org/steve/omniv/eulers_are_evil.html
Very cool article but the scroll-jacking makes this painful to read on an iPhone. :-(
Using chrome on the desktop I see normal scrolling behaviour, what exactly are you talking about?
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Inigo is a pioneer living decades ahead of the field. Utterly futuristic approach to graphics.
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Quilez's concern with how the number fits into 32 or 64 bits may be necessary for performance, but using arbitrary precision numerators and denominators is the only way to achieve the robustness he was initially pursuing.