Re: [eigen] Euler-angles and partial reductions API design |

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*To*: eigen@xxxxxxxxxxxxxxxxxxx*Subject*: Re: [eigen] Euler-angles and partial reductions API design*From*: Benoît Jacob <jacob@xxxxxxxxxxxxxxx>*Date*: Sat, 19 Jul 2008 00:03:51 +0200

Hi again, I thought a bit further about all this in the shower: - about the cast to scalar for 1x1 size: the danger is if it will make certain exprs ambiguous, making compilation fail. For example: 2 * some_1x1_xpr this could be interpreted as product of matrixbase by scalar, or as first casting some_1x1_xpr to scalar and then doing plain old multiplication. I'm afraid the compiler will stop there saying it's ambiguous. - about the rotationX() name: i forgot you proposed the good idea that static functions should start with capital, so indeed RotationX(). Then I'm not a fan of the Rot abbreviation. - about the rowWise() / colWise() names: since -wise is just a suffix there's not need for capital, like in cwise(). So: rowwise(). Cheers, Benoit On Thursday 17 July 2008 15:11:03 Gael Guennebaud wrote: > Hi list, > > I have a couple of questions regarding function names and API design, > all suggestions are welcome. > > > Let's start with the EulerAngles class of the geometry module. An > EulerAngles aims to represent a rotation as a sequence for 3 > elementary rotations around the X, Y or Z axes. However there exist as > many conventions as orders for the axes: since an axis can be used > twice and 2 consecutive axes must be different then we have a total of > 12 possibilities: XYZ, XYX, ZXY, ZXZ, etc... > > Moreover, the represented rotation depends on whether we consider the > rotation of an object around fixed axes (left multiply) or whether we > consider the rotation of a local frame (right multiply). Currently the > EulerAngles class supports only the XYZ-left-multiply convention, > however with only little extra code it is possible to handle all > conventions making it much more useful. How to to specify the desired > convention ? Let's first consider the axis order, we can imagine the > following possibilities: > > * define 12 XYZ-like constants: > EulerAngles<float, ZXZ>(a0, a1, a2); > > * define the three X, Y, and Z constants: > EulerAngles<float, Z, X, Z>(a0, a1, a2); > but they might easily conflict > > * simply name the axes 1, 2, 3: > EulerAngles<float, 3, 1, 3>(a0, a1, a2); > > I prefer the first option but maybe someone has a better idea. About > the the right/left multiply choice, I would make it right-multiply by > default for many reasons, one of them being to be consistent with > OpenGL. Maybe we could even only allow this convention ? (otherwise it > would be specified as an additional (and optional) template > parameter). > > In the same vein, in order to tackle degree vs radian mistakes, we > could define two very small classes Degree and Radian providing > automatic conversion from one type to the other. So a function which > requires a radian would be declared: void foo(Radian angle) and > called: > foo(Degree(180)) or foo(Radian(3.14)), (and we do the same for the > returned type). This is a really safe solution (I saw that in the > rendering engine Ogre3D), but maybe it is a bit overkill and heavy to > use. So I mention it for the record, but I'm not sure at all to like > it. (btw, in eigen all angles are of course assumed to be in radian > but the confusion is easy as soon as you mix it with OpenGL code). > > > For the second point I'll try to be more concise ;), so what about the > partial-redux proposal which is on the Todo page, section Array > module: > http://eigen.tuxfamily.org/index.php?title=Todo#Array_module > > The idea to make partial-reduction usable by adding .columnWise() and > .rowWise() members to MatrixBase giving access to partial-reduction > shortcuts. > I suggested columnWise/rowWise because I think they are much less > ambiguous than "horizontal/vertical reduction". > Also, the current templated functions verticalRedux/horizontalRedux > would be moved to the generic "PartialRedux proxy", and named "redux": > m.columnWise().redux(myfunctor()); > > > Even shorter, what about an automatic cast to scalar operator for 1x1 > fixed size matrix ? I see 2 use cases: > - you can write: float a = vector1.transpose() * vector2; > - you can write generic multimensionnal algorithms without the the > pain of writing [0] for the 1D case. > In practice the cast operator would be protected by a static assert. > > > That's it for now, > cheers, > Gael.

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**References**:**[eigen] Euler-angles and partial reductions API design***From:*Gael Guennebaud

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