Posts by Mike Hewson

11) Message boards : Cafe Einstein : Jumbled Word Game #5 (Message 137905)
Posted 4 days ago by Profile Mike Hewson
Whoops, I won ! :-)


First clue : there is a single hyphen involved.

Cheers, Mike.

12) Message boards : Cruncher's Corner : Write your own Einstein@home screensaver (Message 137904)
Posted 5 days ago by Profile Mike Hewson
That's indeed good to know. Now how about adding quadrics...?

GLM doesn't have that. But I have a Sphere class which generates a vertex listing that yields a geodesic type frame composed of triangles.

I have considered generalising ( because I can !? ) to cylinder and disc, though a disc is really a very short cylinder. A torus/donut would be useful too.

Algorithmically they are much the same, having :

- circularity ie. the end meets the start, along at least one coordinate progression.

- need for normals to light.

- color at the vertex ( perhaps implying extension to the triangle faces )

- need for texture co-ordinates for pasting images on.

.... however they differ with respect to the particular generator of the vertex co-ordinates.

A general quadric then has/needs parameters for :

- overall dimensions eg. a sphere's radius

- granularity of modelling eg. how many slices/partitions along the relevant axes

- what totality of information is desired per vertex eg. position, normals, color, texture co-ords.

- possibly some prescription of base representation eg. float, double etc.

One important issue I have discovered : if you scale the object then DON'T scale the normals ! Keep them unitary.

The 'output' of a quadric format would be a 1D array of records, possibly with another array of indices into that one. Each record is the same structure of vertex attributes. This could be wrapped up into high level calls that invoke generation and rendering. Naturally one would have a 'Quadric' base class, virtual for that matter, plonking the shape specifics into subclasses to solidify the virtual stuff. I guess that is the idea ....

Consider it on my to-do-list ! :-)

Cheers, Mike.

( edit ) As discussed elsewhere, you have to use triangles in the modelling. Quadrilaterals are out in later OpenGL ...

( edit ) Mind you, one can 'fake it' eg. have coplanar triangles ( or possibly a sequence of lines ) but the API doesn't natively support quads. However the fixed parts of the rendering engine may catch you out by producing artifacts if you don't get it truly right though. This then gets down to low level stuff like number representation formats ie. when are two floating points equal, and when do you have the right to expect equivalent mathematical expressions ( notably commuting transforms ) to yield that ?

I'm sure you're familiar with all of that. FWIW GLSL has an 'invariant' qualifier that may be used to ( allegedly ) force equivalence of computation outcomes across programs ( on a given machine ). There's a time cost in that because it requires some degree of introspection and adjustment at runtime. Very clever feature, but IIRC it's a bit like 'inline' ie. at most a hint and might indeed be ignored by the implementation. For graphics production this is probably neither here nor there. I think it was included for regularity of compute shader outcomes, when doing 'multi-pass' techniques especially.

( edit ) One may wonder why I use a 32GB RAM machine. Firstly because I can [ :-) ], but it turns out that 'unrolling' much of the C++ standard library template classes can massively blow out the compiler's memory needs, at least for short periods on g++.
13) Message boards : Cafe Einstein : Cafe Einstein: LPTP Triskaidekaphenia (Message 137903)
Posted 5 days ago by Profile Mike Hewson
That thing looks like a fat fish

I think that's the point .... it needs to lose weight. It's not foreboding, it's you're actual boding.

Of course, then there's the gag : where does he sit ?

Wherever he likes ..... :-)

@David : "I'm not going to hide it in white font anymore."

Well done, hidden in plain sight ! I'll have to go back and look now.

Cheers, Mike.
14) Message boards : Science : Null Gravity? (Message 137852)
Posted 7 days ago by Profile Mike Hewson
So ..... Is gravity Zero at the center of a Black Hole ?

Not that I'd want to go there )


Ooooh .... a corker of a question there Bill. :-)

Classical GR says the density is infinite there. Well, Newton also for that matter. That's a hint that we can't legitimately make any genuine interpretive statement from theory. It's best to say "don't know" here rather than give a figure, or even say infinite gravity or whatever. Leave it as a patch of ignorance to be safe.

Such points in the theory are known by the mathematical term 'singularity', a type of naughtiness ( more or less the difficulty produced when you divide a number by zero ) that some functions have at certain points. It's not just gravitational theory that mentions singularities, these are know to occur in the mathematics of many other areas of physics also.

Sometimes it is not an especial problem for a theory overall but rather for the way a physical situation has been modelled in some example to which theory has been applied, and in particular by choice of a coordinate scheme to define a scenario. A good example of that is the event horizon of a black hole which has singularity features only evident with certain choices of reference frame. The difficulty may be 'transformed away' eg. by taking the viewpoint of an object descending into a black hole. Nothing especial happens at the Schwarzschild radius in that case. The central singularity remains with any/all choices of frame and is if you like a more general roadblock to understanding.

It is of great note that quantum mechanics has this blessed ability to prevent some reasons for singularities occurring. The core idea here is that one can't really pin down positions and movement simultaneously for any entity ( and regardless of whether you label said entity a 'particle' or a 'wave' ). The machinery of QM prevents 'one thing sitting atop another', and hence the sort of stacking up and totalling of things without bound. There must be some separation of things. I guess with a quantum theory of gravitation that would enforce an upper limit on densities.

So the Heisenberg Uncertainty Principle, which is usually disdained for it's imprecise undercurrent, can come into play to protect a theory from yielding singularities. Well that's a rough take on it at least.

[aside]There is a trick used often with quantum field theories called 'renormalisation'. This is a tactic that essentially excludes certain infinite sums from being performed ( as a mathematical procedure ), and that reflects back onto the modelling as only assessing quantities above a certain scale size. Very roughly speaking if you don't look too close around the region of some point then you can't manufacture a singularity ...

... Feynman admitted this was 'sweeping infinity under the rug'. He found in quantum electrodynamics that some parts of a calculation would have a sum which totalled to infinity. The very same sum ie. the mathematical expression, appeared in several places. He decided to simply replace that crappy term with a ( finite ) value as measured by experiment. This divided QED calculations into two parts : that which gave sensible results and that which gave the silly ones. Calculate the sensible bits, straight up ignore the bad bits by replacing them with a valid number from the real world. This is not terrible, more to the point it is a recognition that below a certain distance scale we don't know what is happening, but we can accept a 'summary' of behaviour that occurs at that difficult scale.

So that's sort of like producing a profit & loss statement for a company without going into finer details of it's operations. If you own a group of companies then look at said group in toto by manipulating the various profit/loss statements of each. Anyone who has either dealt with an accountant and/or read The Hitch-hikers Guide To The Galaxy series knows that much useful work can come forth from the minds of accountants ... :-)[/aside]

Cheers, Mike.
15) Message boards : Cafe Einstein : Seti Refuge Bar & Bistro Wing of Cafe Einstein (Message 137847)
Posted 8 days ago by Profile Mike Hewson
I'd like to clarify that a 'zimmer frame' is a perfectly legitimate mode of transport for those in need of assistance with mobility :

... and no one should be making any fun at the expense of those who have one,

... so given that we have that absolutely crystal clear,

... I'm now off to have surgery to fix the hernia I got while trying to keep a straight face when writing this ... :-) :-)

Cheers, Mike.
16) Message boards : Cruncher's Corner : Write your own Einstein@home screensaver (Message 137829)
Posted 8 days ago by Profile Mike Hewson
Now here's nice surprise ie. GLM has provided the matrix transforms which mimic those previously provided by GLU :

5.2. GLM replacements for GLU functions From GLM_GTC_matrix_transform extension: <glm/gtc/matrix_transform.hpp> gluLookAt: glm::mat4 glm::lookAt(glm::vec3 const & eye, glm::vec3 const & center, glm::vec3 const & up); glm::dmat4 glm::lookAt(glm::dvec3 const & eye, glm::dvec3 const & center, glm::dvec3 const & up); gluOrtho2D: glm::mat4 glm::ortho(float left, float right, float bottom, float top); glm::dmat4 glm::ortho(double left, double right, double bottom, double top); gluPerspective: glm::mat4 perspective(float fovy, float aspect, float zNear, float zFar); glm::dmat4 perspective(double fovy, double aspect, double zNear, double zFar); gluPickMatrix: glm::mat4 pickMatrix(glm::vec2 const & center, glm::vec2 const & delta, glm::ivec4 const & viewport); glm::dmat4 pickMatrix(glm::dvec2 const & center, glm::dvec2 const & delta, glm::ivec4 const & viewport); gluProject: glm::vec3 project(glm::vec3 const & obj, glm::mat4 const & model, glm::mat4 const & proj, glm::{i, ' '}vec4 const & viewport); glm::dvec3 project(glm::dvec3 const & obj, glm::dmat4 const & model, glm::dmat4 const & proj, glm::{i, ' ', d}vec4 const & viewport); gluUnProject: glm::vec3 unProject(glm::vec3 const & win, glm::mat4 const & model, glm::mat4 const & proj, glm::{i, ' '}vec4 const & viewport); glm::dvec3 unProject(glm::dvec3 const & win, glm::dmat4 const & model, glm::dmat4 const & proj, glm::{i, ' ', d}vec4 const & viewport);

... the return values of these functions which would hence be available to provide as program uniform variables and so into the shader code.

Cheers, Mike.
17) Message boards : Cafe Einstein : Jumbled Word Game #5 (Message 137826)
Posted 8 days ago by Profile Mike Hewson

... or as Terry Pratchett would say Quezovercoatl ( the god of mass human sacrifice ).

Cheers, Mike.
18) Message boards : Cafe Einstein : Cafe Einstein: LPTP Triskaidekaphenia (Message 137824)
Posted 9 days ago by Profile Mike Hewson
Ah dear. Evidently the clever opponents haven't twigged to the gig yet. It would be a poor broadsheet indeed that couldn't sell copy on both sides of a debate.

Cheers, Mike.

( edit ) I am trying to recall a really old quote about 'the stink of the righteous in our nostrils'. That sort of thing. You so often smell that on both sides of debates these days. From topics ranging from where to ( not ) put a parking space right through to which country to ( not ) bomb. Plus ça change, plus c'est la même chose ....
19) Message boards : Technical News : Uploads disabled (Message 137823)
Posted 9 days ago by Profile Mike Hewson
Black ice at the Battle of Hastings ? Yup, I'd buy that. :-)

Cheers, Mike.

( edit ) I must apologise : I wasn't aware that the Bayeux Tapestries depicted horses' willies.
20) Message boards : Cafe Einstein : This one's for our friend Mike from Oz! (Message 137820)
Posted 9 days ago by Profile Mike Hewson
Nice one TL ! Yes, that is just a way too ugly sucker ... :-)

I like this bit ( my emphasis ) :
The frilled shark was offered to the Commonwealth Scientific and Industrial Research Organization, which identified the bizarre catch, but it declined to take it.

Smooth move there. They should have offered it to Barb Broccoli. I can really see Blofeld stroking one or having a pool full of them. Maybe huge nuclear irradiated mutant versions wrestling a submarine, that sort of thing.

Now since I live only about 100 miles from Lakes Entrance ( been on holiday there in fact ) I should have absolutely no concerns then. But do please watch out for the upcoming summer blockbuster : The Shark That Ate Chum Creek .... :-) :-)

Cheers, Mike.

( edit ) I quite like Greg Kinnear as an actor. Maybe he could play me.

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This material is based upon work supported by the National Science Foundation (NSF) under Grants PHY-1104902, PHY-1104617 and PHY-1105572 and by the Max Planck Gesellschaft (MPG). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the investigators and do not necessarily reflect the views of the NSF or the MPG.

Copyright © 2015 Bruce Allen