User talk:Macchess

Welcome!

Hello, Macchess, and welcome to Wikipedia! Thank you for your contributions. I hope you like the place and decide to stay. Here are some pages that you might find helpful:

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I hope you enjoy editing here and being a Wikipedian! Please sign your messages on discussion pages using four tildes (~~~~); this will automatically insert your username and the date. If you need help, check out Wikipedia:Questions, ask me on my talk page, or ask your question on this page and then place {{helpme}} before the question. Again, welcome! Waldir ^talk 12:28, 11 June 2008 (UTC)

Heliotrope images

Dear Macchess, I just uploaded a few heliotrope images to commons:Category:Heliotrope. Please take a look and replace those in the article as you find fit. Cheers, Waldir ^talk 12:25, 11 June 2008 (UTC)

Great work with heliotrope2-2.jpeg. I think both articles are in a fairly sactisfactory state by now (I did some minor edits to heliotrope), and as you say, Heliotrope can be enhanced by anyone interested in the subject,given the sources there provided. If you continue to improve the Heliograph article, it might well become a featured article soon! Cheers, Waldir ^talk 08:56, 27 June 2008 (UTC)

Yes, I use "Shadows/Highlights" a lot myself. I am pretty sure I used it in heliotrope2.jpg, but perhaps after cropping the image the results could have improved (or maybe it was just a matter of increasing the values). In any case, your enhancement made some glitches in the image more visible. I will probably remove them this weekend. Cheers, Waldir ^talk 13:35, 27 June 2008 (UTC)

Hi Macchess. I just checked the image, it's excellent! I added a template to see if someone so inclined would do a cleanup in the pattern in the background. Look at the links in that template, or see this image (previous version) for an example of what can be done. But nevertheless it already is very good! It was also a nice idea to upload new versions of the other images, cropped to better show the detail of the instrument.
As for Image:Heliotrope2-2.jpg, I edited it to remove the scratches, as I said. Tell me what you think. -Waldir ^talk 12:25, 29 June 2008 (UTC)

I agree with you, the "noise reduction" reduced much of the detail in the original image. I mean, I somehow don't understand how the editor (who usually does a great job) would consider the edited image was better than the original, and overwrite it. I don't know how to perform FFT cleanup on images, but I'll ask him whether he can reprocess the image to remove only the halftone pattern from the printing, leaving the engraving alone. On the other hand, the image without the engraving actually does look more photo-realistic, as you say, and perhaps we could upload a new one without the engraving. The version you uploaded should keep it, for historic consistency. The one without it should be uploaded with a new name, instead. I'll look into this as time permits. You can link to the previous version by clicking in the dates in the upload history table, but you cannot embed them in articles directly. If you wish you can restore the previous version by clicking in the appropriate link on the left to it. But if you do so, make sure to state it is a temporary measure -- and warn the editor too, as he nevertheless spent some of his time working on the image. I would do so myself but I don't have more time today. Probably I could do something during the week. In any case, your reversion would itself be revertible, so no permanent consequences would come from that.

As for the cleanup of the heliotrope image, I used a mixed approach. I Rarely use automatic tools since they tend to destroy details in the image that one wants to keep. So I started with the healing brush but quickly changed to the clone stamp since it was producing patterns that didn't exist (or weren't noticeable) in the surroundings.

Finally, you make a very good point on the cropped versions of images. It would be cool if you could choose a subset of an image to be displayed, kind of a viewbox, instead of uploading a new image, which you have to indicate the source by hand, otherwise is a completely different image to the software. As for the page links -- the software detects image links in the same wiki only. When an image is uploaded locally in Wikipedia, we can see it links on the image page. But when it is uploaded to commons, which is a separate wiki, you have to use the "check usage" tab to view where it is being used. It would be impracticable to have this info on each image page, since they can be used hundreds of times in any of the hundreds of Wikimedia projects... but no, the links are not generated by hand. :) The categories are, though. Could you please indicate which of the images was the troublesome one so I can verify why it wasnt categorized? Cheers, Waldir ^talk 00:03, 30 June 2008 (UTC)

Heliograph

Not sure why I did that. I must have been trying for something else. I re-rolled it back to the ip's edit. something lame from CBW 08:09, 18 February 2010 (UTC)

Fast & Furious Character Pages

The big problem with those articles is that they don't have enough reliable sources. I would help with that, but I really have no idea where to start. --Boycool (talk) 22:14, 14 May 2011 (UTC)

Hooke

My source for hooke's heliograph was from a historical/chronological book called 'British History' by Rodney Castleden. However, I looked through the document of his presentation to the Royal Society and Hooke's device doesn't seem to have operated on the principles of the heliograph, as you pointed out. So, I guess if you think it's wrong then it can be taken out. Thanks. Noodleki (talk) 12:48, 23 April 2012 (UTC)

Thanks, done. Macchess (talk) 07:01, 24 April 2012 (UTC)

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Thanks, fixed. Macchess (talk) 01:03, 5 May 2014 (UTC)

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Thanks! I fixed them. Macchess (talk)

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Re: Quaternions

I threw that note in because a few months ago I myself got the impression from that very article that the relationship between unit quats and SO(3) was a bijection rather than two-to-one. I appreciate the attention you've paid to the subject. However, I'm uneasy about making a distinction between "rotations" and "rotational transformations." To me rotations are a subset of transformations, so this doesn't quite parse for me. Is there a source somewhere that makes a distinction between these two? 11wx (talk) 05:37, 13 December 2022 (UTC)

> I myself got the impression from that very article that the relationship between unit quats and SO(3) was a bijection rather than two-to-one.

Oh, that's not good. Glad you spoke up.

> However, I'm uneasy about making a distinction between "rotations" and "rotational transformations."

I gave an example of two different rotations (physically distinct processes) that produced the same rotational transformation:

"If 0 ⩽ $\theta$ ⩽ $2\pi$, the rotation about ${\vec {u}}$ by $\theta$ and the rotation about $-{\vec {u}}$ by $2\pi -\theta$ both achieve the same final orientation by disjoint paths through intermediate orientations."

Both these rotations result in the same rotational transformation, but are two very different rotations - they are in the opposite sense, and share no orientations other than the common start and end points - if they are achieved as physical rotations that are continuous processes that take finite time, the intermediate orientations each pass through are disjoint sets. What is unclear about that? Would it help if I said "physical rotation" rather than "rotation"? I'll make that change now.

> To me rotations are a subset of transformations, so this doesn't quite parse for me.

I'm using "rotations" in the lay sense.

The rotation is the process that gets you from A to B, "the journey", and the "rotational transformation" is the result of that process - "the destination"

I'm pretty sure if you ask someone not exposed to math what a rotation is, they will understand rotation to describe a continuous, non-instantaneous process, as in "perform a rotation" - they will think of a rotation as "the journey".

They probably won't have a clue what "rotational transformation" is. We've been taught that transformations tell us how vectors in the original frame look at the destination, and if a process is ascribed, it is typically that of projections, not rotations. If I'm given two orientations, and asked to write out the transformation matrix, I was taught to calculate the nine elements by projection of the original axes on the final axes. Projection has is no implication of duration - it is taught as though it were an instantaneous mapping, with each of the nine elements computed completely independently of the other eight.

Before people are taught about vectors, quaternions, or matrices, they are taught about rotations - learning how the hands turn on a clock and how the earth spins on its axis.

They describe rotations by:

- the axis to rotate about

- the sense in which you will rotate about that axis

- the angle about that axis you will rotate in that sense.

And that is the way rotations are described by the Euler Axis/Angle form, which maps directly to the quaternion form, but which, unlike quaternions, is undefined for the identity.

The Euler Axis/Angle form, or quaternion form, directly specify which of the two distinct rotations in SU(2) ("CCW about +X, or CCW about -X), you are going to use to create the SO(3) transformation. Knowing which is which can be critical, as in cases where you need to later interpolate between the start and end.

Rotation matrices directly give you the SO(3) object, but do not tell you which SU(2) object created it. If I have a rotation matrix that corresponds to a 180 degree rotation,

I can't tell you whether it was achieved by a clockwise turn or a counterclockwise one from the matrix, whereas I can from the quaternion.

> Is there a source somewhere that makes a distinction between these two?

It isn't my own creation - it is something I've read in more than one paper.

However, I'm not turning up an example quickly via the indices of the three quaternion texts closest to hand -

I'm not seeing "rotation" used to distinguish elements of SU(2) and SO(3).

Does my explanation clarify things for you? Or would you prefer I find an explicit citation?

Macchess (talk) 08:35, 13 December 2022 (UTC)

Hmmm, I'm still not convinced about the distinction, and a quick search online tells me that "rotational transformation" seems to be used as a synonym for "rotation," but I'm going to let it go for now. Most important is that the two-to-one mapping is communicated, and we've accomplished that. 11wx (talk) 22:33, 13 December 2022 (UTC)

Very good.

The best solid introduction I've found to quaternion math is the 27-page: “Chapter X – Quaternions” from the copyright-expired 1947 text by Louis Brand: “Vector and Tensor Analysis” (Wiley). This can be read online or downloaded as a .pdf from Google Books [1]https://books.google.com/books?id=ZYS4AAAAIAAJ&pg=PA403 (For an even pithier introduction, §181-184 provide the foundation for §187: Rotations, a 13 page total). The 2020 Dover Books paperback reprint of this text (I have no financial interest) costs less than $15 here [2]https://www.amazon.com/Vector-Tensor-Analysis-Dover-Mathematics/dp/0486842835.

For computer graphics, Andrew J. Hanson’s textbook: "Visualizing Quaternions" concentrates on applications from a computer graphics perspective. Hanson has published papers on quaternion applications since 1994 ( [3]https://www.semanticscholar.org/author/A.-Hanson/1730461?q=quaternion&sort=influence ) I have the hardbound version, but it is freely downloadable as a .pdf from the ACM Digital Library: Hanson, Andrew J., Visualizing Quaternions, February 2006, 531pp Morgan Kaufmann Publishers Inc., ISBN:978-0-08-047477-9

[4]https://dl.acm.org/doi/10.5555/2821580 The hardback version is about $70 used.

For aerospace applications, the text: "Quaternions and Rotation Sequences: A Primer with Applications to Orbits, Aerospace and Virtual Reality" (371pp), J B Kuipers, Princeton University Press, 1999 was developed under US govt contract, and a soft-copy preprint version is freely downloadable from the US Govt. here: [5]https://apps.dtic.mil/sti/pdfs/ADA322836.pdf

From a mathematical perspective on quaternions, the latest edition of John Voight's textbook "Quaternion Algebras" (v. 1.0.5, 863pp, posted Dec 7, 2022) can be freely downloaded from his page at Dartmouth here: John Voight's Quaternion Page, together with supplemental material, or the 2021 version can be downloaded or purchased in paperback/hardback form from Springer-Verlang (I have no financial interest) here: Springer-Verlag page for Quaternion Algebras

Macchess (talk) 22:04, 14 December 2022 (UTC)

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