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Scientific Method HSH vs. PTM


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this is my first topic, so hopefully it is at the right place and sorry for all grammar and spelling mistakes. 


We use the highlight method for a few weeks for our palaeontological research and we want to publish our results. However it is much easier to use the method than to understand the details (I have no backround in physics or photos what so ever).

Therefore I want to ask the experts if my understanding of the method is correct and if you could explain me the difference between hsh and ptm.


What I (think I) got so far is:

Usually a pixel contains color information (RGB) in a ptm published by Malzbender et al. (2001) each pixel has additional information about the luminescence (=reflectance??; =shape/relief??), stored by the coefficents a0-a5.

Therefore a PTM contains information which makes it able to show how the light would reflect from each pixel, making it an image with movable light source (simply spoken).  

The first data were .ptm as developed by malzbender, while the "new generation" is the .rti data, which is based on different algorithms. This was developed by the CHI (?) whit a focus on better refelctance but with the lack of other features. 

Does this new algorithm use the same method as the ptm (storing additional surface data to each pixel)? Is hemispherical harmonic a different approach than ptm? Is hsh only a different algorithm?

Do you have any papers regarding the hsh? I only found a very short mention in Mudge et al. (2008) with the picture of a coin used to compare ptm and hsh. And a paper of Wang et al. "Material Classification using BRDF Slices", however I am not sure if this is related to RTI...


Reflectans transformation

If I understood Hammer et al (2002) correctly: "diffuse gain and specular enhancement are both methods of reflectans transformation", which would mean reflectans transformation is a method (?) to change the reflectance of an object, which is possible due to the additional data in a ptm.


Ufff so many questions and so limited understanding of image capurting and processing....


Thank you very much for your help




p.S. I will opend another topic with other questions in the highlight method forum.

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Kai - 


Good questions.  There is a lot in here, and I will make a stab at answering some of it.  For some areas, I'm not the best expert.


First, I'll point you to the description of RTI on the CHI website - which describes surface normals - a key principle in understanding RTI data and it's power.  (see in particular the section called "how does it work" 


Then I'll note that the work to develop the Hemispherical Harmonics (HSH) fitter was done at UC Santa Cruz by then PhD students Prabath Gunawardane and Oliver Wang with professor James Davis.  Tom Malzbender consulted on the project.  This was done with funding from the Institute of Musem and Library Services as part of a larger project you can read about here.  The CHI team operated as a research lead for the project and was involved in a variety of ways in the work across all the different aspects and teams.


The paper you cite by Oliver Wang et.al. came out of this work.


I'm attaching here the file format document for the .rti file format.  This document is marked "draft" because we did not have time to finalize the metadata portion of the format. However, the way that the data for the files produced from the HSHfitter are stored is correct and has not changed.


Hope this begins to answer your questions.




There is a new post about file format specifications with the attachment in the FAQ forum - and it includes a link to the PTM format: http://forums.culturalheritageimaging.org/index.php?/topic/389-where-can-i-find-the-file-format-specifications-for-rti-and-ptm/?p=1143



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You have the basic idea right.


In a normal image, each pixel stores the RGB color value for that pixel.


If we want a relightable image, we could just store the RGB color value for each of 50 lighting directions in each pixel, and then look up the right one when we want to draw the picture. But this wouldn't let us interpolate in the color in between the lighting directions we actually took pictures of. So we fit a polynomial to the 50 RGB values instead. This polynomial has 6 terms and thus 6 coefficients in PTMs, and either 9 or 16 coefficients in the most common RTIs. Spherical harmonics are just a specific set of polynomials. So really PTM and RTi are just doing exactly what you would do in excel if you wanted to make a plot from some scatter data points and told excel to fit a curve for you. When you use the 'normal' PTM or RTI render this is happening.


One additional thing that is often used with PTM and RTI is to calculate the surface normal. The surface normal (the local orientation of the surface) can be used to calculate synthetic lighting that many people find useful for visualizing small scratches and features on objects. When you use this mode the picture is rendered via computer graphics and the PTM coefficients aren't used at all.


There are a variety of rendering modes and some might combine both sets of data.So part of your confusion is there is a set of techniques often used together and the exact method depends on the rendering mode you chose.

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Thanks James!


To add to this, there is a paper about the rendering modes implemented in the RTIViewer for PTM - beyond Specular Enhancement and Diffuse Gain (those are described in papers by Tom Malzbender)  You might want to check this out:



As this was published in an ACM Journal, you need to purchase it if your institution doesn't have access to ACM publications.



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One other thing - folks interested in this topic, may also be interested in another topic describing choosing between HSH and PTM as a practical matter:






PS: I am looking for a better way to cross link in the forum - but I haven't found it.  Am I missing something obvious?  It would be so handy to show related posts.  as a moderator I can move posts to another forum, and I can "merge" them - but it doesn't seem like there is a way to easily link them other than just posting a link.  I guess proper tags helps too.

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  • 9 months later...

Sorry for the 9 month delay, we had to stop our project, but now we have time to pick it up again.


Thank you very much for your replies, you helped me a lot to understand the different aspects.



I think i might come up with a couple of other, more practical, questions in the next weeks.

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