Ankit Pruthi Posted August 13, 2014 Share Posted August 13, 2014 Now i've noticed this on a couple of projects. If we take a camera exported from other softwares such as maya into houdini, and scale it down by a factor of 100, to account for cms to metres conversion, volumes tend to render very very dense, almost as if the density itself has been multiplied by a 100 times.What is the reason behind this behavior?I've noticed the same happen if there are no transforms anywhere, but a transform is done inside of a sop on the volume.Below are the attached images showing the difference. Quote Link to comment Share on other sites More sharing options...
pezetko Posted August 13, 2014 Share Posted August 13, 2014 The problem is scaling.Good habit is always reseting the scale on the camera (and all objects to the 1,1,1), otherwise you get some (unwanted) surprises (different render time, sampling noise, displacement, depth culling, density.. etc.)You didn't scale lights in your examples, so you got different results. Scale matters! Quote Link to comment Share on other sites More sharing options...
koen Posted August 13, 2014 Share Posted August 13, 2014 (edited) If you interpret the density field as a concentration, scaling a volume up or down will increase or reduce the amount of "stuff" a ray travels through. if you look at beer's law: ( http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law ) the absorbtion relates linear to the path length. When you scale the volume, you change the path lenght. Cheers, koen Edited August 13, 2014 by koen 1 Quote Link to comment Share on other sites More sharing options...
Ankit Pruthi Posted August 14, 2014 Author Share Posted August 14, 2014 On 8/13/2014 at 8:21 PM, koen said: If you interpret the density field as a concentration, scaling a volume up or down will increase or reduce the amount of "stuff" a ray travels through. if you look at beer's law: ( http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law ) the absorbtion relates linear to the path length. When you scale the volume, you change the path lenght. Cheers, koen Yes that makes complete sense. It also explains why a volume which is about a 1000 metres in size needs to have a density of the order of 0.001 or so to start looking different. So, if i understand it correctly, it depends on how many units the ray travels according to camera space coordinates. Now, what would be the appropriate way to convert a camera imported using alembic from cm units into metres?? Something that works only on the x,y and z transforms and does not effect the scale? I'm not even sure if it makes sense to do that. As of now the workaround is scaling the density down or up by the same factor used on the camera. Quote Link to comment Share on other sites More sharing options...
hall Posted August 14, 2014 Share Posted August 14, 2014 I used to re-build the camera from Alembic using just the point orientation to the new camera, it's a bit work, but I looking for a better solution as well. Quote Link to comment Share on other sites More sharing options...
eetu Posted August 14, 2014 Share Posted August 14, 2014 (edited) On 8/14/2014 at 5:04 PM, claudiohick said: I used to re-build the camera from Alembic using just the point orientation to the new camera, it's a bit work, but I looking for a better solution as well. You could check Jordi's setup from page 118 of his General Concepts guide at http://www.sidefx.com/index.php?option=com_content&task=view&id=2711&Itemid=132 edit: fixed page number Edited May 10, 2015 by eetu Quote Link to comment Share on other sites More sharing options...
tfreitag Posted May 10, 2015 Share Posted May 10, 2015 (edited) haa! thanks eetu (and of course jordi) for the link ..after spending serveral hours try to rendering volume with max imported camera, this is nice trick, with blend node. ...and its page 118. Edited May 10, 2015 by tfreitag Quote Link to comment Share on other sites More sharing options...
Ryan Posted May 14, 2015 Share Posted May 14, 2015 I am assuming you are using a Null to scale your camera. You could also unhide the scale parameter in the camera and put in scale x, y, and z: origin(opinputpath(".",0), "", "SX") origin(opinputpath(".",0), "", "SY") origin(opinputpath(".",0), "", "SZ") Quote Link to comment Share on other sites More sharing options...
Jebbel Posted September 29, 2016 Share Posted September 29, 2016 Thanks guys...this information solved my problems I was having with camera rays. The bigger your scene is...the more apparent the issues become (weird noise, flickering and clipping). We're scaling our maya scenes to 0.01 before alembic export and I rebuild the camera in houdini using the method described in page 118 -> http://www.sidefx.com/index.php?option=com_content&task=view&id=2711&Itemid=132 cheers Quote Link to comment Share on other sites More sharing options...
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