The Persistence of Vision Raytracer (POV-Ray).
This is the legacy Bug Tracking System for the POV-Ray project. Bugs listed here are being migrated to our github issue tracker. Please refer to that for new reports or updates to existing ones on this system.
Attached to Project: POV-Ray
Opened by Sean Day - 2014-06-08
Last edited by Christoph Lipka - 2015-12-26
Opened by Sean Day - 2014-06-08
Last edited by Christoph Lipka - 2015-12-26
FS#325 - SSLT Glow issue with radiosity
Hi,
I think I may have found a bug in the subsurface/radiosity features of Povray 3.7 attached image showing the issue and sample files required to reproduce. The original files (on povray forum images thread) had different radiosity settings but these ones show the problem and only take a few seconds to render the image without AA so probably better for bug fixing/testing etc.
Thanks
Sean
I have just performed a new test and found that the issue is present even without radiosity so this looks like it is purely a SSLT issue. If I comment out the radiosity settings just leaving the subsurface settings I get the same effect.
Sean
Hi,
I have run some further tests and it seems this issue is to do with the texture_map and SSLT (does not seem to matter which pattern). A further example source file is uploaded.
Sean
I think I have an idea what may be going on here.
The key to it all is probably that you have interpolation between two textures involved, with significantly different pigment and subsurface translucency settings.
Now in subsurface scattering textures the pigment does not do exactly the same as in classic textures: It does not directly govern the surface's colour. Instead, from the pigment and the translucency POV-Ray computes another set of parameters, called the scattering and absorption coefficients. These govern how much light entering the object is subject to being scattered and to being entirely absorbed, respectively.
The translation from pigment and translucency to these internal parameters is chosen in such a way that for a sufficiently large slab the resulting surface colour effectively does match the pigment colour. However, as the object gets smaller – or as you increase the translucency setting – the effect of both scattering and absorption become less and less pronounced. In the end, the object effectively gets transparent.
Now one more thing to note here is that if the subsurface scattering parameters are chosen in such a way that absorption is much stronger than scattering, small objects will appear transparent but tinted.
Now your image has two different textures involved: One of them has comparatively low transparency; this is the texture that governs the overall look of the material, which gives it a quite credible opaque look in general.
The other texture, however, has settings that result in low scattering but high absorption. This means two things: First, the low scattering would give the object a glass-like appearance. Second, the high absorption means that it would be a very dark glass, through which we can see only exceptionally bright stuff. Which in turn means that this texture does not have any effect on the overall appearance of the object (except that interpolating it in means to dim the overall brightness) – except where the very bright sky probe is visible, which due to the high refractive index and the convex shape is at the bottom of the object.
Therefore, I think this is not a bug, just unexpected effects of intended behaviour.