The Persistence of Vision Raytracer (POV-Ray).

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FS#269 - Transparent Objects inside Media Cause Artefacts

Attached to Project: POV-Ray
Opened by Benedikt Adelmann (badelmann) - Tuesday, 29 January 2013, 20:24 GMT
Last edited by William F Pokorny (wfpokorny) - Saturday, 28 January 2017, 15:42 GMT
Task Type Possible Bug
Category Backend → Texture/Material/Finish
Status Tracked on GitHub
Assigned To No-one
Operating System Windows XP
Severity Low
Priority Normal
Reported Version 3.70 RC6
Due in Version Undecided
Due Date Undecided
Percent Complete 0%
Votes 0
Private No


When placing a transparent object inside another object which contains media, artefacts may occur (see attached file). They look similar to specular highlights or are just strange white spots in the image.

I discovered artefacts of that kind first in the image of which MediaArtefactDetail.png is a cropped part. The code I managed to reproduce such artefacts with contained a “starfield” sphere

sphere {
  <0,0,0>, 1
  pigment { rgbt 1 }
  interior {
    media {
      emission rgb 1/10
      density {
        crackle form <1,0,0>
        density_map {
          [0.0 rgb 1]
          [0.05 rgb 0]
        scale 0.002
  scale 1000
  hollow on

and a transparent sphere

sphere {
  <0,0,0>, 1
  pigment { rgbt 1 }
  scale 2
  hollow on

which is, obviously, completely inside the other sphere. So is the camera.

Since the sphere has a pigment { rgbt 1 }, it should be completely invisible, which is correctly rendered as long as the scaling factor is 1 and hollow off (MediaArtefact1.png). Changing hollow to on does not yet produce the artefact, but the right half of the output image seems to be shifted by one pixel (MediaArtefact2.png). Changing the scaling factor to 2 (as it is in the above code) produces the artefact (MediaArtefact3.png). Changing the camera location (MediaArtefact4.png) does not change anything, the artefact just “moves with the sphere”. Changing the sphere size again, however, seems to stir up the “stars” in the “starfield” sphere while not removing the artefacts (MediaArtefact5.png). Changing hollow to off again does neither (MediaArtefact6.png).

The artefacts are definitely no specular highlights. There is not even a light source in the scene that could produce any. I used POV-Ray 3.7 RC6 to render the images, but the artefact shown in MediaArtefactDetail.png already occured in POV-Ray 3.6 which I used to render that image.

This task depends upon

Comment by William F Pokorny (wfpokorny) - Saturday, 28 January 2017, 15:42 GMT
  • Field changed: Status (Unconfirmed → Tracked on GitHub)

Now tracked on github as issue #221.