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.
Opened by Anton Markwart - 2011-11-22
Last edited by Christoph Lipka - 2011-11-22
FS#228 - Emission Media does not show on Alpha background
I tried to render a candle’s flame (an object with interior made of emission media) against an Alpha background. In 3.6, the “flame” appears as a bright object against the alpha background; but in 3.7, the flame disappears against the alpha (as the attached scene will make clear, the “flame” is still visible against opaque background objects). I used the settings “Output_Alpha=true Output_File_Type=N”.
If there is another (better?) way to achieve a similar effect, I am open to using a work-around.
FlameTest.pov
FlameTest 3.6.png
Note that the 3.6 output does not match the intended effect either: The flame does not brighten up the background (as emissive media should), but simply replaces it (partially, depending on density). This is due to a limitation of the PNG file format (for the technically inclined: PNG mandates to use non-premultiplied alpha) which makes it impossible to achieve the desired additive effect unless the background color is known beforehand (which makes the use of transparency rather moot).
The different behaviour of POV-Ray 3.7 is due to an alpha handling bug in 3.6 that has now been fixed. (POV-Ray 3.6 would writing PNG files with premultiplied alpha, in violation of the PNG specification, and also in violation of its own input file handling, where it would - correctly - expect non-premultiplied alpha.)
What you need is a file format that is specified to employ pre-multiplied alpha (both OpenEXR and Tiff qualify) - and also an image viewer that implements this properly (most viewers erroneously expect non-premultiplied alpha).