Hello everyone,

I recently updated my version of Povray (form 3.62 to 3.7) and unfortunately, my


I think that my problem probably comes from the fact that objects are now
blended, taking into consideration the color of the background.

As a result, my pictures containing object with only interior (and no texture)
and transparent background do not have the same aspect than before. They are
completely transparent.

In short, I would like to create a light sword on a transparent background.

Do you know what should I do to solve my problem?

Regards,

JB

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On 01/28/2014 12:55 PM, JB wrote:
> Hello everyone,
>
> I recently updated my version of Povray (form 3.62 to 3.7) and unfortunately, my

>
> I think that my problem probably comes from the fact that objects are now
> blended, taking into consideration the color of the background.
>
> As a result, my pictures containing object with only interior (and no texture)
> and transparent background do not have the same aspect than before. They are
> completely transparent.
>
> In short, I would like to create a light sword on a transparent background.
>
> Do you know what should I do to solve my problem?
>
> Regards,
>
> JB
>
>
>
>

most likely specific to your problem:
http://wiki.povray.org/content/Reference:Background

other changes (summarized) that you /might/ also be of interest:
http://wiki.povray.org/content/Documentation:Tutorial_Section_1#Changes_and_New_Features_Summary

Post a reply to this message

Am 28.01.2014 18:55, schrieb JB:
> Hello everyone,
>
> I recently updated my version of Povray (form 3.62 to 3.7) and unfortunately, my

>
> I think that my problem probably comes from the fact that objects are now
> blended, taking into consideration the color of the background.
>
> As a result, my pictures containing object with only interior (and no texture)
> and transparent background do not have the same aspect than before. They are
> completely transparent.
>
> In short, I would like to create a light sword on a transparent background.
>
> Do you know what should I do to solve my problem?


There are two issues that might come into play here:


(1) What you are seeing might be the difference between two ways of 
alpha handling: "straight" (aka "non-premultiplied") vs. "premultiplied" 
(aka "associated").

When displaying an image with alpha channel in front of some background 
image, the resulting colour of a given pixel may be computed according 
to the following formula:

   R" = A * R + (1-A) * R_back
   G" = A * G + (1-A) * G_back
   B" = A * B + (1-A) * B_back

where R,G,B are the colours the object would have if it was fully 
opaque. This is the so-called "straight" or "non-premultiplied" alpha.

However, the multiplication with A can already be done when the file is 
generated, saving some time when displaying the image:

   R" = R' + (1-A) * R_back
   G" = G' + (1-A) * G_back
   B" = B' + (1-A) * B_back

where R',G',B' are the colours the object would have if it was displayed 
in front of a black background. This is the so-called "premultiplied" or 
"associated" alpha.

Note that the above formulae are what is used in the /displaying/ software.


Previous versions of POV-Ray did a particularly poor and inconsistent 
job with respect to this distinction: Entirely ignoring any file format 
specifications, POV-Ray 3.6 invariably expected input files to use 
"straight" alpha, but also invariably generated output files using 
"premultiplied" alpha.

In POV-Ray 3.7, this has been changed to conform to the PNG file format 
specification, which mandates that "straight" alpha is to be used.

There is currently no way to coax POV-Ray 3.7 to violate the PNG file 
format specification. However, POV-Ray 3.7 also supports the OpenEXR 
file format for output, which is specified to always use "premultiplied" 
alpha handling.


(2) What you are seeing might also be the effect of a change in the 
background and sky_sphere colour handling. POV-Ray 3.6 was an 
inconsistent mess in this respect as well.

However, this should be easy to fix by specifying "#version 3.6;" at the 
start of your scene file.



There is also a fundamental problem with the PNG output file format when 
it comes to emissive-only objects in an otherwise transparent image, 
because of ambiguity as to what the alpha value should actually denote 
in this case: Should it specify (a) how "visible" the foreground object 
is, or (b) how "invisible" the background is?

"Straight" alpha handling insists that both must be the same. There is 
no way to tell PNG spec conformant display software that both the 
foreground /and/ background should be 100% visible.

"Premultiplied" alpha file formats are much better suited for such 
effects, as the "visibility" of the foreground is already accounted for 
in the stored R,G,B values, while the alpha value is unambiguously 
dedicated to specifying the "invisibility" of the background.

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