I have a problem: I want to use sky_sphere to make a sky, but for some reason it
does not change color smoothly, you can cleary see lines that separate different
"regions". I played with Antialias_Threshold, thinking that is the key, but
whatever I do I keep having this ploblem.

Here is my code.
#version 3.5;


// ************************   Camera   ***********************

#local Cam = <-5, 6, 12>;

camera {

        #local to = <2, 0.9, 6>;
        #local rat = 1;

        location rat*Cam + (1-rat)*to
        look_at to
        angle 38
}



// ************************   Lights   ***********************


light_source {
    Cam
    color 0.4*<1,1,1>
    shadowless


}



sky_sphere {
    pigment {
      gradient y
      color_map {
        [ 0.0 color rgb <0.8,0.2,0.2> ]
        [ 0.4 color rgb <0.8,0.8,0.4> ]
        [ 0.8 color rgb <0.3,0.0,0.5> ]
        [ 1.0 color rgb <0.0,0.0,0.0> ]
        }

      scale 2
      translate -1

      rotate 70.2763614*z
      rotate -105.46554492*y

      }
    }


The resulting image: http://dl.dropbox.com/u/1655276/esfera.png


Can anyone give me a clue?

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Zeycus <zey### [at] gmailcom> wrote:
> I have a problem: I want to use sky_sphere to make a sky, but for some reason it
> does not change color smoothly, you can cleary see lines that separate different
> "regions".

  It looks quite smooth to me.

  If you are talking the region where the purple-blue transition changes
to blue-black, where the exact point of change which has the purest blue
stands out from the rest, that's not really a problem with povray. If you
create the same kind of gradient in any program it will look like that.
(The reason is, very technically speaking, that there's an abrupt change
in the slope of the color transition curve.)

  Getting rid of it can be a bit tricky. (Basically it would need for the
color change slope to itself be smooth.)

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> Zeycus <zey### [at] gmailcom> wrote:
> > I have a problem: I want to use sky_sphere to make a sky, but for some reason it
> > does not change color smoothly, you can cleary see lines that separate different
> > "regions".
>
>   It looks quite smooth to me.
>
>   If you are talking the region where the purple-blue transition changes
> to blue-black, where the exact point of change which has the purest blue
> stands out from the rest, that's not really a problem with povray. If you
> create the same kind of gradient in any program it will look like that.
> (The reason is, very technically speaking, that there's an abrupt change
> in the slope of the color transition curve.)
>
>   Getting rid of it can be a bit tricky. (Basically it would need for the
> color change slope to itself be smooth.)
>
> --
>                                                           - Warp

Thanks a lot, warp.

I think I understand what you mean, the color function is not derivable
too many times.

Anyway I see all over the image lines that are like "height lines" in a map. I
just thought maybe it is a problem with my colour configuration. I will try
tomorrow to see the same image in another computer.

Thanks again,

Post a reply to this message

On 16-Aug-11 13:00, Zeycus wrote:
> I have a problem: I want to use sky_sphere to make a sky, but for some reason it
> does not change color smoothly, you can cleary see lines that separate different
> "regions". I played with Antialias_Threshold, thinking that is the key, but
> whatever I do I keep having this ploblem.

You will see this if your display is set up for 16-bit color. It looks smooth to me at
24-bit or 32-bit color.

Post a reply to this message

Zeycus wrote:

> Anyway I see all over the image lines that are like "height lines" in a map.

I can also see these ... but the actual RGB value look reasonable
in GIMP. I think the main reason is that I never managed to calibrate
my monitor properly (it's an old flat screen from 2004).

But I also wonder if there may be a general problem with
interpolating in RGB color space as opposed to something like
CIELAB that better matches human perception of color differences
(BTW, it's not related to linear RGB vs. gamma RGB, I see lines
both with assumed_gamma 1.0 and with 2.2)

I also tried dithering (new in 3.7) but it didn't change much. Doing
the same gradient in GIMP with dithering looks smoother but that is only
because gradients use extremely strong dithering.

Post a reply to this message

It's very interesting, I am in another computer and now I really cannot see
those sharp lines, it is definitely a screen issue.

I was using 24bits, but there must be some other factor.

Thank you very much for your expert comments!

Post a reply to this message

On 17/08/2011 10:22, Zeycus wrote:
> It's very interesting, I am in another computer and now I really cannot see
> those sharp lines, it is definitely a screen issue.
>
> I was using 24bits, but there must be some other factor.
>
> Thank you very much for your expert comments!

are you using a VGA-cable? if so, it's probably a sharpness-setting on 
your monitor.
Try http://www.lagom.nl/lcd-test/ for some manual LCD calibration, but 
be aware that if you're using a TN-panel (i.e. any cheap LCD), any real 
color-calibration is rather useless, because of the very low viewing angles.

Post a reply to this message

Am 16.08.2011 21:00, schrieb Zeycus:
> I have a problem: I want to use sky_sphere to make a sky, but for some reason it
> does not change color smoothly, you can cleary see lines that separate different
> "regions". I played with Antialias_Threshold, thinking that is the key, but
> whatever I do I keep having this ploblem.

That's your graphics hardware and/or image file format limitations you see.

If by "clearly" you mean "when zooming into the picture" or "when having 
an exceptionally close look", then that's perfectly normal for the 
traditional 8 bit per channel (24 or 32 bit per pixel) file formats and 
graphics cards. Yes, depending on the sky sphere gradient they can be 
quite prominent, even to the point where "clearly" would be a fitting term.

In that case...

(A) If you're using POV-Ray 3.6, output the image to a 16 bits per 
channel file format (e.g. "+fn16" for PNG with 16 bpc), then downsample 
to 8 bits per channel using some external piece of software that 
supports dithering (e.g. LILYsoft's IC, 
http://www.lilysoft.org/IC/ic_index.htm).

(B) If you're already using version 3.7(RC3) of POV-Ray, use "+th" or 
"Dither=yes"; this activates POV-Ray 3.7's inbuilt dithering for the 
output file.

"Dithering" is a mechanism that hides color banding artifacts by adding 
noise-like artifacts, which are less obvious to the human eye.


If by "clearly" you mean "at first glance", then you may indeed have set 
your display to 15 or 16 bits per pixel (i.e. 5 to 6 bits per color 
channel); in that case I suggest you simply change that.

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