Drawing a better star

[t00fri]

I haven't done anything with the appearance of nearby stars, though obviously that needs quite a bit of work too.

--Chris

Chris,

another related question to ask is how your stars behave, once the sensitivity is increased from visual to a 200 mm photographic lens, say. In Celestia the automag might be a relevant testing ground for this. Anyway, in this case, the distance to the stars remains large throughout.

As a possible reference, here is a nice photo of alpha Cen and beta Cen, taken with a 200 mm lens by Dr. Noël Cramer, Observatoire de Genève.



We see that the two bright stars really blow up in size with the glare regime being very conspicuous. An uncountable number of background stars is visible with the star size remaining tiny.

Where would the hardware-specific profile enter in your approach? Once I increase the amount of collected light by some device other than the eye, I need to know its spectral sensitivity! A CCD, for example is way more sensitive in the blue regime than normal film.

Fridger

PS: probing your above pixel formula at varying light levels might provide further evidence about its proper normalization and the correct ratio of core / glare size of bright stars.

[chris]

another related question to ask is how your stars behave, once the sensitivity is increased from visual to a 200 mm photographic lens, say. In Celestia the automag might be a relevant testing ground for this. Anyway, in this case, the distance to the stars remains large throughout.

We see that the two bright stars really blow up in size with the glare regime being very conspicuous. An uncountable number of background stars is visible with the star size remaining tiny.

In fact, with glare described as a Gaussian, extremely bright stars did not look like the ones in your image. With a Gaussian, the edges of the star get sharper as the brightness increases. This matches what we see for the core PSF, but the glare behaves differently.

Here's a very relevant paper titled "Physically-Based Glare Effects for Digital Images": http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf

The authors describe the PSF for the eye as the sum of three terms: a central Gaussian, a theta^-2 term, and a theta^-3 term (where theta is the angle from the center of the light source.) Here's a sample image of the Southern Cross where Gaussian glare is replaced by G(1/(1 + k*r^3)), where G is the glare brightness, k is a constant that sets the glare falloff rate, and r is the distance in pixels. I deliberately did not the angular distance, as it's more desirable to resolve stars at high zoom factors than it is to try and realistically reproduce all the quirks of a real imaging system (and I realize that's what I did with the diffraction spikes, but they're aesthetically pleasing ).

New glare function

Where would the hardware-specific profile enter in your approach? Once I increase the amount of collected light by some device other than the eye, I need to know its spectral sensitivity! A CCD, for example is way more sensitive in the blue regime than normal film.

You could multiply the final pixel colors by RGB factors right before the linearToSRGB step.

--Chris

[W0RLDBUILDER]

The diffraction spikes should be an option in the next version of Celestia. Cool!

[chris]

PS: probing your above pixel formula at varying light levels might provide further evidence about its proper normalization and the correct ratio of core / glare size of bright stars.

Here's just such a sequence... Crux again, with a limiting magnitude changing in steps of one magnitude:

Limiting magnitude = 8

Limiting magnitude = 9

Limiting magnitude = 10

Limiting magnitude = 11

Limiting magnitude = 12

There are several free parameters:
- The limiting magnitude
- The 'saturation magnitude'--the magnitude at which a star centered exactly on a pixel will reach the maximum pixel value
- sigma^2 for the PSF
- The glare brightness; arbitrarily set now, but this should be some fraction of the total star light that gets scattered in the optical system
- The glare falloff

In the images above, the limiting magnitude and saturation magnitude both change, but the difference between the two remains constant. Increasing the distance between limiting and saturation magnitudes will allow a greater range of brightnesses to be shown without blowing out the bright stars. The tradeoff is that visual contrast between bright and faint stars is reduced.

Star colors are derived here are derived from Johnson B-V color indices. The color index is converted to an effective surface temperature. From the black body temperature, I use a piecewise cubic approximate to get the CIE xy color coordinates. These are converted to CIE XYZ, transformed to linear sRGB, and then used in the pixel shader. It is important to avoid using gamma-corrected sRGB colors in the pixel shader, as the multiplication of pixel brightness and color must be done in linear color space. My earlier images suffered from this error and thus appeared too unsaturated.

--Chris

[chris]

For comparison with the images in the previous post, here's Crux again with a limiting magnitude of 12 and saturation magnitude 4. The bright stars of the Southern Cross are now not nearly so overexposed. Changing the difference between saturation magnitude and limiting magnitude amounts to gamma correction, while changing the limiting magnitude is adjusting the exposure.

Limiting mag = 12.0, saturation mag = 4.0

--Chris

[CAP-Team]

I really like the new way of drawing stars. Is this going to be an extra option for CTRL+S or will it replace the older methods?

Any chance this wil hit SVN soon?

[chris]

I really like the new way of drawing stars. Is this going to be an extra option for CTRL+S or will it replace the older methods?

I'd like it to replace the older methods on capable hardware. After using this new star rendering technique, none of the old methods seem adequate.

Any chance this wil hit SVN soon?

It's not ready for SVN, but I do have a patch ready for testing. PM if your interested (and are building Celestia from the SVN source.)

--Chris

[Guckytos]

I really like the new way of drawing stars. Is this going to be an extra option for CTRL+S or will it replace the older methods?

I'd like it to replace the older methods on capable hardware. After using this new star rendering technique, none of the old methods seem adequate.

--Chris

Chris,

what is "capable hardware" in this case? Can you give a minimum graphics chip generation? And how would it impact on the FPS of this minimum?

Regards,

Guckytos

[CAP-Team]

Chris, in the pictures above, do you use the standard star database or some other database?

[chris]

Chris, in the pictures above, do you use the standard star database or some other database?

It's the Tycho 2 catalog down to magnitude 10--about a million stars, versus the 100,000 or so in the standard Celestia catalog.

--Chris

[chris]

I really like the new way of drawing stars. Is this going to be an extra option for CTRL+S or will it replace the older methods?

I'd like it to replace the older methods on capable hardware. After using this new star rendering technique, none of the old methods seem adequate.

--Chris

Chris,

what is "capable hardware" in this case? Can you give a minimum graphics chip generation? And how would it impact on the FPS of this minimum?

Any NVIDIA card from the GeForce FX on will work. ATI Radeon cards will work as long as they are from the X1xxx series or later (anything 2005 or later should be OK.) The new stars should actually be faster, since more work gets pushed onto the GPU. In fact, with some modifications to the octree processing, the new star code could be quite a bit faster. The amount of CPU work could be reduced to a bare minimum; I recall that when I last profiled the star code, a significant amount of time was spent computing apparent magnitudes for stars. This step could be eliminated altogether if the star octree traversal and shader were written to work with absolute luminosities instead of magnitudes.

--Chris

[CAP-Team]

A disadvantage of this new star rendering is that it's perfect as long as you don't travel between stars.

If you select a star and then go to it, it brightens twice. First you approach the glare and when that's full screen, you approach the star it self.
But I guess it's hard to find a decent way to transit from a big glare to the star itself.

[Boux]

I have been playing with Chris's code (big thanks for sharing!).
The new star rendering is very nice and can be gorgeous with some tweaking.
First, I have got rid of the 1 Ly solar system size limit so that the transition between shader and mesh rendering is smooth.
Then I have been playing with the variables and I found that brightnessBias is the most controllable.
The screenshots below - /!\ big pics! - are made with brightnessBias set to 0.5, 0.7 and 0.8

EDIT Aug 12 --> brightnessBias back to (0.0f)

The first shot simulates a bare-eyes sight from someone's backyard:


The second shot simulates the same scene as seen through hypothetic binoculars:


Now, let's try with a digital camera on some sort of equatorial stand with a few minutes long exposure time:


Next, this small telescope that we just found at the nearby pawn shop will allow us to do some zooming in:


How about fitting a ccd sensor to our telescope?


OK, enough experimentation for the time being and let's go for more eye candy with our pocket Hubble toy:

Galactic center


Carina


Andromeda


Southern Cross


Now I am going to tweak some more variables

[Boux]

As a reference to t00fri's post re alpha cent picture...
Some not so bad shots...
Celestia's sensor is doing pretty well and does not leak light
Done with brightnessBias(0.65f)
EDIT Aug 12 --> brightnessBias back to (0.0f)
What is weird is that there are more visible differences between fuzzy points and point rendering on my (big) display than on the smallish cropped screen captures.

Fuzzy points:


Points:


Scaled discs:


Scaled discs with a tad more glare effect would be very close. Will check that.
BTW, we need more stars in Celestia!
 

[chris]

I have been playing with Chris's code (big thanks for sharing!).
The new star rendering is very nice and can be gorgeous with some tweaking.
First, I have got rid of the 1 Ly solar system size limit so that the transition between shader and mesh rendering is smooth.
Then I have been playing with the variables and I found that brightnessBias is the most controllable.
The screenshots below - /!\ big pics! - are made with brightnessBias set to 0.5, 0.7 and 0.8

The new star rendering is only enabled when point stars mode is selected. The brightnessBias setting will only affect the old star rendering. The adjustable parameters for the new star rendering are the limiting magnitude (adjusted with the square bracket keys) and the various settings in the renderShaderStars method. The three variables that control how star brightness are mapped to pixel values are the limiting magnitude, saturation magnitude, and the just visible pixel value. Here are the values from the code:

Code: Select all

// Saturation magnitude is the magnitude at which a star centered on a pixel                                                                               
// will have the maximum pixel value.                                       
float saturationMag = faintestMag - 4.5f;

// Minimum pixel value (wihout gamma correction) that is visible; this will vary                                                                           
// based on display device and viewing conditions                                                                             
float justVisiblePixelValue = 1.0f / 300.0f;


Roughly speaking, stars with an apparent magnitude equal to the limiting magnitude will have a pixel value equal to the just visible value, and stars at the saturation magnitude will have a pixel value of 1.0. Gamma correction and that the stars are Gaussians rather than points complicates things slightly, but this should give an idea of the physical meaning of the parameters.

One thing missing from your screenshot is a background of faint stars. By adjusting the brightnessBias to a very high value, every star appears bright. For realistic images, there should be a rich field of faint stars in the image.

--Chris