Black Marble - Earth at Night

New data from the Suomi NPP satellite allows NASA scientists to create new global views of Earth's city lights at night. More here.

Centennial

The new Universal Centennial Logo which you might have already seen in the cinema this year. Great CG by Weta Digital:

Making of a Planet - Part 4 - Simple atmosphere shader

Here's a good place to start if you need some instant texture maps for creating a CG Earth: JHT's Planetary Pixel Emporium. The color, bump and specular maps are self-explanatory, although in my case I'm using true displacements instead of bump. You can also get the Blue Marble textures from NASA's Visible Earth here.

The tricky part is to create the Earth's atmosphere. From reference photos, we can see that due to the curvature of the planet, the atmosphere appears more visible along the circumference compared to the center.

To achieve this effect, we can make use of the facing ratio of the surface toward the camera. Referring to the diagram below, when the view vector and the surface normal is in line, the facing ratio is 1, when they are perpendicular, the facing ratio is 0. Surfaces that face away from the camera are not rendered, so need not be considered. As such, the facing ratio at any point always fall between 0 and 1:

There are different ways to obtain the facing ratio. Maya has a very useful Sampler Info utility node which will provide that information. In Houdini, it is simple to build using the Eye vector (I) and the surface Normal vector (N). Just do a dot product, which will give you the cosine of the angle between them as long as the vectors are normalized. When the angle is 0° (I aligned with N), the cosine is 1. When the angle is 90° (I perpendicular to N), the cosine is 0°. Remember to normalize both N and I, and also, since I is the direction from Eye (camera) to the surface, it has to be negated. Finally, an Absolute node is used to ensure that the output values are positive. The SHOP network is shown below:

The network above will shade any object as if there is a headlight attached to the camera (surfaces facing the camera are brighter and surfaces facing away are darker). From there, we will adjust the output into a suitable type and range using a bit of math. The diagram below illustrates what we are trying to achieve:

The following SHOP network does exactly the above:

The Complement node inverts the black and white facing ratio, and the Power node (exponent of 6 used in this example) adjusts the intensity. A Constant of type "color" set to a suitable shade of blue is multiplied with the output of the Power node to get the blue tinted version, which is then output as the Surface Color. The output from the Power node is fed into the Surface Opacity output to allow for transparency.

Let's apply this shader to a new sphere object slightly larger (1% larger in this example, using a uniform scale factor of 1.01) than the original Earth sphere :

Of course, this is just a starting point. There is immediately the problem of the atmosphere being lit even along the shadowed side of the Earth. This is easily remedied by multiplying the output color by a shadow term, or by a self-shadowing diffuse shader such as Lambert:

The example below uses a Lambert diffuse intensity of 2

While this is more physically correct, the previous unshadowed version looks aesthetically more pleasing, which is why many planetary artists invoke artistic license to bend the laws of physics. Alas, our goal is photorealism, so we will stick with the correct look for this kind of exposure levels. Just remember it is a viable option because the stars (and sometimes the Moon) always provide faint illumination to the night side of the Earth.

Long way to go...

There's still so much that needs to be done to improve the quality of my CG Earth, and I've been exploring options and learning that I really don't have enough know-how to get there yet.

Clouds are the biggest challenge and I've never been happy with my solution, so I'm going back to the drawing board with them. It's a tough nut to crack so it might be a while before I can achieve a proper cloud-filled Earth.

Next, surface detail. Considering I'm using at best 500m/pixel data from BMNG, I began experimenting with adding fractal noise to the displacement and other channels in the hopes of being able to do more close-up shots.

Since I'm starting with a displacement map, just adding displacement noise without regard for the existing topography will result in very artificial output - the same way that simply combining multiple orders of fBm noise will not make a fractal landscape look natural. I'm convinced I need some kind erosion algorithm (fractal, hydraulic, thermal...) but I haven't been able to find any good reference on the subject.

Sometimes when I look up one subject, I find that I need to understand several other topics before I can proceed. Other times, one subject leads to several other related issues that I had not been aware of. There's so much to study, understand and implement that I have to become less of an artist and more a weekend programmer, mathematician and scientist. Gradually, I hope to summarize what I've learnt that is relevant to the CG Earth project within the pages of this blog.

Meanwhile, here's another WIP shot. It has subtly better atmosphere and surface detail than a similar shot I created last year.

2012 version with better terrain detail, specular (rivers) and atmospherics (no more volumetric hack).

2011 version, took some hacking to get it looking like this.

Another making-of

Ever wondered about the Earth image on the screen background of every new iPhone? Turns out it's another Blue Marble image and here is an article on how it's made written by the artist Robert Simmon himself: Crafting the Blue Marble.

Iconic Earth images

While researching, I stumbled across this great article about 10 Iconic Images of the Earth from Space. Images featured include the Blue Marble and the Pale Blue Dot.

One of the most iconic photos ever is the Earthrise taken by astronaut William Anders on Dec 24, 1968 during the Apollo 8 mission. To quote Anders: "We came all this way to explore the Moon, and the most important thing is that we discovered the Earth"

The first public image of an Earthrise from Moon orbit (Dec 24, 1968)

In 2007, JAXA (Japan Aerospace Exploration Agency) launch a lunar orbiter SELENE (nicknamed Kaguya after the moon princess from Japanese folklore) whose on-board HDTV cameras recorded high-definition video footage of a newer Earthrise:


We are indeed lucky to be living at a time when technology allows us to appreciate our home planet from a perspective that must have once been unimaginable.

Shader upgrades ^^

Necessity can be a great motivator. A recent freelance opportunity meant I had to create much faster shaders to get the job done in time. I dove head first into VEX code writing for the first time, and managed to adapt Sean O'Neil's real-time atmospheric scattering algorithm into my new Earth shaders.

My method uses the CPU instead of GPU and is not real-time, but is still multiple times faster than my old method while looking visually very similar. Some volumetric shadow effect is lost, but not really noticeable if you are not looking for it, and the amount of speed up is well worth tradeoff.

Naturally, the implementation is not straightforward and took me almost 3 days to get it working properly. Perhaps I'll post more details under the "making-of" label in future.

Making of a Planet - Part 3 - Shader Basics

In this post I'll be describing what I learnt about shaders when building the CG Earth, with specific examples of Houdini VEX shader networks. First off, a quick introduction to how shaders work.

What are Shaders?

Shaders can be thought of a set of program instructions that tells a renderer how to determine the color of a surface (or volume) at any given location. In 3D programs, they are also referred to as materials.

Shaders are actually program code written in a shading language. However, Houdini allows the code to be build visually as a node network (called a SHOP network, for SHader OPerators). The most famous industry shader language for rendering CGI is the RenderMan shading language. For real-time graphics (e.g. games and 3D applications) you will hear names like GLSL and HLSL. Houdini's shading language is called VEX (Vector EXpressions).

Regardless of shading language used, the simplest shader returns a constant color at every point, making the object appear uniformly flat. In the following example, a constant color blue is being fed into the surface output of a Material Shader Builder node in Houdini's SHOP network:

The constant shader instructs the renderer...

 ...to use the same color for every point on the surface

Let's consider how light may affect how the surface looks.

The following example is the basic Lambert shader (which mathematically is the dot product of the normalized surface normal vector, N, and the normalized light vector, L). It's amazingly elegant that this single dot product formula is enough to describe how the light falls off a surface depending on its angle to the light source:

A classic Lambertian shader...

...makes the teapot surface react correctly to light

Lighting Inside a Loop

In Houdini, this lighting calculation must take place inside what is called an illuminance loop. A loop in computer programming is a sequence of instructions that repeats itself until particular conditions are met. Think of it as repeatedly calculating the same lighting formula (dot product) for all the points on the surface that appears in the final image, using the corresponding N and L vectors for each point location.

So we now have a properly lit teapot, but what if we want it in blue color? All we need to do is to multiply the result of the illuminance loop with a constant (blue color) before passing it to the output:

The Lambert shading network from before is actually hidden inside the illuminance loop node above

Only one dot product and one multiplication is needed to get this blue teapot

Houdini already includes a selection of basic CG shaders (such as Lambert) as a network node so we don't really need to create our own illuminance loops. However, writing your own illuminance loop function allows specific low-level control over the look of your material. For the Earth shader, I adapted a light wrap shader inside the illuminance loop to simulate atmospheric scattering.

Building Visual Complexity

When shaders take into account various factors such as lighting and texturing, more complex looks can be achieved. Notice also how we can take the first two separately rendered images (the constant blue and the grayscale lit teapots) and combine them using a multiply blending in a 2D program to get the same results (blue lit teapot). Therein lies the flexibility and power of render passes and compositing, because we can tweak the amount of blending without having to re-render in 3D.

The example below is a slightly more complex shader including specular highlights and texture mapping, but the basic concept is the same as the previous examples. Notice that the texture map (diffuse color) is multiplied with the Lambert shading, and the specular is added to the results.

No illumination loop used here

More complex shaders will result in more realistic surface appearance

Shader writing may not be everyone's cup of tea, but with some effort anyone can write their own shader. There's a definite amount of mathematics and logical thinking involved, but with it comes the power to go beyond what is available off-the-shelf.

If you're keen to find out more, I recommend the following books:

1) Texturing and Modeling: A Procedural Approach
2) The RenderMan Companion: A Programmer's Guide to Realistic Computer Graphics
3) Advanced RenderMan: Creating CGI for Motion Pictures

That's it for today. Next post I'll elaborate more about my Earth shaders.

Enjoy a few more rendered images before the next installment of "making-of" ^^;

Making of a Planet - Part 2

In this post, I'll be describing in more detail how the CG Earth is constructed.

The whole planet was broken down into several key components: land, water, clouds, atmosphere and an atmospheric "rim". The atmosphere has two components because it's not a true volumetric simulation, but a kind of hack ^^;

Land 

Nothing fancy here that's not been done for decades. A simple sphere primitive with polar (spherical) UV projection so that the Blue Marble textures can be wrapped around it. Of course, it's well known that the Earth is not a perfect sphere, but an oblate spheroid. This is approximated using an Equatorial radius of 6378.14km and Polar radius of 6356.75km

An early "making-of" montage showing the key components of the Earth setup

A displacement shader is used with elevation data to create the mountains and terrain above ground, and bathymetry data is used to create the ocean floor below sea level.

Water

This is just another sphere primitive with an ocean shader applied. Water reflects the sun with a much stronger specular glint than the land or clouds. In the final version, however, I ditched both the bathymetry and water components and instead included the ocean shader into the single land shader for the entire planet. This was done to optimize rendering since the sea is really not that transparent from space, therefore calculating displacement for the sea floor is a waste of render time.

Clouds

After much experimentation with various volumetric hacks, I eventually settled upon the same age old technique everyone else is using - a sphere (surprise, surprise) mapped with transparency data from NASA's cloud texture, with the same cloud map doubling as a displacement map to give the clouds actual thickness. On top of this, I added procedural noise for detail, since the cloud map is of noticeably lower resolution than the land texture. To be honest, I feel that I have failed in the implementation of this component most miserably, so future versions will likely be done differently.

Displacement mapping to give the clouds thickness; procedural noise to add detail

Atmosphere and Rim

Another key component that makes or breaks the look of a CG planet is the atmosphere. The atmosphere of the Earth interacts with sunlight to produce a multitude of lighting effects and localized color shifts. Indeed, it is reason why the sky is blue and why the Sun appears yellowish when viewed from the ground. 

No atmosphere

With atmosphere

The volumetric effect is created using a set of concentric sphere of increasing size and decreasing opacity. The Rayleigh scattering effects is creating using ramp lookup based on a modified Lambert angle. As the shading approaches the terminator, the light color turns reddish.

The other component is the atmospheric rim, which uses another ramp lookup to create the blue "outline" around the limb of the Earth.

I must confess the most of the effects are hacked - they are "physically informed", but not physically based. At this point in time I'm still researching true volumetric solutions for the clouds and true scattering for the atmosphere.

The sane Rayleigh ramp lookup is used to color the cloud layer. A crude but effective technique ^^;;

Size Matters

As mentioned previously, all modeling was done to relative scale as much as possible in the spirit of "realism". Initially I used a unit of 1km but the comparatively small details of Earth and the astronomical scale of the solar orbit appear to cause precision problems when rendering shadows. No amount of tweaking the raytracing bias worked. In the end, through trial and error I found that using a base unit of 10km seem to minimize the glitches. So, for example, instead of 6378.14 for the Equatorial radius, I used 637.814

In the next post, I'll talk about the various shaders.

Making of a Planet (version 0.2b) - Part 1

As promised, I shall attempt to summarize the techniques I used to create the CG Earth from the previous post. This version represents another milestone in my efforts to create a decent photorealistic Earth using computer graphics.

Before I begin, it seems more and more people are interested in Earth observation. One emerging hobby is for people to build and launch home-made high altitude balloons containing low-cost GPS systems and digital cameras up into the stratosphere to capture videos and photos of the Earth from "space". These balloon capture amazing imagery of the Earth from altitudes upwards of 30km, but is still well within the Kármán line (altitude of 100km) commonly considered the boundary between Earth's atmosphere and outer space. For comparison, the International Space Station (ISS) orbits between 278-460km above ground. Still, the balloon footage are a great source of additional reference material ^_^

Scene Setup

Okay, back to the production report. In this first post I will outline the features I have built into my scene setup and the type of options they offer for creating Earth renders.

Houdini scene

Looking at the scene network above, you can see that I have not only created the Earth, but also the Moon, Sun and the ISS. I've been careful to scale all the objects as well as their orbits to relatively accurate proportions using data from the internet. In reality, all orbits should be elliptical, but circular orbits are easier to setup for now. However, I did apply correct inclination and axial tilt parameters for both the Earth and Moon.

The Sun is 1,380,000 km across in diameter (about 110 times that of Earth), and 149,600,000km from Earth. It takes light 8.32 minutes to reach Earth from the Sun! The Earth is way much smaller than 1 pixel at this scale ^^;;

The Moon looks tiny bacause it is 384400 km away from Earth. It takes light 1.3 seconds to reach Earth from the Moon! We can make the Moon appear bigger in shot by increasing the focal length

The setup is basic CG101: just use null objects and correct parenting. So, the Earth rotates about it's axis, the ISS and Moon rotates around the Earth, and the Earth-Moon system rotates about the Sun. The animation controls are set up for properly relative orbital periods - i.e. Earth rotates on it's own axis once a day, ISS makes one revolution round the Earth in 90 minutes, Moon revolves round the Earth every 27.3 days, Earth goes round the Sun in 365.25 days. Of course, time can be allowed to speed up, slow down or stop in the CG world. Offset controls allow independent control over exactly where to position the objects in orbit to allow fine tuning for shot composition.

Cameras and Animation Controls

All this trouble to be "realistic" actually makes setting up cameras and shot composition even more difficult. So I took a more sensible approach by setting up orbital cameras around the Earth (attached to the ISS) and Moon, with controls to adjust their location and orientation. Again, it's all null objects and parenting, with expressions to drive rotation channels to simulate orbit.

Although properly set up, I ended up using this only for reference and using Maya to render the ISS because it was too much work to surface the model properly in Houdini. It was tedious in Maya too, but the workflow is much less painful.

Centering the Earth Cam on any location is as easy as entering their latitude and longitude. Here's London at 51.5171° N, 0.1062° W

View from the Moon ^^

In the next post, I'll describe how the Earth is constructed. Until then... ^^/

The story so far...

A video compilation of my work-in-progress is now on Vimeo. I will follow up with a "making of" post in this blog, hopefully in the near future. Comments and critique are most welcome. Any pointers on how I can improve the look will be much appreciated ^_^;

Bella Gaia

More Earth appreciation news ^^

BELLA GAIA, which means "Beautiful Earth" is a multimedia audio-visual presentation expressing the deeply moving beauty of planet Earth as seen through the eyes of astronauts.Through data visualizations, dance and music performances, it tells the story of the living universe and explore humanity's challenges as we enter a new human epoch called the Anthropocene. We can see real-time display of the Earth rendered by the Uniview visualization platform.

Correcting colors

It's been almost a year since I rebooted this project (switching to Houdini and learning how to write VEX shaders), during which I have spent countless hours making minute tweaks to various shader properties in an attempt to get the right look and feel.

Alas, I feel I have pushed the shaders far enough. I've decided that it's time to apply color correction and adding effects in post to bring the final look home. At times it seem the changes I keep making are too miniscule to be even noticeable, but then again, I find it hard to put things down as long as something doesn't look right. The problem is - something always look out of place :(

Anyway, I constantly remind myself that this project is a learning process, and ultimately worth the effort at least from the educational angle ^^;;

Good Morning Eden!

After weeks of re-working the VEX shaders, I am finally back on track. Here are a sequence of WIP shots (dated 27 Mar) highlighting one of my initial goals of the project - to recreate a sunrise from space sequence:

At first glance, the shots look similar compare to my early versions (created using Maya and After Effects compositing trickery in 2010), but believe me there is a world of difference, especially when things start moving. If you scrub through the sequence, you can see a hint of atmospheric scattering at work as the sunlight penetrates the atmosphere.

Naturally, I will have to add in the Sun with accompanying flares and effects during post-production in the eventual final version.

Seas and skies

In the last post I mentioned how my shaders crashed under the new version of Houdini. I'm glad to report that I have re-worked them to work with H12, and in the process deepened my understanding of VEX shader networks. I've also managed to remove the rendering artifacts and shader errors, mainly by not assuming that unconnected inputs will have the correct default values applied. Subsequently, I have also refined various aspects and added new features to my Earth shaders.

The two most significant additions are the simulation of forward scattering in the atmosphere and anisotropy in the specular highlight of the ocean, as seen in the following work-in-progress snapshots:

The atmosphere scatters more light as the Sun gets behind it, an effect attributed to Mie scattering of light by particles in the air

The specular reflection of the Sun is stronger at more glancing angles, a behavior called the Fresnel effect. The shape of the highlight also elongates as the Sun nears the horizon, mimicking how the Sun reflected in the ocean looks during sunsets

Progress update

Just last week, Side Effects Software launched Houdini 12 and it's full of cool bells and whistles. I downloaded the Apprentice version and did a quick test with my Earth scene, expecting rendering speed improvements but my shaders just keep crashing the Mantra renderer. More on that in a later post...

Meanwhile, here are the latest updates with NASA photos for comparison. All are Houdini 11 renders with only exposure and gamma adjustments, mild sharpening and no color correction, down sampled to 8-bit JPG from 32-bit EXR renders.

For the more observant, those tiny white dots are a strange rendering artifact which I have yet to figure out how to remove :(

This is a closeup of the above shot. It may not be obvious, but the land, clouds and atmosphere are lit with sunset colors near the terminator

NASA photo

Still in the process of balancing colors, but I'm reaching a point where setting the right exposure alone allows me to match different reference photos without the need for much color correction

NASA photo (Apollo 11)

 

NASA photo (Apollo 11)

  

I drastically reduced the amount of cloud coverage. Remember all my data is based on NASA's BMNG 2005 release, and they have tons of clouds everywhere. I need new data and a new way of implementing atmosphere and clouds ^^;;

NASA photo

 Let's check the baseline I set in September 2011:

1. Accurate proportions - approximated using equatorial and polar radii
2. Axial tilt - this is a no brainer ^^;
3. Terminator reddening - faked
4. Atmospheric scattering - faked
5. Volumetric clouds - faked
6. Displacement mapped topography - done

In addition I've also implemented:

1. Location sensitive direct sunlight and ambient lighting colors (sunset lighting at the terminator)
2. Faked atmospheric refraction of sunlight causing the terminator line to shift back a few degrees
3. Atmosphere with altitude-based density falloff (non-physically based)
4. Specularity for inland rivers and lakes
5. City lights for night exposures

I'm happy with my progress considering I have no shader writing skills and very little Houdini experience prior to taking up this project. Still, it's a long way to go before the closeups can look convincing.