# SSS all the things!!

Jon Valdes did this super-funny image for the Beta Testing team after we released an internal build with SSS enabled.

One of the most remarkable improvements introduced by the new BSDF system I’ve been working on for the last weeks is the very complete and flexible physically-based SSS system which we announced some days ago: http://blog.randomcontrol.com

# Reflection/refraction schematics

This is a visualization of the behavior of a ray of light as it hits a dielectric interface.

Some key phenomena which show up in the video are:

• The Fresnel term (reflection vs. refraction).
• The Index of Refraction.
• The critical angle.
• Total Internal Reflection (TIR).

As nd increases the Index of Refraction becomes higher, and so does the Fresnel term, which defines the proportion between reflected and refracted light. The critical angle becomes higher too, so there is more Total Internal Reflection.

When a ray of light hits an interface (assuming an ideal surface), all incident light must be either reflected or refracted. The Fresnel term (controlled by nd) tells how much light is reflected at a given incident angle. All the light that is not reflected is refracted, so both amounts (reflection and refraction) always add up to the total amount of incident light.

The Fresnel term approaches 1 at grazing angles (all light is reflected and nothing is refracted, regardless of nd) and is low (the lower the smaller the nd) at perpendicular angles (more light is refracted).

As a rule of thumb:

• The lower the nd, the lower the Index of Refraction, and the more transparent the surface (more glass/liquid-like).
• The higher the nd, the higher the Index of Refraction, and the more reflective the surface (more metallic/mirror-like).

For example:

• Void: nd=1.
• Air: nd=1.1.
• Water: nd=1.33.
• Glass: nd=1.51.
• Diamond: nd=2.5.
• Metals: nd=20+. (approx. complex IoR)
• Ideal mirror: nd=infinity.

When a ray of light enters a medium with an nd lower than the nd of the previous medium, there is an angle at which the Fresnel term becomes 1 and beyond which light won’t refract anymore. This angle is called critical angle, and beyond it, the surface behaves like a perfect mirror, reflecting back all incident light. This effect is called Total Internal Reflection (TIR).

NOTE: The video was generated with an odd aspect ratio as it is a concatenation of 3 interfaces with different nd values. There’s also some text overlaid, so the video is best viewed full-screen / 720p HD.

# What do you normally do?

So true.

Erwan Loison put up this image for me. I guess that it explains very well what an average day in my life feels like.

# Lua vs. Python

The Lua logo

These days we’ve been adding scripting support to RCSDK. We constrained the answer to the question ‘what scripting language shall we support?‘ to Python and Lua.

Lua won by a long shot. Here’s why:

Both APIs (Python and Lua) are multi-platform, and are more than powerful enough to meet our demands. Both are also very popular and hence they can be considered highly reliable.

Python seems to be much more complete (higher level) and provides a comprehensive set of libraries. However, Lua is much more light-weight and there is some strong evidence that it is also much more efficient (see the related links below).

Last, but not least (and this is what I will talk about) the cost of embedding each of them into your own project happens to be very different.

Put in short:

– Lua is low-level, light-weight, clean and simple. Embedding it into our own SDK was trivial and took half an hour or so.
– Python is high-level, but fat, and its sources seem to be way more hellish/dirty. Embedding it took several hours of work.
– Lua increased the size of our .lib in 1.1 MB. Python did in 13.0 MB.

After taking a look at some benchmarks comparing both runtimes (speed/memory) we decided to choose Lua over Python.

The goal (in our case):

– Add scripting support to RCSDK.
– Avoid any external DLLs. RCSDK is designed to be just one file (a statically-linked library).
– Avoid any external LIBs. RCSDK is deployed in different operating systems, in 32-bit and 64-bit flavors, so it would be a terrible idea to keep many different .LIB files. So Python or Lua sources must be embedded in our own source code project. We always do this for the few exceptions where we use external libraries (libjpg, libpng, libtiff, libexr, libzlib, libfreetype).
– The same source codes must compile seamlessly across multiple platforms (Windows, Mac OS X, Linux) and in multiple compilers (MSVC, Intel C++ and GCC) without any warnings in /W4 at least.
– Short compilation times are a plus, so the fewer/simpler the files, the better.
– The scripting system should have no external dependencies whatsoever. It should not need the end user to install any additional packages or force us to add any files in our setups. Everything should be self-contained in the source codes.
– Last, but not least, efficiency (speed/memory) is a plus as well.

How to embed Lua-5.1.4 into your own C/C++ source project:

– Pick the sources package from the Lua website.
– Inflate the package wherever you like and get rid of everything but the /src folder. In my case I copied those files into a folder in my source project. Make sure to copy all the .c and .h files, but exclude lua.c and luac.c as those define a main()function themselves.
– (Optional step) Rename all the files, adding some prefix to them. It may be important to do this so you avoid future collisions with other .obj files, particularly if your project is large enough. In my case I used the prefix sys_lua_*. Then I ran a Search & Replace in the Lua files to replace #include “ by #include “sys_lua_.
– Lua is plain C, so if your own project is C++ you need to extern “C” the Lua #includes.

Also (bravo, Lua) the newly added source codes compile without any warnings at /W4. In 32-bit I only needed to use a single #pragma (C4996) to shut up those annoying _CRT_SECURE_NO_WARNINGS deprecations in MSVC (not Lua’s fault anyway). I needed a couple more #pragmas in 64-bit.

```lua_State* L = lua_open() ;

luaL_openlibs( L              ) ;
luaL_dofile  ( L , "test.lua" ) ;
lua_close    ( L              ) ;
```

You’re set to go. This code will load and run a test Lua script.

How to embed Python-3.2 into your own C/C++ source project:

From the very first moment that you download and inflate Python you notice the following:

While Lua is made of a small bunch of well-organized files, Python’s package has uncountable folders and subfolders with several trillions of files. Ok, Python is bigger and it may be able to do more stuff, but… come on. To begin with, the sources that you need are spread in 8 or 10 different folders where source/non-source files co-exist. Also, you need to hand-pick them selectively, as adding everything to your project will just fail to compile.

– Pick the sources package from the Python website.
– Inflate the package wherever you like and go to the /PCBuild folder.
– You will find a big mess in that folder. Open up the .sln file in Visual Studio (2008) and then remove all the .vcproj entries but pythoncore. This is the one producing the Python .DLL.
– Big hell ahead. Now open up your own source project (VS2010 in my case) and add to it each and every file in the pythoncore project. Be warned that you need to add many, many files. You may try to create a macro and transport the files from the original .vcproj to yours.
– Now spend some time fine-tuning the file pyconfig.h. You need to do this per platform, as Python deals with the OS native sockets, threading, etc… (Not the case in Lua). Python comes with a Windows-ready pyconfig.h which worked for me.
– (Optional step) Rename all the files, adding some prefix to them. It may be important to do this so you avoid future collisions with other .obj files, particularly if your project is large enough. In my case I used the prefix sys_pyt_*. Then I ran a Search & Replace in the Python files to replace the #includes accordingly.
– Make sure to #define Py_NO_SHARE_SHARED so the files are compiled for static (instead of dynamic) linking.
– Now get ready to get hundreds of warnings in VS2010 using /W4. #pragma party. I must say that I took a look at the code locations where those warnings were happening, and… some dirty things were happening down there.

Note that the newly added sources in your project won’t compile in other platforms unless you review the file pyconfig.h. So if you want to use just one config file, you need to #ifdef it per platform.

Also note that Python won’t run (it will throw an error and then crash) unless you copy the /Lib folder you will find in the Python setup into your execution folder. You need to do either that, or set the proper environment variable so Python knows the path of that folder.

The conclusion based on my experience:

– Lua’s impact in your source project is much more gentle (fewer and cleaner files).
– Lua is trivial to embed and highly portable. In our particular context, it took a surprisingly high amount of effort until the compilers swallowed Python, then some extra effort to shut up the warnings, and last, even more effort to get it to work in each of the platforms our SDK is designed to support.
– By default, Lua runs without the need for any external files, environment variables, etc…
– The size of Lua’s runtime wins hands down. Its performance and memory usage are remarkably better. This is the main reason why Lua has become so popular among game developers.
– Python is better equipped, but that extra equipment comes at a steep price.
– Python has more language features. Lua is more barebone which is both good (flexibility) and bad (may need some more effort on your side to implement things).

So, if you need the extra equipment that Python offers, go for it. Otherwise, you should go for Lua. Much more so if you’re paranoid about things such as performance, portability, flexibility, cleanness, executable size, etc…