Advanced Shading and Lighting for Mobile
Hello everyone. You have probably already heard about Shadowgun, the new mobile game developed by MADFINGER Games. You might also have seen Shadowgun being demoed at WWDC on stage during the Graphics State of the Union talk.
Needless to say, we’re very proud of the next great creation made in Unity, and we love how it delivers console quality to the mobile with the help of Unity’s awesome tech. It runs at an impressive 60fps on the iPad 2 and 30fps on the iPad, iPhone 4 & 3GS. It will also be released on Android. Since lighting is one of the main elements making the game look so good, here are some notes about the techniques we used.
All characters in the game use the same per pixel normal mapped isotropic BRDF pixel shader, which goes beyond the usual Phong shading. It gives a great effect on the skin of the character, metal and cloth. Our solution combines off-line precomputed texture which describes how surface interacts with lighting and run-time per-pixel calculations to achieve both artistic control and superb performance.
Light probes are an advanced technique for lighting dynamic objects and characters with high-quality baked lighting. It is possible to capture incoming direct and bounced lighting within a scene at a number of points – light probes. As a character moves through the scene, nearby probes are picked, interpolated, and the result is then passed to the shader to efficiently light the surface. There it can be sampled per-vertex or per-pixel, if you want to use the normalmap-perturbed normal to do so.
Choosing and interpolating the right probes (and doing it fast!) is a hard problem, though. We have developed a technique, which takes an arbitrary set of probes and performs Delaunay tetrahedralization on them to divide the entire space into tetrahedron-shaped cells.
That gives us fast interpolation with great properties and no light popping – when you’re in a given tetrahedron, weights for the four corner probes are your barycentric coordinates! Finding the right tetrahedron can also be done efficiently by checking the one from last frame – if it’s not a hit (most of the time it is!), we check it’s neighbours. Finally we make sure interpolation works outside of the volume of the probes just as fast and as good with a clever projection onto the hull surface.
I’m especially proud of the light probes, since we managed to shape them up to be extremely fast and have low memory footprint – perfect even on mobiles and giving so much bang for the buck. As far as we know, it’s also the first time it has been used to such extent (at all?) on mobile devices.
As a bonus, since we’re already rendering every character with a light probe, we can project any additional lights onto the probe as well. We do that for the muzzle flash effect. For a very low CPU cost and no GPU cost characters get lit by dynamic light.
God rays, light shafts, light flares and fog
God rays, light shafts, light flares and fog are rendered using both static and animated alpha-blended geometry with very optimized vertex and pixel shaders to save fillrate.
Lightmaps were baked with one click in Unity using Beast to capture all the beautiful Global Illumination on static geometry. To give the objects a more detailed look while still keeping the shaders extremely fast, we calculate approximate specular reflections per vertex and then modulate them per pixel with a gloss mask to differentiate between shiny and rough parts. Reflective objects additionally use a pre-baked cube map matching the surroundings.
Caustics and character shadows
Caustics from the light passing through the green fluid, character shadows and rotating ventilator shadows were simulated with textures projected onto tight geometry (to keep the fillrate requirements low) and then animated in the shaders.
We plan to release light probes with Unity 3.5; all the other effects can already be achieved in Unity!