* The Tesseract Rendering Pipeline

This article is a brief overview of the rendering pipeline in Tesseract, an open-source FPS game and level-creation system. It assumes basic knowledge of modern rendering techniques and is meant to point out particular design decisions rather than explain in detail how the renderer works.

* Platform

The Tesseract renderer ideally targets OpenGL 3.0 or greater, using only Core profile functionality and extensions, as its graphics API and using SDL 2.0 as a platform abstraction layer across Windows, Linux, *BSD, and MacOS X. However, it also functions on OpenGL 2.1 where extensions are available to emulate Core profile functionality. Mobile GPUs with OpenGL ES are, at the moment, not targeted. GLSL compatibility is provided across various incompatible versions using a small preprocessor-based prelude of macros abstracting texture access and attribute/interpolant/output specification that would otherwise be incompatible across GLSL versions.

* Motivation

The main design goal of Tesseract is both to support high numbers of dynamically shadowmapped omnidirectional point-lights and minimize the number of rendering passes used with respect to its predecessor engine, Sauerbraten. While improved visuals over Sauerbraten were somewhat desired, it was more important to make the rendering process more dynamic with at least comparable visual fidelity while still emphasizing performance/throughput, although at a higher baseline than Sauerbraten required. So while other potential design choices might have resulted in further visual improvements, they were ultimately discarded in the service of more reasonable performance.

Sauerbraten made many redundant geometry passes per frame for effects such as glow, reflections and refractions, and shadowing that multiplied the cost of geometry intensive levels and complex material shaders. So, where possible, Tesseract instead chooses methods that factor rendering costs, such as deferred shading which allows complex material shaders to be evaluated only once per lighting pass, or screen-space effects for things such as reflection that reuse rendering results rather than require more subsequent rendering passes.

Further, Sauerbraten was reliant on precomputed lightmapping techniques to handle lighting of the world. This approach posed several problems. Dynamic entities in the world could not fully participate in lighting and so never looked quite "right" due to mismatches between dynamic and static lighting techniques. Lightmap generation took significant amounts of time, making light placement a painful guess-and-check process for mappers, creating a mismatch between the ease of instantly creating the level geometry and the not-so-instant-process of lighting it. Finally, storage of these lightmaps became a concern, so low-precision lightmaps were usually chosen, at the cost of appearance to reduce storage requirements. Tesseract instead chooses to use a fully dynamic lighting engine to resolve the mismatch between lighting of dynamic and static entities while making better trade-offs between appearance and storage requirements.

Certain features of Sauerbraten's renderer that were otherwise functional such as occlusion culling, decal rendering, and particle rendering have mostly been inherited from Sauerbraten and are not extensively detailed in this document, if at all. For more information about Sauerbraten in general, see http://sauerbraten.org

* Shadows

Tesseract's shadowing setup is built around observations originally made while implementing omnidirectional shadowmapping in the DarkPlaces engine.

The first observation is that by use of the texture gather (a.k.a. fetch4) feature of modern GPUs, it is possible to implement a weighted box PCF filter of sufficient visual quality that generally performs better than all other competing shadowmap filters while also requiring less bandwidth-hungry shadowmap formats, especially if 16bpp depth formats are used. This is advantageous over competing methods such as variance shadowmaps or exponential shadowmaps that prefer high-precision floating-point texture formats or prefiltering with separable blurs that can become quite costly when shadowmaps are atlased into a larger texture as well as suffering from light bleeding artifacts that plain old PCF does not have any problems with. A final benefit of relying only on plain old depth textures and PCF is that depth-only renders are generally accelerated on modern GPUs and so provide a speedup for rendering the shadowmaps in the first place over other techniques before they are ever sampled.

For general information about PCF filters, see http://www.gdcvault.com/play/10092/Efficient-PCF-Shadow-Map

It was later discovered that this same weighted box filter could be approximated with the native bilinear shadowmap filter (originally limited to Nvidia hardware under the "UltraShadow" moniker, but now basically present on all DirectX 10 hardware when using a shadow sampler in combination with linear filtering) so that no texture gather functionality is even required, and allowing further performance enhancements. The particular approximation avoids use of division/renormalizing blending weights while only causing a slight sharpening of the filter result that is almost indistinguishable from the aforementioned weighted box filter. This method in general, though, allows the (approximated) NxN weighted box filters to be implemented in about (N+1)/2*(N+1)/2 taps. The default shadowmap filter provides a 3x3 weighted box filter using only 4 native bilinear taps, providing a good balance between performance and quality.

The final 3x3 filter utilizing native bilinear shadow taps contains some non-obvious voodoo and was largely found by experimenting with fast approximations for renormalizing filter weights in the weighted box filter. Ultimately it was discovered that just the seed value for iteration via Newton's method (http://en.wikipedia.org/wiki/Newton's_method) was more than sufficient to compute filter weights and did not significantly impact the look of the result. Texture rectangles are also used where possible instead of normalized 2D textures to avoid some extra texture coordinate math. The filter (with the unoptimized yet more precise box filter in comments) is listed here for posterity's sake:

    #define shadowval(center, xoff, yoff) float(shadow2DRect(shadowatlas, center + vec3(xoff, yoff, 0.0)))
    float filtershadow(vec3 shadowtc)
    {
        vec2 offset = fract(shadowtc.xy - 0.5);
        vec3 center = shadowtc;
        //center.xy -= offset;
        //vec4 size = vec4(offset + 1.0, 2.0 - offset), weight = vec4(2.0 - 1.0 / size.xy, 1.0 / size.zw - 1.0);
        //return (1.0/9.0)*dot(size.zxzx*size.wwyy,
        //    vec4(shadowval(center, weight.zw),
        //         shadowval(center, weight.xw),
        //         shadowval(center, weight.zy),
        //         shadowval(center, weight.xy)));
        center.xy -= offset*0.5;
        vec4 size = vec4(offset + 1.0, 2.0 - offset);
        return (1.0/9.0)*dot(size.zxzx*size.wwyy,
            vec4(shadowval(center, -0.5, -0.5),
                 shadowval(center, 1.0, -0.5),
                 shadowval(center, -0.5, 1.0),
                 shadowval(center, 1.0, 1.0)));
    }

This idea is extended to larger filter radiuses but is not shown here.

After experimenting with different projection setups for omnidirectional shadows such as tetrahedral (4 faces) or dual-parabolic (2 faces), it was found that the ordinary cubemap (6 faces) layout was best as the larger number of smaller frustums actually provides better opportunities for culling and caching of faces while providing the least amount of projection distortion. However, for multi-tap shadowmap filters, the native cubemap format is insufficient for easily computing the locations of neighboring taps. Also, despite texture arrays allowing for batching of many shadowmaps during a single rendering pass, they do not allow adequate control of sizing of individual shadowmaps and their partitions. 

For further information about the basics of rendering cubemap shadowmaps, see page 42+ of ftp://download.nvidia.com/developer/presentations/2004/GPU_Jackpot/Shadow_Mapping.pdf

Both of these problems may be addressed by unrolled cubemaps, or rather, by emulating cubemaps within a 2D texture by manually computing the offset of each "cubemap" face within an atlas texture. The face offset needs only to be computed once and then any number of filter taps can be cheaply computed based on that offset. The perpective projection of each frustum must be slightly wider than the necessary 90 degree field-of-view, to allow the filter taps to sample some texels outside of the actual frustum bounds without crossing any face boundaries. A filter with an N texel radius needs a face border of at least that many texels to account for such out-of-bounds taps.

Further, it becomes trivial to support custom layouts based on modifying the unrolled lookup algorithm, or to allow other types of shadowmap projections to co-exist with the unrolled cubemaps in a single texture atlas. Yet another advantage of the cubemap approach in general, not limited to unrolled cubemaps, is that rather than sampling omnidirectional shadows frustum-by-frustum (requiring as many as 6 frustums) as some other past engines do and needing complicated multi-pass stenciling techniques limit overdraw, the omnidirectoinal shadowmap may be sampled in a single draw pass over all affected pixels.
 
Initially, this emulation was done by use of a cubemap (known as a "Visual Shadow Depth Cube Texture" or VSDCT) to implement the face offset lookup to indirect into the texture atlas. Later, an equally efficient sequence of simple, coherent branches was discovered that obviated the need for any lookup texture and removed precision issues inherent in the lookup texture strategy. The lookup function that provided the best balance of performance across Nvidia and AMD GPUs is listed here:

    vec3 getshadowtc(vec3 dir, vec4 shadowparams, vec2 shadowoffset)
    {
        vec3 adir = abs(dir);
        float m = max(adir.x, adir.y);
        vec2 mparams = shadowparams.xy / max(adir.z, m);
        vec4 proj;
        if(adir.x > adir.y) proj = vec4(dir.zyx, 0.0); else proj = vec4(dir.xzy, 1.0);
        if(adir.z > m) proj = vec4(dir, 2.0);
        return vec3(proj.xy * mparams.x + vec2(proj.w, step(proj.z, 0.0)) * shadowparams.z + shadowoffset, mparams.y + shadowparams.w);
    }

This function overall maps a world-space light-to-surface vector to texture coordinates within the shadowmap atlas. A useful trick is used in the first few lines for computing a depth for the shadowmap comparison - the maximum linear depth along the 3 axial projections is ultimately the linear depth for the cubemap face that will be later selected - and allows the depth computation to happen before the resulting projection is found via branching giving slightly better pipelining here. Note that this lookup function assumes a slightly non-standard orientation for the rendering of cubemap faces that avoids the need to flip some coordinates relative to the native cubemap face orientations. It otherwise lays out the faces in a 3x2 grid. Various math has been baked into uniforms and passed into this function to transform to post-perspective depth from linear depth for the later actual shadowmap test in the filtershadow function. The function is only listed here to give an idea of the performance of the unrolled cubemap lookup, so reader beware, it is not quite plug-and-play and some investigation of the engine source code is required for more details. The result of this lookup function is then passed into the filtershadow function listed above. These two little functions are rather important and inspired Tesseract's design; they represent its beating heart and make large numbers of omnidirectional shadowed lights possible.

All of the shadowmaps affecting a single frame are further aggregated into one giant shadowmap atlas, currently 4096x4096 using 16bpp depth texture format. This better decouples the shadowmap generation and lighting phases and allows lookups for any number of shadowmaps to be easily performed in a single batch or many shading passes. Various types of shadowmaps are stored in the atlas: unrolled cubemap shadowmaps for point lights, a simple perspective projection for spotlights, and cascaded shadowmaps for directional sunlight.

For cascaded shadowmaps for sunlight, Tesseract uses an enhanced parallel-split scheme with rotationally invariant world-space bounding boxes rounded to stable coordinate increments for each split as originally detailed for Dice's Frostbite engine. This allows for somewhat less waste of available shadowmap resolution than the standard view-parallel split scheme as well as combats temporally instability/shadow swim that would otherwise occur. For further information, see http://dice.se/publications/title-shadows-decals-d3d10-techniques-from-frostbite/
 
"Caching is the new culling." Lights can often have large radiuses that pass through walls and other such occluders, often making occlusion culling or view-frustum culling of light volumes ineffective. As an alternative to never the less greatly reduce shadowmap rendering costs for such lights, the shadowmap atlas caches shadowmaps from frame to frame, down to the granularity of individual cubemap faces, if no moving objects are present in the shadowmap. Lights in Tesseract usually only affect static world geometry, at least when individual cubemap faces are considered, so the majority of shadowmapped lights are not more expensive than unshadowed lights, adding only the cost of the shadowmap lookup and filtering itself. To further optimize the rendering of shadows for static geometry, for each frustum of each light, an optimal mesh is generated of all triangles contained only within that frustum and omitting all backfacing triangles. To avoid moving textures around within the atlas, cached shadowmaps attempt to retain their placement from the previous frame within the atlas. To combat fragmentation, if the atlas becomes overly full, cached shadowmaps are occasionally evicted from a quadrant window of the atlas that progresses through the atlas from frame to frame.

* Deferred shading and the g-buffer

After evaluating many alternatives, given the small range of materials used in Tesseract maps, it was decided that deferred shading, in contrast to competing methods such as light pre-pass or light-indexed, was the most sensible method for the actual shading/lighting step. Deferred shading provides other benefits such as easy blending of materials in the g-buffer before the actual shading step takes place.

Further, by use of tiled approaches to deferred shading, the cost of sampling the g-buffer can be largely amortized, to the extent that Tesseract's renderer is, in fact, compute bound by the cost of evaluating the actual per-light lighting equation on lights that pass culling/rejection tests, rather than bound by bandwidth or culling costs as other deferred renderers and related research claim to be.

For further information about the trade-offs involved in various deferred rendering schemes, see http://c0de517e.blogspot.com/2011/01/mythbuster-deferred-rendering.html or http://software.intel.com/en-us/articles/deferred-rendering-for-current-and-future-rendering-pipelines or http://aras-p.info/blog/2012/03/27/tiled-forward-shading-links/

Tesseract breaks the screen up into a grid of 10x10 tiles aligned to pixel group boundaries. Lights are then inserted into per-tile lists by computing a 2D bounding box of affected tiles. Finally, lights are batched into groups of at most 8 lights of equivalent type (shadowed or unshadowed, point or spot light) which are then evaluated per-tile in a single draw call. As many calls to the tile shader are made as necessary to exhaust all the lights in the per-tile list. Other lighting effects such as sunlight, ambient lighting, or global illumination is also optionally applied by the tile shader.

It was found that beyond coarse light culling and per-tile bucketing, more complicated schemes in the fragment shader for culling lights, possibly involving dynamic light lists, yield little to no actual speedups when measured against simpler rejection tests that involve static uniforms. The bandwidth costs of accessing dynamic light lists tend to actually exceed the costs of accessing the g-buffer in a tiled renderer thus motivating a simpler tile shader. As even using uniform buffers to store light parameters imposes a cost similar to a texture access, it is strongly preferred to use statically indexed uniforms to supply light parameters to minimize the cost of iterating through light parameters in the tile shader.

For information about computing accurate screen-space bounding rectangles for point light sources, see http://www.terathon.com/code/scissor.html

The actual g-buffer is composed a depth24-stencil8 depth buffer, an RGBA8 with diffuse/albedo in RGB and specular strength in A, an RGBA8 with world-space normal in RGB and either a scalar glow value or an alpha transparency or a multi-purpose anti-aliasing mask/depth hash value in A. When rendering transparenct objects, an extra packed-HDR (or RGBA8 if required) texture is used with additive/emissive light in RGB. On some platforms, a further RGBA8 texture is used to store a piece-wise encoded linear depth where directly accessing from the depth buffer texture is either slow or buggy. RGBA8 textures are used for all layers of the g-buffer both to support older GPUs that can't accept multiple render targets of varying formats and because RGBA8 textures provide a good trade-off between size and encoding flexibility.

World-space RGB8 normals are chosen as they are both temporally stable (no frame-to-frame jitter artifacts) and require almost no encode/decode costs like other eye-space normal encodings might. The additive/emissive layer allows for easy handling of environment maps or reflection effects. Overall, this layout provides a reasonably compact g-buffer while handling the range of materials used by Tesseract maps. For more information about g-buffer normal encodings, see http://aras-p.info/texts/CompactNormalStorage.html

* Mesh rendering

Where possible, Tesseract utilizes support for half-precision floating point in modern GPUs to reduce the memory footprint of mesh vertex buffers. Instead of the usual tangent and bitangent representation, Tesseract utilizes quaternion tangent frames (QTangents) for compressed tangent-space specification of mesh triangles, which combine well with the existing use of dual-quaternion skinning for animated meshes. For more information about QTangents, see http://www.crytek.com/cryengine/presentations/spherical-skinning-with-dual-quaternions-and-qtangents

* Decal rendering

Before final shading, any decaling effects are applied to the scene by blending into the g-buffer.

* Material shading/Light accumulation

The shading is evaluated into a light accumulation buffer, containing the final shaded result, that preferably uses the R11G11B10F packed floating-point format. When the GPU hardware does not support packed float format or is otherwise buggy (as observed on some older AMD GPUs that do not properly implement blending of packed float format render targets), a fallback RGB10 fixed-point format is used that is scaled to a 0..2 range to allow some overbright lighting and still provide somewhat better precision than an LDR RGB8 format. For more information about the packed floating-point format and its limitations, see http://www.opengl.org/registry/specs/EXT/packed_float.txt

Linear-space lighting and sRGB textures have also been avoided here because, during experimenting, it was found they unavoidably produce lighting values that quantize poorly in these lower precision formats, and the bandwidth cost of higher-precision formats was ultimately not worth the perceived benefits. The gamma-space lighting curves and values are well understood by Sauerbraten mappers, providing for better fill lighting due to softer/less harsh lighting falloff, interoperating better with a wealth of pre-existing textures optimized to look appealing under gamma-space lighting, and producing fewer banding artifacts with lower-precision HDR texture formats.

While the lighting thus still happens in gamma-space, overbright lighting values are never the less supported and utilized, motivating later tonemapping and bloom steps.

For more information about the trade-offs involved in working in gamma-space vs. linear-space, see http://filmicgames.com/archives/299

* Screen-space ambient obscurance

To help break up the monotony of those indoor areas of a map that may rely on ambient lighting and to help reduce the burden of requiring lots of point lights to provide contrast in such places, Tesseract implements a form of SSAO.

After the g-buffer has been filled, but before the shading step, the depth buffer is downscaled to half-resolution and SSAO is computed, utilizing both the downscaled depth and the normal layer of the g-buffer, into a another buffer packing both the noisy/unfiltered obscurance value and a copy of the depth in each texel. This buffer is then bilaterally filtered, efficiently sampling both the obscurance and depths in a single tap due to the aforementioned packing scheme. The final resulting buffer is then used to affect sunlight and ambient lighting in the deferred shading step.

In particular, Tesseract makes use of the Alchemy Screen-Space Ambient Obscurance algorithm detailed here: http://graphics.cs.williams.edu/papers/AlchemyHPG11/
Tesseract further incorpates improvements to the algorithm suggested here: http://graphics.cs.williams.edu/papers/SAOHPG12/

* Global illumination

One of the primary motivations for including global illumination in Tesseract was not so much to increase visual quality, but instead to actually increase performance. While Tesseract can support a large number of shadowed lights, eventually mappers with the best of intentions can defeat the best of engines. So having some form of indirect/bounced lighting allows for light to get to normally dark corners in a map that would otherwise require a lot of "fill" lights to brighten them up or otherwise rely on ugly/flat ambient lighting. Tesseract provides a form of diffuse global illumination for the map's global, directional sunlight that can thus help to brighten up maps, without requiring a lot of point light entities, so long as a mapper is careful to allow sunlight into the map interior.

Diffuse global illumination is computed only for sunlight using the Radiance Hints algorithm, which is similar to but distinct from Light Propagation Volumes. First, a reflective shadowmap is computed for the scene from the sun's perspective, storing both the depth and reflected surface color for any surface the sunlight directly hits. Then using a particular random sampling scheme, the reflective shadowmap is gathered into a set of RGBA8 cascaded 3D textures storing low-order spherical harmonics. 3D textures are used for both Radiance Hints and LPV algorithms as they allow for cheap trilinear filtering of the spherical harmonics. However, Radiance Hints still differs from the LPV approach in that it gathers numerous samples from the reflective shadowmap in one shading pass, rather than injecting seed values into a 3D grid and iteratively refining it, offering some performance and simplicity advantages for the case of single-bounce diffuse global illumination.

During this process, an ambient occlusion term is also computed beyond what is detailed by the basic Radiance Hints algorithm. Where possible, these 3D textures are cached from frame to frame. These cascaded 3D textures are then sampled in the shading step to provide both the sunlight global illumination effect as well as using the ambient obscurance term to implement an atmospheric/skylight effect.

For further information on Radiance Hints, see "Real-Time Diffuse Global Illumination Using Radiance Hints" at http://graphics.cs.aueb.gr/graphics/docs/papers/RadianceHintsPreprint.pdf or http://graphics.cs.aueb.gr/graphics/research_illumination.html

* Transparency, reflection, and refraction

Sauerbraten supported an efficient alpha material for world geometry, where first only the depth of world geometry was rendered, and then finally shading of the world geometry was rendered with alpha-blending enabled. This allowed only the first layer, and optionally before that a back-facing layer, to be rendered cheaply with no depth-sorting involved and is essentially a limited and cheaper form of the more general-purpose depth-peeling approach. This was sufficient for making props like windows or similar glass structures in levels by mappers. Though there is the drawback that transparent layers can't be seen behind other transparent layers, when used in moderation this drawback is not debilitating.

Tesseract expands upon this notion by shading transparent geometry in a separate later pass from opaque geometry, though both are accumulated into the light accumulation buffer. This has both the above-mentioned benefits, as well as allowing transparencies to be easily shadowed and lit just like any other opaque geometry and avoiding the need for a separate forward-renderer implementation. Because transparent geometry is first rendered into the g-buffer as if it were opaque, there is no need to do a prior depth-only rendering pass to isolate the front-most transparency layer like in Sauerbraten. Careful stenciling and scissoring is used to limit the actual shading step to only the necessary screen pixels that will have transparent geometry blended over them. The A channel in the normal layer of the g-buffer is used to store the alpha transparency value to control the blending output of this shading step.

This separate shading pass for transparent geometry also allows the light accumulation buffer from the previous opaque geometry pass to be easily resampled for screen-space reflection and refraction effects on materials like distorted glass or water, providing for a greater range of reflective and refractive materials than Sauerbraten was previously capable of. The emissive layer of the g-buffer is used for handling the refractive/reflective component of a material's shading.

Refraction effects are done by sampling the light accumulation buffer with added distortion, limited by a mask of refracting surfaces to control bleed-in of things outside the refraction area. For more information about the refraction mask technique, see http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter19.html

Reflection poses some difficulty since a separate render pass for every reflecting plane was no longer desired and would anyway be too expensive given the heavyweight deferred shading pipeline. A screen-space ray marching approach is instead used that in a small number of fixed steps walks through the depth buffer until it hits a surface and then samples the light accumulation buffer at this location. Some care is needed to fade out the reflections when either looking directly at the reflecting surface or when it might otherwise sample outside of screen borders or valid reflection boundaries in general. This approach has some potential artifacts when objects are floating above reflective surfaces, but on the other hand allows any number of reflective objects in the scene without requiring a separate render pass for every distinct reflection plane in the scene. For materials where screen-space reflections are inadequate, environment maps are instead used.

For further information on screen-space reflections, see http://www.crytek.com/cryengine/presentations/secrets-of-cryengine-3-graphics-technology or http://www.gamedev.net/blog/1323/entry-2254101-real-time-local-reflections/

* Particle rendering

Before the tonemapping step, Tesseract does particle rendering. Unlike in Sauerbraten, the depth buffer and shading results are more easily available without having to read back the frame buffer or using special-cased kluges to avoid such read-backs, making effects like soft or refractive particles cheaper and simpler to implement.
 
* Tonemapping and bloom

The light accumulation buffer is first quickly downscaled to approximately 512x512. A high-pass filter is run over this and then separably blurred to yield a bloom buffer that will be added to the lighting. This downscaled buffer (the non-blurred one) is also converted to luma values and iteratively reduced down to 1x1 to compute an average luma for the scene. This average luma is slowly accumulated into a further 1x1 texture that allows for the scene's brightness key to gradually adjust to changing viewing conditions. This accumulated average luma is fed back (via a vertex texture fetch in the tonemapping shader) into a tonemapping pass which maps the lighting into a displayable range. Note that the gamma-space lighting is converted temporarily into linear-space before tonemapping is applied and then converted back to gamma-space.

To better preserve color tones and in contrast to tonemapping operators that unfortunately tend to "greyify" a scene such as filmic tonemapping, Tesseract uses a simpler "photographic" tonemapping operator suggested by Emil Persson a.k.a. Humus, but applied to luma. See http://beyond3d.com/showthread.php?t=60907 or http://beyond3d.com/showthread.php?t=52747

For more information about the trade-offs involved in various tonemapping operators, see http://filmicgames.com/archives/category/tonemapping or http://mynameismjp.wordpress.com/2010/04/30/a-closer-look-at-tone-mapping/

* Generic post-processing

Before the final anti-aliasing and/or upscale step, any generic post-processing effects are applied. Currently this stage is not extensively utilized.

* Anti-aliasing

In contrast to Sauerbraten's forward renderer, Tesseract's performance is strongly impacted by resolution. Many schemes were evaluated for reducing shading costs, such as inferred lighting or interleaved rendering, but ultimately they were more complicated and no more performant or visually pleasing than simply rendering at reduced resolution and anti-aliasing the result with a final upscale to desktop resolution. Since Tesseract relies upon deferred shading, simply using MSAA by itself does not provide adequate performance due to increasing memory bandwidth usage from large multisampled g-buffer textures, though Tesseract does, in fact, implement stand-alone MSAA in spite of deferred shading. To this end, Tesseract provides several forms of post-processing-centric anti-aliasing, though mostly in the service of implementing one particular post-process anti-aliasing algorithm, Enhanced Subpixel Morphological Anti-Aliasing by Jorge Jiminez et al, otherwise known as SMAA.

The baseline SMAA 1x algorithm provides morphological anti-aliasing utilizing only the output color buffer. While this algorithm is an improvement over competitors such as FXAA, it still suffers from some temporal instability visible as frame-to-frame jitter/swim. To combat this, Tesseract implements temporal anti-aliasing that combines with SMAA to provide the SMAA T2x mode. The SMAA T2x mode, and temporal anti-aliasing in general, however, are often inadequate when things move quickly on-screen. Temporal anti-aliasing reprojects the rendering output of prior frames onto the current frame, and when the scene changes quickly, this is often not possible, so the temporal anti-aliasing fails to anti-alias in such cases. The A channel of the g-buffer's normal layer is used to provide a mask of all pixels belonging to moving objects in the scene, as distinguished from static world geometry, instead of requiring a more costly velocity buffer. Ultimately, only static geometry that is subject only to camera-relative movement participates in the temporal anti-aliasing which allows cheap computation of per-pixel velocity vectors from the global camera transforms without requiring storing object velocities.

To overcome the particular movement limitations of temporal anti-aliasing, SMAA also provides several modes that combine with multisample anti-aliasing, SMAA S2x and SMAA 4x, utilizing 2x spatial multisampling to provide temporal stability. SMAA 4x further combines temporal anti-aliasing and 2x multisample anti-aliasing with the baseline morphological anti-aliasing to provide a level of post-process anti-aliasing that can rival MSAA 8x modes while using far less bandwidth (only requiring 2x MSAA textures) and being much faster. 

Overall, SMAA gracefully scales up and down both in terms of performance and visual quality according to the user's tastes with its ability to incorporate all these disparate anti-aliasing methods, and while still interacting well with deferred shading. For more information about SMAA, see http://www.iryoku.com/smaa/ and also for further improvements recently suggested by Crytek see http://www.crytek.com/cryengine/presentations/cryengine-3-graphic-gems

Tesseract's deferred MSAA implementation renders into multisampled g-buffer and light accumulation textures. Before shading, an edge detection pass is run using information contained in the normal/depth hash layer of the g-buffer to fill the stencil buffer with an edge mask. The depth hash value, stored in the A channel of the normal layer of the g-buffer, is simply an 8 bit hash combining information about linear depth and material id that when combined with the world space normal stored in the RGB channels of this same texture provides reasonable and cheap determination of pixels that only very occasionally mispredicts edges. Single-sample shading is evaluated at internal/non-edge pixels, and multisample shading is evaluated at edge pixels. The tonemapping pass is able to run before the MSAA resolve to properly support the multisampled SMAA modes; however, this is not done by default for stand-alone MSAA usage as it can decrease performance for mostly imperceptible benefits in quality.

For more information about implementing MSAA in combination with deferred shading, see http://software.intel.com/en-us/articles/deferred-rendering-for-current-and-future-rendering-pipelines

As a final low-quality but highly performant backup and comparison basis, Tesseract provides an implementation of FXAA, though the baseline SMAA 1x is ultimately preferred.

For higher quality upscaling of the anti-aliased result than the usual linear filtering, Tesseract also provides a bicubic filter such as used for upscaling video. This can alleviate some blurring a linear filter would otherwise cause. For more information about fast bicubic filtering, see http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter20.html

* Further information

Homepage - Tesseract, http://tesseract.gg
Developer - Lee Salzman, http://sauerbraten.org/lee/

Last revised November 7, 2013
 
